Issue 149 December 2002
Wireless Temperature Sensor Stew

FRED’S rfPICDEM1

The rfPICDEM1 comes ready to roll with functional firmware loads in both the transmitter and receiver. My rfPICDEM1 worked as designed when I powered up the boards. Although the rfPICDEM1 is a fun gadget to play with, you’re not supposed to stop playing with it after you power it up and verify the temperature of the room you’re standing in.

From the preliminary marketing package I received from Microchip, I knew I could use the ICSP adapter on the receiver to twiddle bits on the PIC16C925 by replacing it with a windowed JW reprogrammable part. However, I was reasonably concerned about how I was going to manipulate the transmitter code, especially because there weren’t any alternatives other than replacing the surface-mounted rfPIC or putting something there in its place.

In anticipation of not being able to keep my hands off the code or the physical electronics, I obtained a couple of windowed PIC16C925 devices and a tube of rfPIC12C509AG units. I also picked up an ICSP adapter and a PIC16C925 programming adapter for my Microchip Pro Mate II programmer. I had considered getting some of the windowed rfPICs but that would mean performing major surgery on the little transmitter stick.

After I became accustomed to what the rfPIC demo package could do, I decided to leave the receiver alone; it was already equipped to show the contents of the data received from the remote transmitter as well as the temperature of its on-board TC74 senses. Because the transmitter was without any user feedback by itself, I thought it would be neat to crack into the transmitter and display its station ID and temperature without having to transmit all of that to the receiver and view it on the receiver’s LCD.

Notice that I didn’t mention displaying the push button status. That’s because I really want to replace those tiny buttons with a more ergonomic user interface. Now, I’m at the crossroads and the challenge has been laid down: Do I build customized hardware to do this conversion, or can I assemble a set of prefabricated electronic building blocks and weasel out of doing a bunch of soldering and coding?

One problem would be what to do about the surface-mounted rfPIC. It seems I would have to solder in a new one each time I changed the code or cut the traces and install the optional 8-pin DIP socket. Assuming I can come up with a viable solution to deal with the physical elements involved with converting the rfPIC code, I’m still in the soup as to how to convert the firmware.

The original code for the rfPICDEM1 was completed using the Hi-Tech PIC C compiler. I don’t own a copy of Hi-Tech’s C, but I do own a copy of the CCS PICC, and you can be sure there will be differences in the source that I’ll have to interpret and compensate for. And, if I did use the 8-pin DIP socket and another 8-pin part, could I get all of my new code to fit? The real showstopper is that I would have to remove the PIC to program it, because there is no provision for ICSP on the transmitter board. There will be no surrogate PIC12C509 if I can get around it.

Because the transmitter’s push-button switches simply take an I/O pin from the rfPIC to ground, I had originally thought I could replace the transmitter’s switches with logic levels from an external microcontroller that would take the place of the PIC12C509 portion of the rfPIC. The external microcontroller would allow me to use the existing rfPIC in its entirety without having to compromise the rfPIC’s original firmware or PC board traces.

The problem, however, is that even though I would gain the I/O lines and program memory space necessary to drive an LCD, I’d still be stuck using push buttons or a keypad to actuate the transmitter. No matter what, I’m going to have to alter the rfPIC’s firmware to obtain any data from the transmitter’s rfPIC. The ideal solution would be a touch-sensitive display that would integrate easily to an external microcontroller and handle the data-gathering (and conversion) duties needed to drive the LCD, as well as the RF tasks.