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Reverse Engineering Electronics

Reverse engineering an electronic system can be a rewarding yet challenging endeavor. In the February 2015 edition of Circuit Cellar, engineer Fergus Dixon presents four reverse engineering projects and explains how he overcame a variety of challenges.

Dixon details the first project below:

One of my colleagues, who is the biomedical manager at a large hospital, was having issues with hospital gas panels failing and wanted a cheaper repair option. The gas panels were designed and manufactured by a local company that had gone bankrupt several years earlier. After taking a unit away to look it over, I found that the gas panel had a bright green vacuum fluorescent display with connectors for up to 24 inputs. Each input would show whether the gas supply was normal or in alarm, and thanks to some clever design would also show on the display an open or short circuit on the cable to the gas cylinder. There were 0 to 5-V analog inputs. There was a rechargeable 3.6-V battery on each gas panel to save RAM memory on power off (now this is usually done with EEPROM memory). The problem was that the gas panel would lose its memory when the battery failed or dropped below 2 V. Random characters would then appear on the screen, and the system error light would illuminate (see Photo 1).Dixon Photo 1 Hosquip Panel

The suggestion was to look at the microcontroller since this is usually where the memory was stored. The microcontroller was the popular but now obsolete Motorola 6805. A quick glance at the datasheet showed that it had no EEPROM or nonvolatile memory (i.e., memory that is not affected by a power-off cycle). Looking at the chips, one of the eight-pin chips was a Philips PCF8570 I2C memory chip with 256 bytes of memory and there were five of these making up to 1,280 bytes of memory. Since the display had one line of 40 characters and there were 24 alarm inputs each with an alarm message, a start-up message, and a normal operation message, there needed to be at least 26 messages × 40 characters or 1,040 characters, so this had to be where the message data was stored. The battery was the backup for this RAM, so it appeared the memory was battery-backed RAM (BBRAM). The memory voltage supply was held up by the battery, but when the battery failed, it dragged down the voltage supply rail. A quick inspection of the battery terminals showed some fuzziness and fine crystals indicating that it was leaking and was probably not operational any more.

To read the memory required an I2C reader. The easiest way to do this at the time was to make a prototype board using a Atmel ATmega32 and use two pins to drive the SDA and SCL lines. The output data was ported through a RS-232 converter to a computer. I wish I had more research here since I2C reader/writers are very cheap and I did not realize that the Atmel TWI port was actually an I2C port but with a different name due to the Philips trademark. Anyway, I read the datasheets for the I2C interface and made a small circuit which could read and write to one of the I2C memory chips. The I2C interface consists of Start bits, Write bits, Read bits, and Stop bits with the SCL clock line always being driven from the microcontroller but the SDA line being bidirectional (i.e., an input or an output).

After building the prototype and reading and writing to memory, the circuit managed to read and write the whole 1,280 bytes of memory in the gas panel, which was quite easy since the memory chips addresses lines were sequential (i.e., 000 001 010 011 100). The microcontroller was removed from the PLCC socket during this process to prevent any spurious I2C communications. The next part was to read the memory from a working machine since the gas panel I had was now full of corrupted data. After a few trips to the hospital later, I had the memory in a file, and straight away, the alarm messages could be seen as ASCII data. Each message was preceded by one byte which determined whether the gas alarm input was a warning, an alarm or turned off (see Photo 2).

Gas panel programmer

Gas panel programmer

The last challenge was the system error light. Even though the gas panel could now be programmed with the correct messages, the system error light remained on. A quick solution was to remove the driving resistor to this light, but then that meant any real system error would be missed. Looking through the gas alarm panel memory again showed that each alarm message had a trailing byte which looked like a checksum. The simplest checksum can be found by adding up all the bytes and this almost worked. Then I realized that the trailing spaces in the alarm messages were also used in the checksum and the game was over. Since then, a lot of gas panels have been able to repaired using the prototype circuit.

The complete article is available in Circuit Cellar 295.

AIR Module Enabled by Broadcom’s WICED Smart Bluetooth Technology

Anaren’s Wireless Group announced the release of its first AIR for Wiced Smart Bluetooth module and Atmosphere on-line developer platform as part of a strategic engagement with Broadcom Corp. This new relationship advances the goal of both companies to support designers, innovators, and end-equipment manufacturers looking for intuitive, easy-to-use developer tools, like Atmosphere, that will simplify the challenge of going wireless and speed up their time to market.AnarenDevKit

In beta trials of its new module and Atmosphere tool, customers were able to get their product “proof of concept” running in 90 minutes or less. The small, low-cost module comes pre-certified to global standards and includes comprehensive technical support to the mass market group of customers.

The Anaren’s Bluetooth Smart Development Kit’s (A20737A-MSDK1) features and advantages include:

  • Broadcom’s BCM20737 SoC
  • A20737A AIR for WICED module
  • Pre-certified to FCC/IC and ETSI compliant
  • Low power consumption
  • Works in conjunction with Anaren’s online Atmosphere development tool.
  • Generates and loads embedded code on the Multi-Sensor Development Board and creates an app that can be loaded onto a Bluetooth Smart mobile device

Source: Anaren

New Oscilloscopes with Capacitive Touch Screens, Zone Triggering

Keysight Technologies recently introduced the InfiniiVision 3000T X-Series digital-storage and mixed-signal oscilloscopes  with intuitive graphical triggering capability. This new oscilloscope series delivers capacitive touch screens and zone triggering to the mainstream oscilloscope market for the first time. The scopes help engineers overcome usability and triggering challenges and improve their problem-solving capability and productivity.

