Using the PIC 18F2455 as the UBW (USB Bit Wacker)This little one has promise
What is not to like about this device? It is low cost, fairly sophisticated, low power, and accessible. It's magic is not the hardware, it is the firmware. This is also it's weakness. Be very careful to NOT run firmware D version 1.2... more on this later. The UBW (USB Bit Wacker) was purchased from http://www.sparkfun.com. While sparkfun has reproduced this board, the concept and the firmware came from here.
The UBW and its Firmware
The real heart of the UBW is not the hardware (after all, there is not much to it). The firmware concept, however, is really something. While the concept is great, we have to wonder about some aspects of its implementation. When things are setup correctly, the device shows up as a "CDC RS-232 Emulation Device" when attached to a Windows USB port. The father of the UBW, Brian Schmalz, wrote us to correct an error in our earlier commentary. We had complained about the need to load a driver before one can use the UBW on windows. Brian pointed out that this is not strictly the case. The CDC(Communication Device Class) driver required is already built in to Windows and the additional file simply tells Windows to use the the driver for this device. Notably, plugging the UBW into a Redhat Enterprise Linux 4 system yields immediate results and the UBW appears as /dev/ttyACM#.
The UBW can then be operated by speaking to the created com port. There is a set commands which can be used to read/write DIO lines, or read analog values, change whether a pin is input or output, etc. This allows for very fast setup. As a side comment, what if the UBW exposed its internals as a filesystem? Something like the /proc fs in Linux where one writes values into "files" and reads then back. Then any system that knows how to open FAT16 files would work... and no drivers!
The C firmware works good, but is DIO only. The D r1.2 firmware is terrible. It rarely shows up on the host computer in a usable way. I have had it actually crash the system on plugin. The D r1.3 firmware works great and allows analog functions as well.
The Low-cost USB Thermometer
Those of you familiar with the site may be know of our continuing effort to provide cheap, reliable temperature sensing for machine room environmental monitoring. We have built these several times in the past on various hardware platforms searching for the holy grail of environmental monitoring. Naturally, we will apply the UBW to this problem as a litmus test of its capabilities!
The LM35 has worked so well in the past we will keep that (see our previous work here). In this case, the LM35 Vcc and GND are connected to the UBW Vcc and GND, the Vout is connected to RA0 which we pull low with a 100k ohm resistor (mounted on the back).
We wrote a perl script to fetch the values. The script makes use of the Device::SerialPort CPAN module for Linux. The script extends a sample script that attempts to be cross platform by loading the Win32:SerialPort module if it is run on windows. The script has not been tested on Windows. The script can be found here.