CCS C Compiler Manual PCB / PCM / PCH

Why is the RS-232 not working right?
1. The PIC® is Sending Garbage Characters.
A. Check the clock on the target for accuracy. Crystals are usually not a problem but RC
oscillators can cause trouble with RS-232. Make sure the #USE DELAY matches the
actual clock frequency.
B. Make sure the PC (or other host) has the correct baud and parity setting.
C. Check the level conversion. When using a driver/receiver chip, such as the MAX 232,
do not use INVERT when making direct connections with resistors and/or diodes. You
probably need the INVERT option in the #USE RS232.
D. Remember that PUTC(6) will send an ASCII 6 to the PC and this may not be a visible
character. PUTC('A') will output a visible character A.
2. The PIC® is Receiving Garbage Characters.
A. Check all of the above.
3. Nothing is Being Sent.
A. Make sure that the tri-state registers are correct. The mode (standard, fast, fixed) used
will be whatever the mode is when the #USE RS232 is encountered. Staying with the
default STANDARD mode is safest.
B. Use the following main() for testing:
main() {
while(TRUE)
putc('U');
}
Check the XMIT pin for activity with a logic probe, scope or whatever you can. If you can
look at it with a scope, check the bit time (it should be 1/BAUD). Check again after the
level converter.
4. Nothing is being received.
First be sure the PIC® can send data. Use the following main() for testing:
main() {
printf("start");
while(TRUE)
putc( getc()+1 );
}
When connected to a PC typing A should show B echoed back.
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