As digital speeds and device complexity continue to increase, signals under test are getting more complex, and engineers are more challenged to isolate anomalies in their devices. Intuitive graphical triggers, previously unavailable in mainstream oscilloscopes, help engineers debug and characterize their cutting-edge devices faster and more easily. With graphical triggers, engineers can use a finger to draw a box around a signal of interest on the instrument display to create a trigger.Keysight InfiniiVision

 

The new oscilloscope series offers upgradable bandwidths from 100 MHz to 1.0 GHz and several benchmark features in addition to the touch screen interface and graphical zone triggering capability. An uncompromised update rate of one million waveforms per second gives engineers visibility into subtle signal details. The series comes with six-instruments-in-one integration, including oscilloscope functionality, digital channels (MSO), protocol analysis capability, a digital voltmeter, a WaveGen function/arbitrary waveform generator, and an eight-digit hardware counter/totalizer. Finally, the 3000T X-Series delivers correlated frequency and time domain measurements using the gated FFT function for the first time in this class, to address emerging measurement challenges.

The 3000T X-Series supports a wide range of popular and emerging serial bus applications: MIL-STD 1553 and ARINC 429, I2S, CAN/CAN-FD/CAN-Symbolic, LIN, SENT, FlexRay, RS232/422/485/UART and I2C/SPI. The new gated FFT function allows engineers to correlate time and frequency domain phenomenon on a single screen. Finally, the power analysis, video analysis and hardware-based mask test option makes the 3000T X-Series a comprehensive mainstream oscilloscope.

The InfiniiVision 3000T X-Series includes 100-MHz, 200-MHz, 350-MHz, 500-MHz and 1-GHz models. The standard configuration for all models includes 4 Mpts of memory, segmented memory, advanced math, and 500-MHz passive probes. Keysight InfiniiVision 3000T X-Series oscilloscopes are now available starting at $3,350.

Source: Keysight Technologies 

Infineon Technologies Acquires International Rectifier

Infineon Technologies recently announced the closing of the acquisition of International Rectifier. As of January 13, 2015, the El Segundo-based company has become part of Infineon following the approval of all necessary regulatory authorities and International Rectifier’s shareholders.Infineon-Executive-Board

The combined company gains greater scope in product portfolio and regions, especially with small and medium enterprise customers in the US and Asia. The merger taps additional system know-how in power management. It expands the expertise in power semiconductors, also combining leading knowledge in compound semiconductors, namely Gallium Nitride. Furthermore, the acquisition will drive greater economies of scale in production, strengthening the competitiveness of the combined company.

Source: Infineon Technologies

Labcenter Proteus Version 8 Offer

In the January 2015 edition of Circuit Cellar, Labcenter Electronics is offering readers a 10% discount on Proteus Design Suite Version 8 (until March 2015). The Proteus Design Suite is a PC-based CAD tool that includes Schematic Capture (ISIS), Microprocessor Simulation (VSM) Advanced Simulation (ASF) for graphing of mixed mode designs and PCB Layout (ARES).logo with background_1

WHY SHOULD CC READERS BE INTERESTED?

The Proteus Design Suite integrates Schematic Capture, Circuit Simulation and PCB Layout. Circuit simulation is of the full schematic, including microprocessors from many of the major vendors, including, Microchip, Atmel, TI, ARM7, Cortex, etc., as well as analog and digital devices. Items such as LCDs, GLCDs, switches, and sensors are all simulated and operate in the schematic just as they would in a circuit on your work bench. During the simulation the user can interact with the design, stop, single step the code in the micro to verify its function. Debugging tools such as I2C and SPI are included. The PCB Package is easy to use, and intuitive. Proteus is an excellent tool for rapid design.

SPECIAL OFFER – PROTEUS VERSION 8

Proteus Version 8 Offer for Circuit Cellar—Save 10%. Use Promo Code CCIPRF2015 when placing your order. Offer expires March 31, 2015.

 

Circuit Cellar prides itself on presenting readers with information about innovative companies, organizations, products, and services relating to embedded technologies. This space is where Circuit Cellar enables clients to present readers useful information, special deals, and more.

12-W Receiver IC for Wireless Mobile Device Charging

At CES 2015, Toshiba America Electronic Components introduced its newest IC enabling wireless mobile device charging. The TC7765WBG wireless power receiver controller IC can manage the 12-W power transfer required for the wireless charging of tablet devices. The TC7765WBG is compatible with the Qi low-power specification version 1.1 defined by the Wireless Power Consortium (WPC). It delivers a user experience comparable to that of conventional wired charging for tablets, as well as smartphones and other portable devices.Toshiba TC7765WBG

The TC7765WBG was built with Toshiba’s mixed-signal process using a high-performance MOSFET design that maximizes power efficiency and thermal performance. The IC combines modulation and control circuitry with a rectifier power pickup, I2C interface, and circuit protection functions. Compliance with the “Foreign Object Detection” (FOD) aspect of the Qi specification prevents heating of any metal objects in the path of wireless power transfer between the receiver and the transmitter.

The 12-W TC7765WBG is designed in a compact WCSP-28 2.4 mm × 3.67 mm × 0.5 mm package. This further facilitates design-in and contributes to the new chipset’s backward compatibility with the lower-power receiver IC. Combining the TC7765WBG with a copper coil, charging IC, and peripheral components creates a wireless power receiver. Joining the receiver with a Qi-compliant wireless power transmitter containing a Toshiba wireless power transmitter IC (e.g., TB6865AFG Enhanced version) forms a complete wireless power charging solution.

