A few years ago, the Android Open Accessory initiative was announced with the aim of making it easier for hardware manufacturers to create accessories that work with every Android device. Future Technology Devices International (FTDI) joined the initiative and last year introduced the FTD311D multi-interface Android host IC. The goal was to enable engineers and designers to make effective use of tablets and smartphones with the Android OS, according to Circuit Cellar columnist Jeff Bachiochi.
The FTD311D “provides an instant bridge from an Android USB port(B) to peripheral hardware over general purpose input-out (GPIO), UART, PWM, I2C Master, SPI Slave, or SPI Master interfaces,” Bachiochi says.
In the magazine’s December issue Bachiochi takes a comprehensive look at the USB Android host IC and how it works. By the end of his article, readers will have learned quite a bit about how to use FTDI’s apps and the FT311D chip to turn an Android device into their own I/0 tool.
Bachiochi used the SPI Master demo to read key presses and set LED states on this SPI slave 16-key touch panel.
Here is how Bachiochi describes the FT311D and its advantages:
The FT311D is a full-speed USB host targeted at providing access to peripheral hardware from a USB port on an Android device. While an Android device can be a USB host, many are mobile devices with limited power. For now, these On-The-Go (OTG) ports will be USB devices only (i.e., they can only connect to a USB host as a USB device).
Since the USB host is responsible for supplying power to a USB peripheral device, it would be bad design practice to enable a USB peripheral to drain an Android mobile device’s energy. Consequently, the FT311D takes on the task of USB host, eliminating any draw on the Android device’s battery.
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All Android devices from V3.1 (Honeycomb) support the Android Open Accessory Mode (AOAM). The AOAM is the complete reverse of the conventional USB interconnect. This game-changing approach to attaching peripherals enables three key advantages. First, there is no need to develop special drivers for the hardware; second, it is unnecessary to root devices to alter permissions for loading drivers; and third, the peripheral provides the power to use the port, which ensures the mobile device battery is not quickly drained by the external hardware being attached.
Since the FT311D handles the entire USB host protocol, USB-specific firmware programming isn’t required. As the host, the FT311D must inquire whether the connected device supports the AOAM. If so, it will operate as an Open Accessory Mode device with one USB BULK IN endpoint and one USB BULK OUT endpoint (as well as the control endpoint.) This interface will be a full-speed (12-Mbps) USB enabling data transfer in and out.
The AOAM USB host has a set of string descriptors the Android OS is capable of reading. These strings are (user) associated with an Android OS application. The Android then uses these strings to automatically start the application when the hardware is connected. The FT311D is configured for one of its multiple interfaces via configuration inputs at power-up. Each configuration will supply the Android device with a unique set of string descriptors, therefore enabling different applications to run, depending on its setup.
The FT311D’s configuration determines whether each application will have access to several user interface APIs that are specific to each configuration.
The article goes on to examine the various interfaces in detail and to describe a number of demo projects, including a multimeter.
Many of Bachiochi’s projects use printable ASCII text commands and replies. This enables a serial terminal to become a handy user I/O device. This current probe circuit outputs its measurements in ASCII-printable text.
Multimeters are great tools. They have portability that enables them to be brought to wherever a measurement must be made. An Android device has this same ability. Since applications can be written for these devices, they make a great portable application tool. Until the AOAM’s release, there was no way for these devices to be connected to any external circuitry and used as an effective tool.
I think FTDI has bridged this gap nicely. It provided a great interface chip that can be added to any circuit that will enable an Android device to serve as an effective user I/O device. I’ve used the chip to quickly interface with some technology to discover its potential or just test its abilities. But I’m sure you are already thinking about the other potential uses for this connection.
Bachiochi is curious to hear from readers about their own ideas.
If you think the AOAM has future potential, but you want to know what’s involved with writing Android applications for a specific purpose, send me an e-mail and I’ll add this to my list of future projects!
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You can e-mail Bachiochi at jeff.bachiochi@imaginethatnow.com or post your comment here.
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Circuit Cellar's editorial team comprises professional engineers, technical editors, and digital media specialists. You can reach the Editorial Department at editorial@circuitcellar.com, @circuitcellar, and facebook.com/circuitcellar
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