Electronic – How to build Forth for embedded devices

After reading a few posts and doing a little testing with a DMM, I believe I have solved my questions concerning how a home thermostat works. First the posts/info that led to the conclusion:

RiteTemp Installation Guide (pg 17 - 4 Wire Heat/Cool No Pump)

JohnSAZ from EEVBlog:

Of course your thermostat may be different but every one that I've ever looked at worked the same (and your color codes pretty much indicate an industry standard setup).

Think of the thermostat as nothing more than three switches. The "RH" is your common. If you short "RH" to "G", your blower fan will turn on. If you short "RH" to "Y", the air conditioning compressor will turn on. If you short "RH" to "W", the heater system will turn on. This could be a gas burner solenoid, or relays to energize resistive electric heaters. If the unit is a heat pump, it will turn on the air conditioning compressor, but it will also energize a reversing valve that will make the freon run the other way around (kinda).

Obviously you need to ensure that the "G" and "Y" leads are NEVER turned on at the same time. You also need to make sure that the eater system can not be turned on with the blower off. Also, you need to make sure that when the compressor turns off, it is not allowed to turn back on for some time (10 minutes? don't know the time limit for sure). Also, higher quality thermostats keep the blower running for some time after the compressor (or heater) has turned off to suck every last Btu out of the system before shutting down.

I grabbed my trusty DMM and confirmed that W, Y, G, and C all measure 30VAC+ when tested against RH (or R) - GND. When the system is running, G drops to 0.05 for both heat and cold (meaning the fan is running). W drops to 0.05 if the heater is running. Y drops to 0.05 if the Cooler is running.

I still need to research the appropriate timing for cooler/heater shutdown etc.

If anyone can think of any additional circuit protection outside of what JohnSAZ noted, please let me know. I am interested in adding a fail safe to the circuit design in case the unit fails. Not sure if relays in off positions during power outage would take care of business or not. Either way, I do plan on providing battery backup to the project just in case (IE. detect power failure, disable all controls).

I know this is old but might be helpful incase anyone stumbles apon it like I did.

I was having issues with overrun errors in my PIC18F452 communicating to a beaglebone black over USART. With quite a busy program going on in both the beagle and the PIC, there was lots of data being sent between the two constantly. After some amount of time after using the instrument the PIC stopped receiving all messages.

After being stuck and baffled by this for more than 3 weeks, I found this:

http://www.piclist.com/techref/postbot.asp?by=thread&id=%5BPIC%5D+PIC18+UART+stops+receiving&w=body&tgt=post

which described PIC code to handle and clear Overrun Errors:

if(OERR)
{
    do
    {
        temp = RCREG;
        temp = RCREG;
        temp = RCREG;
        CREN = 0;
        CREN = 1;

    } while(OERR);
}

if(FERR)
{
    temp = RCREG;
    TXEN = 0;
    TXEN = 1;
}

Running this snippet every now and then in the PIC program allowed me to recover from occasional overrun errors without restarting the PIC.

Hope that helps anyone with similar issues