Adding a display to rfid time tracking

More than a year ago, I blogged here about using RFID to track presence times in the BORM ERP system. I used the system a lot since then. But the BlinkM was really limited as the only immediate feedback channel. To use it with multiple users, a display was needed. The default Arduino compatible displays seemed a bit overpriced, and the Nokia phone that I disassembled didn’t have the same display as I used for the spectrum analyzer. But these displays are available for a bargain from china. The only problem was that the bifferboard didn’t have enough GPIO pins available to drive the “SPI plus extras” interface. But i2c was already configured for the BlinkM.

So, the most obvious solution was to use an AtMega8 as an intermediary. I defined a simple protocol and implemented it as i2c and uart on the AVR. I also wrote a small python class to interface it from the client side. As I buffer only one complete command, I had to add some delays in the python script to make sure the AVR can complete the command before the next one arrives. Apart from that, it all worked well when testing on an Alix or RaspberryPi. But i2c communication refused to work entirely when testing with the bifferboard. Not even i2cdetect could locate the device. That was bad, since I wanted to use it with the Bifferboard, and the other two were only for testing during the development. I checked with the oscilloscope, and found out that the i2c clock on the bifferboard runs with only 33kHz while the other two run at the standard 100kHz. So I tried to adjust the i2c clock settings on the AVR, as well as different options with the external oscillators and clock settings, but I was still still out of luck. Then I replaced the AtMega8 with an AtMega168 and it immediately worked. Next, I tried another AtMega8 and this one also worked with the Bifferboard. I switched back and forth and re-flashed them with the exact same settings. Still, one of them worked with all tested linux devices, while the other one still refused to work with the Bifferboard. So I concluded, one of these cheap AVR’s from china must be flaky, and I just used the other one. Seems like that’s what you get for one 6th of the price you pay for these chips in Switzerland.

Apart from the display, I also added an RGB LED that behaves like the BlinkM before. And on top of that a small piezo buzzer. But since I could hardly hear it’s sound when driven with 3.3V, I didn’t bother re-soldering it when it fell off.

Now, my co-workers also started logging their times with RFID.

The code is still on github.

Leave a Reply

Your email address will not be published. Required fields are marked *