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RS232 baud rate 57600, receiving by PIC XTL 20MHz, ok?

 
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Oliver Lai



Joined: 12 Dec 2003
Posts: 16

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RS232 baud rate 57600, receiving by PIC XTL 20MHz, ok?
PostPosted: Mon Apr 05, 2004 11:21 pm     Reply with quote
Hi, I have got a problem:

From a RS232 output, the baud rate is 57600, I can read data from hyperterminal by PC, but had problem with PIC18F452 (XTL: 20MHz).

I code:

#use rs232 (baud=57600, parity=N, xmit=PIN_C6, rcv=PIN_C7)

BYTE x;

void main() {

x = getc()


}

it stuck in the line x = getc();

It seems nothing has been received, is it the baud rate too high? Can anybody tell me the reason?

Thanks a lot,
Oliver
ttelmah
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Re: RS232 baud rate 57600, receiving by PIC XTL 20MHz, ok?
PostPosted: Tue Apr 06, 2004 2:19 am     Reply with quote
Oliver Lai wrote:
Hi, I have got a problem:

From a RS232 output, the baud rate is 57600, I can read data from hyperterminal by PC, but had problem with PIC18F452 (XTL: 20MHz).

I code:

#use rs232 (baud=57600, parity=N, xmit=PIN_C6, rcv=PIN_C7)

BYTE x;

void main() {

x = getc()


}

it stuck in the line x = getc();

It seems nothing has been received, is it the baud rate too high? Can anybody tell me the reason?

Thanks a lot,
Oliver

I would have expected the chip to receive 'data', but the probability of it being garbage would be high, because of the very large timing error at this baud rate. The fact that it does not see the data at all, suggests that the problem is perhaps in something 'hardware', like your receive buffer.
That having been said, receiving the right data, is less likely. Unfortunately the clock and baud rate you have selected gives a very large timing error (read the MicroChip data sheet for how to calculate this). The error will be about 8%, which is beyond the normally accepted 'max' for reliable reception. I don't know which compiler you have, but latter versions will normally complain about this.
You can solve this problem, by chosing a crystal frequency that is a nicer multiple of the baud rate. The recommended crystals would be either 22118400Hz, or 18432000Hz. This latter crystal s a common part for exactly this reason (a lot of baud rate generators need this type of multiple).
So you probably have two problems to fix. One with the receive hardware, and a second one with the baud rate. Fix the receive hardware one first.
I am assuming that your test program does do more than you show, since what you type, would just receive the garbage, and then sleep.

Best Wishes
SherpaDoug



Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA

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PostPosted: Tue Apr 06, 2004 6:58 am     Reply with quote
What do you mean by "stuck"?
The only way to get stuck at x=getc() is if there is no start bit. That is pretty independant of baud rate. Getc() will wait forever for that start bit and then grab bits at the next 9 bit intervals and return, even if the bits are garbage. This indicates you have a hardware problem.
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Ttelmah
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PostPosted: Tue Apr 06, 2004 2:52 pm     Reply with quote
SherpaDoug wrote:
What do you mean by "stuck"?
The only way to get stuck at x=getc() is if there is no start bit. That is pretty independant of baud rate. Getc() will wait forever for that start bit and then grab bits at the next 9 bit intervals and return, even if the bits are garbage. This indicates you have a hardware problem.


Exactly.
This is why I think there is a hardware problem, independant of the baud rate problem.

Best Wishes
Oliver Lai



Joined: 12 Dec 2003
Posts: 16

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if it is the hardware problem, why other program works?
PostPosted: Tue Apr 06, 2004 3:18 pm     Reply with quote
I have tried other programs, like ex10 and ex11 (CCS gave example program), it works well. The RS232 can getc() receiving data from key hit.

Any suggestion how can I solve this problem?

I got suggestion from CCS technical support, using:

#use rs232 (baud=57600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)

I have tried, the program is stucked on this line now.

Oliver
SteveS



Joined: 27 Oct 2003
Posts: 126

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PostPosted: Wed Apr 07, 2004 7:19 am     Reply with quote
Shouldn't you be specifying BITS in the #USE RS232 comand? Probably not the problem, but I find it best to be explicit in all my declarations - don't assume defaults.

If it works with examples, what is different with your program? Is it the baud? If so try changing to that baud in the example. Is it something else -try changing that.

- SteveS
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