Toshiba announced that samples of the TC7765WBG wireless power receiver IC will be available at the end of January, with mass production set to begin in Q2 2015.

Translating Values: Number-to-ASCII Conversion (EE Tip #147)

There is a neat trick for performing the number-to-ASCII conversion. (We are not sure of this trick’s origin. We’d love to hear from anyone who saw it long ago and can us more about it.)ASCII-iStock_000007340231_Large

One starts by defining an array in the 8051 family microcontroller’s code segment using the following command:

HEX_NUM: DB ‘0123456789ABCDEF’

Following the conventions that now seem to be pretty standard for 8051 family assemblers, this command cause an array whose values are the ASCII values of the characters within the single quotes to be stored in the code segment (assuming that that is the segment in which you are working). To convert a number between 0 and 15 to the relevant ASCII value, you now need only store the value to be converted in the accumulator, A, and the location of the array HEX_NUM in the data pointer, DPTR. You then give the single command MOVC A, @A+DPTR. This command uses the current value of the accumulator, the value you are looking to translate into an ASCII value, as an offset and reads the relevant value of the array into A (thus overwriting the value you wanted to “translate” with the correct translation). Thus, if A starts off with a 5, it ends up with the sixth element of the array (as there is an element with a zero offset)—which is just the ASCII value of 5, and all of this takes place without any hard work on the part of the programmer.—Ahron Emanuel, Shlomo Engelberg, and Dvir Ophir (“Virtual Instrumentation, Circuit Cellar 294, 2015)

Microcontroller-Based Duodecimal Clock Project

Programmers and embedded circuit designers know that decimal is not the preferred number system for digital devices. Binary, octal, and hexadecimal numbers rule the day, so they’re comfortable using strange number systems. Clocks are essentially duodecimal, or base-12, devices. Devlin Gualtieri designed a two-digit clock that only an engineer would love (and know how to read).

The seven-segment LED displays were mounted on the foil side of a single-sided circuit board. I used single-row, in-line sockets to facilitate soldering. The sockets also elevated the displays above the circuit board and the SMT resistors placed beneath the displays.

The seven-segment LED displays were mounted on the foil side of a single-sided circuit board. I used single-row, in-line sockets to facilitate soldering. The sockets also elevated the displays above the circuit board and the SMT resistors placed beneath the displays.

In Circuit Cellar 294, Gualtieri writes:

Digital timekeeping has supplanted mechanical timekeeping not simply because it’s usually more accurate, but because it’s inexpensive. Digital alarm clocks are available everywhere for about $10. You can build the clock in this article for a few dollars more, but with a larger share of satisfaction.

Many years ago, I started designing with Microchip Technology PIC microcontrollers. Since I now have the necessary tools for PIC development, it would be hard for me to change. Fortunately, there’s no reason for change, since PIC microcontrollers are adequate for most embedded designs, and they’re quite inexpensive. This design uses the PIC16F630 microcontroller.

PIC microcontrollers have a built-in oscillator with about a percent accuracy, which is reasonable for many embedded applications. This accuracy comes from digitally trimming an internal resistor-capacitor oscillator on the chip. One percent accuracy is not adequate for a clock, since that would be a quarter of an hour’s error each day.

It is possible to use an inexpensive 32.768-kHz tuning fork crystal with a PIC. This frequency is useful for clocks, since a power of two will divide this frequency to 1 Hz. However, this slow clock rate prevents rapid computation and some useful functionality, so I didn’t use such a crystal in this circuit. Instead, I used a half-sized 10.000-MHz TTL canned oscillator. Canned oscillators are easy to use, highly accurate, and not too expensive.

Seven-segment LED displays are common circuit items, and this clock has a two-digit LED display. One digit is for hours, and the other digit is for the twelfth fraction of the hour. It’s straightforward to display the numbers, zero through nine, on a seven-segment display, but 10, 11, and 12 pose a slight problem. The problem of the twelfth hour disappears when we use zero for that digit. This convention is familiar to most technical people.

In the hexadecimal convention, 10 is represented by an “A,” and 11 is represented by a “B.” A capital letter “A” is easy to show on a seven-segment display, but a capital letter “B” looks just like an eight. As a compromise, we could use a lowercase “b,” instead.

We could also use a capital “E,” for 11, but having an “E” follow an “A” disturbs my hexadecimal sensibility. Although I’m not dyslexic, when I see a “b” on a seven-segment display, I think “six.” Another common notation, which I learned in my elementary school “new math” courses, is to use “t” for 10 and “e” for 11. Figure 1 shows possible number representations for ten and eleven, including the ones I chose for this clock.

Representing 10 and 1 on a seven-segment display. The “t” representations are somewhat abstract, but they can’t be confused with other characters. The last pictured in each category are my choices, since they’re easiest to read.

Representing 10 and 1 on a seven-segment display. The “t” representations are somewhat abstract, but they can’t be confused with other characters. The last pictured in each category are my choices, since they’re easiest to read.

Gualtieri goes on to describe the circuitry.

