A Coding Interface for an Evaluation Tool

John Peck, a test engineer at Knowles Electronics in Itasca, IL, has used ASCII interfaces to test equipment since he was a graduate student.

“I love test equipment with open, well-documented, ASCII command sets,” he says. “The plain text commands give a complicated instrument a familiar interface and an easy way to automate measurements.”

So when Peck needed to automate the process of reading his ultrasonic range finder’s voltage output, he wanted an ASCII interface to a voltmeter. He also wanted the meter to convert volts into distance, so he added an Atmel AVR Butterfly microcontroller into the mix (see Photo 1). “I thought it would be easy to give it a plain text interface to a PC,” he says.

Atmel AVR Butterfly

Atmel AVR Butterfly

The project became a bit more complex than he expected. But ultimately, Peck says, he came up came up with “a simple command interface that’s easy to customize and extend. It’s not at the level of a commercial instrument, but it works well for sending a few commands and getting some data back.”

If you would like to learn more about how to send commands from a PC to the AVR Butterfly and the basics of using the credit card-sized, single-board microcontroller to recognize, parse, and execute remote commands, read Peck’s article about his project in Circuit Cellar’s May issue.

In the italicized excerpts below, he describes his hardware connections to the board and the process of receiving remote characters (the first step in receiving remote commands). Other topics you’ll find in the full article include setting up a logging system to understand how commands are being processed, configuring the logger (which is the gatekeeper for messages from other subsystems), recognizing and adding commands to extend the system, and sending calibration values.

Peck programmed his system so that it has room to grow and can accommodate his future plans:

“I built the code with AVR-GCC, using the -Os optimization level. The output of avr-gcc –version is avr-gcc (Gentoo 4.6.3 p1.3, pie-0.5.1) 4.6.3.

“The resulting memory map file shows a 306-byte .data size, a 49-byte .bss size, and a 7.8-KB .text size. I used roughly half of the AVR Butterfly’s flash memory and about a third of its RAM. So there’s at least some space left to do more than just recognizing commands and calibrating voltages.”

“I’d like to work on extending the system to handle more types of arguments (e.g., signed integers and floats). And I’d like to port the system to a different part, one with more than one USART. Then I could have a dedicated logging port and log messages wouldn’t get in the way of other communication. Making well-documented interfaces to my designs would help me with my long-term goal of making them more modular.”

These are the connections needed for Atmel’s AVR Butterfly. Atmel’s AVRISP mkII user’s guide stresses that the programmer must be connected to the PC before the target (AVR Butterfly board).

Figure 1: These are the connections needed for Atmel’s AVR Butterfly. Atmel’s AVRISP mkII user’s guide stresses that the programmer must be connected to the PC before the target (AVR Butterfly board).

WORKING WITH THE AVR BUTTERFLY
The AVR Butterfly board includes an Atmel ATmega169 microcontroller and some peripherals. Figure 1 shows the connections I made to it. I only used three wires from the DB9 connector for serial communication with the PC. There isn’t any hardware handshaking. While I could also use this serial channel for programming, I find that using a dedicated programmer makes iterating my code much faster.

A six-pin header soldered to the J403 position enabled me to use Atmel’s AVRISP mkII programmer. Finally, powering the board with an external supply at J401 meant I wouldn’t have to think about the AVR Butterfly’s button cell battery. However, I did need to worry about the minimum power-on reset slope rate. The microcontroller won’t reset at power-on unless the power supply can ramp from about 1 to 3 V at more than 0.1 V/ms. I had to reduce a filter capacitor in my power supply circuit to increase its power-on ramp rate. With that settled, the microcontroller started executing code when I turned on the power supply.

After the hardware was connected, I used the AVR downloader uploader (AVRDUDE) and GNU Make to automate building the code and programming the AVR Butterfly’s flash memory. I modified a makefile template from the WinAVR project to specify my part, programmer, and source files. The template file’s comments helped me understand how to customize the template and comprehend the general build process. Finally, I used Gentoo, Linux’s cross-development package, to install the AVR GNU Compiler Collection (AVR-GCC) and other cross-compilation tools. I could have added these last pieces “by hand,” but Gentoo conveniently updates the toolchain as new versions are released.

Figure2

Figure 2: This is the program flow for processing characters received over the Atmel AVR Butterfly’s USART. Sending a command terminator (carriage return) will always result in an empty Receive buffer. This is a good way to ensure there’s no garbage in the buffer before writing to it.

HANDLING INCOMING CHARACTERS
To receive remote commands, you begin by receiving characters, which are sent to the AVR Butterfly via the USART connector shown in Figure 1. Reception of these characters triggers an interrupt service routine (ISR), which handles them according to the flow shown in Figure 2. The first step in this flow is loading the characters into the Receive buffer.

Figure 3: The received character buffer and pointers used to fill it are shown. There is no limit to the size of commands and their arguments, as long as the entire combined string and terminator fit inside the RECEIVE_BUFFER_SIZE.

Figure 3: The received character buffer and pointers used to fill it are shown. There is no limit to the size of commands and their arguments, as long as the entire combined string and terminator fit inside the RECEIVE_BUFFER_SIZE.

Figure 3 illustrates the Receive buffer loaded with a combined string. The buffer is accessed with a pointer to its beginning and another pointer to the next index to be written. These pointers are members of the recv_cmd_state_t-type variable recv_cmd_state.

This is just style. I like to try to organize a flow’s variables by making them members of their own structure. Naming conventions aside, it’s important to notice that no limitations are imposed on the command or argument size in this first step, provided the total character count stays below the RECEIVE_BUFFER_SIZE limit.

