January 2006, Issue 186

Electronic Scarecrow

REMOTE UNIT SOFTWARE

The application code, which I wrote in C language with CodeWarrior 3.1 from Metrowerks (now Freescale Developer Technology Organization), uses the SMAC 4.0 protocol stack. This isn’t ZigBee- or 802.15.4-compliant because I don’t need compatibility with other devices.

When I started development, I found that the software for downloading the boards wouldn’t run on Windows 98. The demonstration programs and the compiler were fine, but I couldn’t download new code. There weren’t any alternatives (short of upgrading my PC), so I made my own. Because I didn’t need any of the ZigBee embedded bootloader’s special features, I found a generic serial bootloader and borrowed a P&E Microcomputer Systems BDM Multilink download cable so I could try it out. The bootloader also came with a DOS program (hc08sprg.exe) that talks to it over a serial port. (You may download the hc08sprg.exe file from the Circuit Cellar FTP site.) I used the bootloader for the rest of the development process.

The entire application fits in 7 KB of flash memory, leaving 8.5 KB available to store a PCM sound file. This provides about one 1 s of sound at 8 kHz. I wanted to be able to download a new sound file or update the running code over the radio link. The latter required some changes to the serial bootloader, which sits in a 512-byte protected area at the top of the flash memory. To free up enough space to add the update function, I replaced the bootloader’s flash memory routines with smaller ones from a MC9S08GB/GT serial monitor. The remote download feature enabled me to update both the base and all of the remotes through the base unit’s serial port.

The radio link operates with a simple request and reply protocol. A remote unit sends a request whenever it has something to report such as a sensor trigger event or the press of a switch. It also sends ping packets with the results of sensor tests performed at regular intervals. The base unit always responds by sending back a reply packet. The remote unit handles the resending of missed packets. Because packets are sent infrequently, any external RF interference is usually different from one packet to the next. Therefore, searching for the quietest channel didn’t make sense, so I just went with a fixed channel.

The SMAC stack had all the features required for this simple protocol. I added a few functions for system control, including support for Timer2 and Doze mode, the ability to read GPIO pins, and control over the ATTN pin interrupt. I also made a few changes to decrease the execution time of some frequently used routines.