One way to minimize I/O pin count in a microcontroller circuit is to use multiplexing whenever possible. If we were to drive each seven-segment display and their decimal points individually, we would need 16 output pins. If we multiplex, we need just nine; that is, eight for the segments and decimal point, and one extra for the digit select.
The segments on the displays need a reasonably high current, and they also need about 4.5 V drive. That’s because they’re 1″ displays with two LEDs connected in series for each segment. For this reason, the display chips are driven from a 12-V supply, and the microcontroller interfaces with two 7406 TTL open-collector inverter/driver chips. Green displays were used, but other colors are available.

Digit select is easily accomplished with two transistors, also driven by sections of a 7406. The 5-V power for the PIC microcontroller and the clock oscillator is derived from a voltage regulator powered by the 12-V supply. A 12-V “wall wart” is a safe and convenient way to power this circuit, which is shown in Figure 2. The maximum current draw for my circuit was 250 mA. The circuit has two buttons for setting the clock by ramping the digits up or down.

This is the duodecimal clock’s circuitry. One nice thing about most embedded systems projects is that the software allows reduction in hardware complexity.  The particular displays used were green Lumex LDS-AA12RI displays.

This is the duodecimal clock’s circuitry. One nice thing about most embedded systems projects is that the software allows reduction in hardware complexity. The particular displays used were green Lumex LDS-AA12RI displays.

The nearby photo shows the the clock mounted in a custom case. You can see an AM/PM dot and the seconds flashing dot. The time setting buttons are mounted at the rear of the enclosure, but you could place them anywhere. A green filter increases the digit contrast.

The clock mounted in a custom case

The clock mounted in a custom case

The complete article appears in Circuit Cellar 294 (January 2015).

New Ultra-Low Power Precision Op-Amps

ON Semiconductor recently unveiled a new family of ultra-low power precision operational amplifiers. The precision NCS325 and NCS333 CMOS op-amps deliver zero drift operation and quiescent current for front-end amplifier circuits and power management designs.NCS325-333-Hires

The op-amp devices enhance the accuracy of motor control feedback and power supply control loops, thereby contributing to higher overall system efficiency. These devices are complemented by the new NCV333 automotive-qualified (AEC-Q100 grade 1) op-amp offering similar functional performance for power train, braking, electronic power steering, valve controls, and fuel pump and fuel injection system applications.

Features include:

  • High DC precision parameters, such as the 10 µV maximum input offset voltage at ambient temperature and the 30 nV/°C of offset temperature drift
  • Minimal voltage variations over temperature along with close to zero offset
  • Rail-to-rail input and output performance and are optimized for low voltage operation of 1.8 volt (V) to 5.5 V, with a best in class quiescent currents of 21 µA and 17 µA respectively at 3.3 V.
  • Operate with a gain bandwidth of 350 kHz with ultra-low peak-to-peak noise down to 1.1 µV from 0.1 Hz to 10 Hz.

The NCS325 is available in a 3 mm × 1.5 mm five-pin TSOP package. It costs $0.35 per unit in 3,000-unit quantities.

The NCS333 comes in a 1.5 mm × 3 mm SOT23-5 package or in a 2 mm × 1.25 mm SC70-5. It costs $0.5 per unit in 3,000-unit quantities.

Microchip Joins Linux Foundation & Automotive Grade Linux

Microchip Technology recently announced that it joined The Linux Foundation and Automotive Grade Linux (AGL), which is an open-source project developing a common, Linux-based software stack for the connected car. Additionally, Microchip has begun enabling designers to use the Linux operating system with its portfolio of MOST network interface controllers.Microchip MOST

AGL was built on top of a stable Linux stack that is already being used in embedded and mobile devices. The combination of MOST technology and Linux provides a solution for the increasing complexity of in-vehicle-infotainment (IVI) and advanced-driver-assistance systems (ADAS).

The MOST network technology is a time-division-multiplexing (TDM) network that transports different data types on separate channels at low latency and high quality-of-service. Microchip’s MOST network interface controllers offer separate hardware interfaces for different data types. In addition to the straight streaming of audio or video data via dedicated hardware interfaces, Microchip’s new Linux driver enables easy and harmonized access to all data types. Besides IP-based communication over the standard Linux Networking Stack, all MOST network data types are accessible via the regular device nodes of the Linux Virtual File System (VFS). Additionally, high-quality and multi-channel synchronous audio data can be seamlessly delivered by the Advanced Linux Sound System Architecture (ALSA) subsystem.

Support is currently available for beta customers. The full version is expected for broad release in October.

Source: Microchip Technology

CY8CKIT-042-BLE Bluetooth Low Energy Pioneer Kit

At just $49, Cypress Semiconductor’s CY8CKIT-042-BLE Bluetooth Low Energy Pioneer Kit is an affordable tool for creating BLE applications with a PSoC 4 and PRoC BLE devices. The kit supports system-level designs using the PSoC Creator IDE. It is compatible with Arduino shields hardware.IMG_20150106_080852956_HDR

The kit comprises:

  • BLE Pioneer Baseboard preloaded with CY8CKIT-142 PSoC 4 BLE module
  • CY5671 PRoC BLE Module
  • CY5670 – CySmart USB Dongle (BLE Dongle)
  • A quick start guide
  • USB Standard A-to-Mini-B cable
  • Four jumper wires (4 inches) and two proximity sensor wires (5 inches)
  • A coin cell (3-V CR2032)

Source:  Cypress Semiconductor

GP691 ZigBee Radio Chip and GPM6000 Modules for IoT

At CES 2015, GreenPeak Technologies announced a new GP691 ZigBee communication controller chip and GPM6000 integrated ZigBee modules for Internet of Things applications and smart home devices.IoTGreenPeak

The GP691 ZigBee communications controller provides IEEE Standard 802.15.4-compliant robust spread spectrum data communication in the worldwide 2.4-GHz band. It can run the full stack and application for ZigBee applications, including ZHA and ZLL profiles. In addition to a radio transceiver, the GP691 comprises a real-time Medium Access Control (MAC) processor, microcontroller, security engine, 16-KB RAM, and 248-KB flash memory. The GP691 is over the air upgradable and includes support for new 802.15.4-based standards upon availability, such as Thread.