When a combined string in the Receive buffer is finished with a carriage return, the string is copied over to a second buffer. I call this the “Parse buffer,” since this is where the string will be searched for recognized commands and arguments. This buffer is locked until its contents can be processed to keep it from being overwhelmed by new combined strings.

Sending commands faster than they can be processed will generate an error and combined strings sent to a locked parse buffer will be dropped. The maximum command processing frequency will depend on the system clock and other system tasks. Not having larger parse or receive buffers is a limitation that places this project at the hobby level. Extending these buffers to hold more than just one command would make the system more robust.

Editor’s Note: If you are interested in other projects utilizing the AVR Butterfly, check out the Talk Zombie, which won “Distinctive Excellence” in the AVR Design Contest 2006 sponsored by ATMEL and administered by Circuit Cellar. The ATmega169-based multilingual talking device relates ambient temperature and current time in the form of speech (English, Dutch, or French). 

Arduino Uno Blueprint — Free Download

Elektor.Labs recently produced an Arduino Uno blueprint poster for element14. The poster details everything you need to know about the Arduino Uno.

Download it for free here.

Download your Arduino Uno poster

Download your Arduino Uno poster

The poster also includes coding notes that will get you working with your Arduino Uno in no time.

About the Arduino Uno:

  • Core Architecture: AVR
  • Core Sub-Architecture: megaAVR
  • Silicon Core: ATmega328
  • Features: The Arduino Uno is powered via USB or an external supply. It’s programmed with Arduino software.

Recent Arduino-related articles from Circuit Cellar:

 

Microcontroller-Based Markov Music Box

Check out the spectrogram for two FM notes produced by FM modulation. Red indicates higher energy at a given time and frequency.

Cornell University senior lecturer Bruce Land had two reasons for developing an Atmel AVR micrcontroller-based music box. One, he wanted to present synthesis/sequencing algorithms to his students. And two, he wanted the challenge of creating an interactive music box. Interactive audio is becoming an increasingly popular topic among engineers and designers, as we recently reported.

Land writes:

Traditional music boxes play one or two tunes very well, but are not very interactive. Put differently, they have a high quality of synthesis, but a fixed-pattern note sequencer and fixed tonal quality. I wanted to build a device which would play an interesting music-like note sequence, which constantly changed and evolved, with settable timbre, tempo, and beat… To synthesize nice sounding musical notes you need to control spectral content of the note, the rise time (attack), fall time (decay), and the change in spectral content during attack and decay.  Also it is nice to have at least two independent musical voices. And all of this has to be done using the modest arithmetic capability of an 8-bit microcontroller.

Land’s students subsequently used the music box for other projects, such as an auto-composing piano, as shown in the following video.

In early 2013 Circuit Cellar will run Land’s in-depth article on the Markov music box project. Stay tuned for more information.

From the IBM PC AT to AVRs & Arduinos (CC 25th Anniversary Preview)

During the last 25 years, hundreds of the world’s most brilliant electrical engineers and embedded developers have published articles in Circuit Cellar magazine. But only a choice few had the skill, focus, creativity, and stamina to consistently publish six or more articles per year. Ed Nisley is a member of that select group. Since Issue 1, Nisley has covered topics ranging from a video hand scanner project to X10 powerline control to Arduino-based designs to crystal characterization.

In the upcoming Circuit Cellar 25th Anniversary Issue—which is slated for publication in early 2013—Nisley describes some of his most memorable projects, such as his hand Scanner design from Issue #1. He writes:

The cable in the upper-left corner went to the serial port of my Genuine IBM PC AT. The hand-wired circuit board in front came from an earlier project: an 8031-based video digitizer that captured single frames and produced, believe it or not, RS-232 serial data. It wasn’t fast, but it worked surprisingly well and, best of all, the board was relatively inexpensive. Having built the board and written the firmware, I modified it to output compressed data from hand images, then wrote a PC program to display the results.

Combining a TV camera, a prototype 8031-based video digitizer, and an IBM PC with custom firmware and software produced a digital hand scanner for Circuit Cellar Issue 1. The aluminum case came from an external 8″ floppy drive!

The robust aluminum case originally housed an external 8″ floppy drive for one of my earlier DIY “home computers” (they sure don’t make ‘em like they used to!) and I assembled the rest of the hardware in my shop. With hardware and software in hand, I hauled everything to Circuit Cellar Galactic HQ for a demo.

Some of the work Nisley details is much more modern. For instance, the photo below shows the Arduino microcontroller boards he has been using in many of his recent projects. Nisley writes:

The processors, from the Atmel AVR microcontroller family, date to the mid-1990s, with a compiler-friendly architecture producing good performance with high-level languages. Barely more than breakout boards wrapped around the microcontrollers, Arduinos provide a convenient way to mount and wire to the microcontroller chips. The hardware may be too expensive to incorporate in a product, but it’s ideal for prototypes and demonstrations.

The Arduino microcontroller project provides a convenient basis for small-scale projects like this NiMH cell tester. Simple interconnections work well with low-speed signals and lowcurrent hardware, but analog gotchas always lie in wait.

Even better, a single person can still comprehend all of a project’s hardware and software, if only because the projects tend to be human scaled. The Arduino’s open-source licensing model fits well with my column’s readily available hardware and firmware: you can reproduce everything from scratch, then extend it to suit your needs.

Circuit Cellar’s Circuit Cellar 25th Anniversary Issue will be available in early 2013. Stay tuned for more updates on the issue’s content.