ZigBee 3.0 supports a wide range of applications (e.g., home, industrial automation, and smart energy). IEEE 802.15.4-compliant, it can cover a complete home with multiple floors. Plus, it can manage dead spots and Wi-Fi interference via mesh networking. ZigBee 3.0 also supports large networks comprising thousands of devices, which also makes it suitable for industrial applications and building automation. ZigBee 3.0 also includes Green Power, part of the ZHA and ZLL profiles, which supports energy harvesting and battery-free applications. Without requiring batteries, these self-supporting devices typically generate (harvest) just enough power to transmit a brief command to the network via ZigBee.

With its partner Universal Scientific Industrial (Shanghai) Co, Ltd (USI), GreenPeak developed “an integrated module for the GP691 that reduces product design company’s time to market without having to solve RF product integration challenges or to worry about international wireless certification.”

The 25 x 17 x 2.5 mm pre-integrated, pre-certified module adds a power stage/LNA providing up to 20-dBm output power, special transmit and receive circuitry, and an integrated antenna plus a connector for a second external antenna enabling antenna diversity configurations, which all together, allow for greater range and robustness, providing coverage throughout an entire home. This module will be offered as the first in the GPM6000 module series optimized for smart home solutions.

Source: GreenPeak

Tips for Measuring Small Currents

Most inexpensive hand-held multimeters have measurement ranges from several amps to single-digit milliamps. While generally handy, such meters are insufficient for sensitive current measurements. There is a solution though. With the following project, you can extend the current measurement range from milliamps down into the nanoamp and picoamp range with simple, low-cost circuits.

In his January 2015 article (Circuit Cellar 294), David Ludington writes:

Most inexpensive hand-held multimeters have current measurement capability. The measurement range for these meters extends from several amps down to single-digit milliamps and sometimes into the microamp range. While this measurement capability is sufficient for many applications, there are times when more sensitive current measurement is required. There are meters available which measure much lower levels of current, but these meters are also more expensive and are often dedicated to just this one measurement function.

The goal of this article is to extend the basic hand-held current measurement range from milliamps down into the nanoamp and picoamp range with relatively simple, low-cost circuits. First, I’ll describe several types of current sources with their relevant performance characteristics that affect measurement circuits. Then, I’ll present practical circuits that deliver high performance at low cost. Each of these circuits will be analyzed to determine what level of measurement performance can be expected. General design issues common to all of the circuit techniques will also be discussed and recommended circuit component and layout techniques will be provided. Finally, two of the circuits will be discussed that were built and tested to demonstrate the desired goal of measuring nanoamp and picoamp currents.

CURRENT SOURCES

Figure 1 shows several types of current sources. Figure 1a is the symbol of what electrical engineers call an ideal current source. It can have any level of current at the output and the output impedance is infinite. The result is that the output current is not affected at all by the characteristics of the measurement circuit. Of course no actual current source is ideal, but this is still a useful concept for approximating actual circuits and is used as a source in circuit simulation programs such as PSPICE.

Figure 1: The current sources: ideal (a), semiconductor (b), and resistive (c)

Figure 1: The current sources: ideal (a), semiconductor (b), and resistive (c)

Figure 1b shows a current source which uses semiconductor transistors (either bipolar or FET). In this circuit, the output impedance is not infinite but can still be quite high (megohms). This means that varying voltages at the collector of the bipolar transistor or drain of the FET transistor have little effect on the output current as long as the voltage is not large enough to affect transistor operation.

Figure 1c is the least ideal of the current sources. The current is generated by the voltage difference across the resistor R2. Any measurement voltage developed by the measurement circuit directly affects the voltage difference across the source resistor. This changes the current that is measured which can result in measurement error. Having said that, Keithley Instruments in Low Level Measurements Handbook (6th edition, page 2-20) uses this current source model in defining what they call the feedback ammeter (also transimpedance amplifier). It may well be that, in real-world circuits, this model is the one that describes the majority of practical applications.

Although the current sources just described are not part of the measurement circuit itself, it is helpful to understand their limitations so that the measurement circuits can be designed to disturb the current source as little as possible. In this way, measurement error is minimized.

CURRENT MEASUREMENT CIRCUITS

In the past, current was measured directly with a moving coil meter. Now using semiconductor technology, voltage is the parameter that is measured directly. The current to be measured is first converted to a voltage by flowing through a load resistor. The resultant voltage is then measured and along with the load resistor is used to calculate the input current.

Figure 2: Current measurement circuits: resistive (a), transimpedance (b), and integrator (c)

Figure 2: Current measurement circuits: resistive (a), transimpedance (b), and integrator (c)

Figure 2 and Figure 3 show several circuit techniques which are used to convert current to a voltage. Figure 2 shows the basic techniques, while Figure 3 shows modifications to two of these basic circuits, which give more accurate results and extend the measurement range. The symbol for the input current used in these circuits is the same as the symbol for the ideal current source used in Figure 1; but in this case, it is used to show where the input current connects to the measurement circuit and can represent any of the described current sources.

Figure 3: Modified current measurement circuits: modified resistive (a) and modified transimpedance amplifier (b)

Figure 3: Modified current measurement circuits: modified resistive (a) and modified transimpedance amplifier (b)

RESISTIVE CIRCUIT

Figure 2a is the least complicated of the measurement circuits. In this circuit, the current source is connected to one end of the load resistor R1 and the other end of the resistor is connected to ground or some other reference point. The voltage developed across this resistor is measured with the voltmeter and used to calculate the input current. This circuit is very simple and is often used on the spot with an available resistor for quick measurements at the workbench or in the field.
The voltage that is developed across the load resistor is called the burden voltage. For a current source that is nearly ideal (such as the transistor source), the burden voltage has relatively little effect on the current being measured unless it is large enough to change the internal working of the current source. For resistive current sources, the burden voltage can directly interact with the current source and give erroneous current readings. This occurs because the load resistor becomes part of the current generating resistance which reduces the current. To minimize this interaction, the load resistor should be much smaller that the output resistance of the current source. The corresponding burden voltage will then also be small.
When a simple hand-held voltmeter is used, the measured voltage cannot be too small because these meters rarely measure below 1 mV. Thus, a compromise is needed between measurement accuracy and a low voltage burden.

Most hand-held meters have a 10-MΩ input impedance on the voltage scale. The load resistor R1 will be in parallel with the meter impedance and needs to be selected appropriately to give the desired equivalent measurement resistance. An example in Table 1 shows the resistor values needed to give a measurement voltage of 50 mV for the given currents. This fairly low value of measurement voltage significantly decreases the burden voltage while at the same time providing enough voltage to give measurement accuracy on the order of 10%.

Table 1: Resistor Values versus Input Current for Resistive Circuit (*Rounded off value. Less than 0.1% error.)

Table 1: Resistor Values versus Input Current for Resistive Circuit (*Rounded off value. Less than 0.1% error.)

When making a current measurement with the resistive circuit, it is always a good idea to try several resistors of different values to see what voltage results. If changing resistor values by a certain amount changes the measurement voltage by the same amount, then the source current is not being affected by the measurement (burden) voltage. In this case you can use the higher value of resistance to get more output voltage and more measurement accuracy. Conversely, if the corresponding measurement voltage increases less than the amount of the resistor value change, the source current is being affected by the measurement circuit and the smaller resistor value should be used.
A small modification to the circuit as shown in Figure 3a gives improved performance by removing the burden voltage at the expense of adding a variable power supply. This can be particularly useful as a quick measurement tool using an available workbench power supply. The power supply is adjusted until VOUT is 0 V. The value of the current is then obtained by dividing the measured value of the power supply voltage by the resistance R1. In this way the burden voltage is removed and the load resistor can be increased so that the power supply voltage can be greater than 50 mV. This will give more accuracy in the measurements.
Because VOUT is zero, the leakage current going into the hand-held voltmeter is zero and the finite input impedance (10 MΩ) of the meter does not affect the measurement. Even when VOUT is not exactly zero, the leakage current is still small. For example, for VOUT < 5 mV, the leakage current will be less than 500 pA. This gives measurement accuracy of 1% or better for input currents greater than 50 nA. Since the resistor R1 in this modified circuit is not developing a burden voltage, the resistor value is decoupled from the input current and can be any practical value depending only on the maximum voltage of the power supply.

TRANSIMPEDANCE AMPLIFIER

Figure 2b shows the circuit for a transimpedance amplifier. This is perhaps the most versatile of the current measurement circuits in that it can cover a large current measurement range using a simple circuit. In this circuit, the output from the current source is connected to the negative input of the operational amplifier while the positive input of the amplifier is connected to a reference voltage. This reference voltage is typically the circuit ground when there are bipolar power supplies and some intermediate voltage when there is a single power supply.
The inputs of an operational amplifier have very high input impedances (greater than 1 GΩ) so that little current goes into the amplifier. Thus, the input current drives the negative input toward one of the power supply voltages depending on the polarity of the input current. This causes a voltage difference between the amplifier inputs which is then amplified with the large internal open loop gain of the amplifier. As a result, the amplifier output voltage moves in a direction to provide current through resistor R2 which is opposite to the input current. Equilibrium is achieved when the amplifier output voltage is such that the current through R2 is equal in magnitude to the input current. With an ideal amplifier and no offset voltage, this results in 0 V at the negative terminal matching the voltage at the positive terminal. Only the value of resistance R2 and the amplifier output voltage are needed to determine the input current. Since we know the resistance value and can measure the output voltage, we can calculate the current through R2 which will equal the magnitude of the input current.
Since there is no burden voltage, the input current is not affected by the value of the feedback resistor R2 or the magnitude of the output voltage. The output voltage is constrained by the power supply voltages, but in principle there is no constraint on the value of the feedback resistor. Table 2 shows R2 resistor values for several nominal input currents for an output voltage (VOUT) of 1 V.
As seen in Table 2, the resistor values get quite large for small currents. These large value resistors are expensive and are often also physically large. Also, circuit constraints like stray capacitance can have an appreciable effect on the circuit when the resistor value is large.

Table 2: Resistor values for nominal input currents and VOUT = 1 V

Table 2: Resistor values for nominal input currents and VOUT = 1 V

There are two ways to reduce the resistor value required for a particular input current. One way is to allow smaller voltages than 1 V to represent the input current. This is acceptable as long as all anomalous voltages in the circuit due to circuit imperfections are calibrated out. This calibration can be either physical using potentiometers to cancel offset voltages. Alternatively, data calibration can be used by measuring the output voltage without the input current and then subtracting that data from the output voltage with the input current. Thus, as the input current (and corresponding output voltage) are reduced, the measurement voltage will still have sufficient accuracy.
The second way to measure lower values of current with a lower resistor value is to use the modified transimpedance amplifier shown in Figure 3b. Here, the output voltage is reduced by the voltage divider consisting of R3 and R4 before driving the feedback resistor R2. If the feedback resistor here is the same as the feedback resistor without the divider, the current flowing to the negative input terminal will be less than before. The internal gain in the amplifier will make the output voltage larger to compensate. For a voltage division of 10, the input current is 10 times lower for the same output voltage as before. One caution: although this circuit does give flexibility in the design, care is needed because there is an amplifier voltage gain equal to the divider ratio. The internal offset voltage and noise voltage of the amplifier are multiplied by this amplifier gain along with the current signal.

The complete article appears in Circuit Cellar 294 (January 2015).

 

Seven Engineers on the Future of Electrical Engineering

The Circuit Cellar staff thought it would be interesting to kick off 2015 by asking several long-time contributors about the future of electrical engineering and embedded systems. Here we present the responses we received to the following questions: What are your thoughts on the future of electrical engineering? What excites you? Is there something in your particular field of interest that you think will be a “game changer”?

STEVE CIARCIA: Frankly speaking, if I was smart enough to accurately predict the future, I wouldn’t be doing all this again. Seriously, “What excites me in the future?” shouldn’t be the question I’m answering here. Instead, it should be  how much does all this embedded stuff we’re seeing and talking about today look like a classic case of déją vu to me. Circuit Cellar started 40 years ago in BYTE to promote my enthusiasm for professional-level DIY computer applications (albeit mostly embedded). The names have changed to Maker this and that and Raspberry Pi whatever, but what once was, still is. Solder fumes aside, Circuit Cellar has always been about nurturing the talented engineer who designs the game changer. (Steve is an electrical engineer who founded Circuit Cellar in 1988.)

DAVID TWEED: Embedded technology is becoming more pervasive, appearing in more and more places in our lives. Embedded processors have become as powerful as desktop machines were just a few short years ago, and the their ability to connect to the world at large through high-bandwidth wireless communications has grown to match this. This is both exciting and scary, because it becomes a powerful enabler for both positive and negative changes in how we live our lives. Take the ubiquitous “smart phone” as an example. It can process two-way audio, video, GPS data, and an Internet connection simultaneously in real time. This enables powerful applications such as GPS-based route finding that can give you verbal and pictorial directions to get you where you want to go. But, as anyone who watches the popular crime drama N.C.I.S. knows, that same technology can be used to track your phone’s location, along with everything it can “see” and “hear,” including the phone calls you have made. While that kind of surveillance can be used it positive ways, such as to aid you in an emergency, it can also be used to invade your privacy. Can you really be sure that everyone in law enforcement and other areas of government has only your best interests in mind when accessing your data? The increased power of embedded systems means that autonomous mechanisms gain capabilities they didn’t have before. Fully-autonomous vehicles—cars, trucks, trains, and aircraft—will be able to carry people and goods long distances over arbitrary routes. Factory automation will become more generic, because complex general-purpose mechanisms will be as easy to use as purpose-built mechanisms that only do one thing, because the software will manage all of the low-level details of “training” the system. Machine vision will be an important part of this, giving the system the feedback it needs to interact with objects and people. “With great power comes great responsibility.” This has never been more true. I’m excited by the possibilities that increasingly powerful embedded technology will open up for us, but let’s make sure that it is used responsibly! (David is a professional electrical engineer and long-time Circuit Cellar author and technical editor.)

ROBERT LACOSTE: I think the most significant change in embedded systems these last years is the nearly mandatory inclusion of Internet connectivity. It’s called the Internet of Things (IoT). Just enter those three words in Google and the 752 million results you get will show it’s a quite hot topic. When a customer meets with us to discuss a potential new product (whatever it is), the question is no longer: “Should it be connected?” The question is: “How should it be connected?” Having said that, the key difficulty is the long list of wireless protocols trying to become the ubiquitous solution for IoT: Wi-Fi, Bluetooth, Bluetooth Low Energy, ZigBee, Zwave, 6LowPan, and a hundred others. Bluetooth seems the clear winner for smartphone-based products, but what about the other applications like home automation, logistics, smart metering, or dog tracking? Which protocol(s) will be the winner(s)? Which one will be natively supported on our Internet access gateways or even rolled-out worldwide? Will it be Thread, sponsored by Google itself? Or will it be another derivative of Bluetooth, due to its huge predominance? (The overall sales of Bluetooth-capable chips already exceed four times the human population on earth.) Or could it be one of the machine-to-machine variants of 3G/4G cellular standards being studied? Or perhaps it will be one of the solutions proposed by one of the many startups working on the technology? Or maybe it will be a completely new protocol that we’ll invent? I don’t know the answer, but the result will be the next game changer! (Robert is an electrical engineer and Circuit Cellar columnist. In 2003, he founded ALCIOM, an electrical engineering firm near Paris, France.)

CHRIS COULSTON: While tech will companies continue to evolve existing technologies to offer more features, with lower power and at a lower cost, I think that the most exciting and revolutionary technology is to be found in the Internet of Everything (IOE) concept. Hardware supporting the IOE offers up the tantalizing potential to free our designs from physical interconnects, giving our designs world wide access, allowing us to interact with our designs in real time, and allowing our design to access the almost unlimited diversity of services available on the Internet. I am excited to explore a design space that enables me to connect something trivial like my key-ring to the Internet. The Raspberry Pi was the first breakthrough with companies like Intel redefining the cutting edge with their Edison module. There are several limiters to the IOE concept including power consumption and standardization. As these issues are addressed, the potential of the IOE concept will only be limited to the creativity of engineers and makers everywhere. (Chris is a professor of electrical and computer engineering at Penn State, Behrend. He’s also technical reviewer for Circuit Cellar.)

GEORGE NOVACEK: Embedded controllers are essential components of automatic systems. Without  automation, many products could not even be manufactured. Machines, such as aircraft, medical equipment, power generators, etc. could not be operated without the assistance of smart control systems. Until some, not yet invented, technology makes electronics obsolete, the future of embedded controllers will remain bright. In the coming years, more and more engineers will be focusing on system design, while only the brightest ones will be developing microelectronic components for those systems—more sophisticated, more integrated, faster, smaller, hardened to environment, consuming less power. There continues to be a trend towards universal embedded controllers. These, equipped with the appropriate sensors and actuators and loaded with a particular application software, could be used for fly-by-wire, or for control of an industrial machinery or just about everything else. Design engineers need to be cautious not to put powerful, yet inexpensive controllers into new products just because it  can be done. There is already a proliferation of simple  consumer products equipped, without any sensible need, with microcontrollers. This often leads to lower reliability, shorter life and, because these products are usually not repairable, to greater cost of ownership and waste. (George is professional engineer and Circuit Cellar columnist who served as president of a multinational manufacturer of embedded control systems for aerospace applications.)

ED NISLEY: The rise of the Maker Movement changes everything in the embedded systems field: Makers take control over the devices in their lives, generally by repurposing embedded hardware in ways its designers never intended. The trend becomes clear when dirt-cheap USB TV tuners become software defined radios. Embedded systems must eventually sprout exposed (and documented!) interfaces, debugging hooks, and protocols, because collaboration with Makers who want to turn the box inside-out and build something better can enrich our world beyond measure. Excluding those people won’t work over the long term: just as DRM-encumbered music became unacceptable, welded-shut embedded systems will become historic curiosities. You can make it so! (Ed is an electrical engineer and long-time Circuit Cellar columnist and contributor.)

KEN DAVIDSON: Twenty-five years ago, while developing the Circuit Cellar Home Control System (HCS) II, our group created a series of interface boards that could be placed around the house and communicate using RS-485. Tons of discrete wire running throughout buildings was the norm at the time, and the idea of running just a single twisted pair between units was novel and exciting. This all predated inexpensive Ethernet and public Internet. Today, such distributed intelligence has only gotten better, smaller, and cheaper. With the Internet of Things (IoT) everybody is talking about, it’s not unusual to find a wireless interface and embedded intelligence right down to the level of a light bulb. There was an episode of The Big Bang Theory where the guys set up the apartment lights so they could be controlled from anywhere in the world. Everyone got a laugh when the “geeks” were excited when someone from Japan was blinking their lights. But the idea of such embedded intelligence and remote access continuing to evolve and improve truly is exciting. I look forward to the day in the not-too-distant future when such control is commonplace to most people and not just a geeky novelty. (Ken is an embedded software engineer who has been contributing to Circuit Cellar for years as an author and editor.)

These responses appear in Circuit Cellar 294 (January 2015).

Expanded Multi-Range Programmable DC Power Supply Offerings

B&K Precision recently announced the 9200 Series, which is its newest multi-range programmable DC power supply line. The series includes four 200- to 600-W models that can deliver power in any combination of the rated voltage and current up to the maximum output power of the supply. With voltage and current ranges up to 150 V and 25 A, the programmable DC power supplies are well suited for electronics manufacturing, R&D, and more.BK-9201_front

 

Multi-range power supplies provide greater flexibility than traditional power supplies. The supplies can provide any combination of higher voltage or higher current along a maximum power curve. This design helps save both bench space and cost by eliminating the need for having multiple power supplies on your workbench.

 

On the front panel, the 9200 Series features a high-resolution, 1 mV/0.1 mA display, output on/off control, and a handy user interface with numerical keypad, cursor keys, and rotary control knob for adjusting voltage and current settings. The power supplies also provide internal memory storage to save and recall up to 72 different instrument settings.

 

For programming and remote control, the 9200 Series offers list mode programming, remote sense, and standard USBTMC-compliant USB, RS-232, and GPIB interfaces supporting SCPI commands. Remote control software is provided for front panel emulation, execution of internal and external program sequences, and logging measurements via a PC. The application software can also be integrated with National Instrumnets’s Data Dashboard for LabVIEW app (available for Android and iOS), allowing users to create custom dashboards on smartphones and tablets for additional monitoring functions.

 

the 9200 Series models are all backed by a standard 3-year warranty and list at the following prices:

  • 9201 (60 V, 10 A, 200 W): $680
  • 9202  (60 V, 15 A, 360 W): $880
  • 9205 (60 V, 25 A, 600 W): $1,380
  • 9206  (150 V, 10 A, 600 W): $1,495