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PIC UART problem with long strings

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AndrewM

Wed Jul 07 2010, 02:59PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

I've got a PIC16F747 and have enabled the UART module for talking to my PC.

It works great, except for if I exceed ~5 bytes back to back, at which point the receipt becomes unreliable - the received message is often incomplete or contains errors. I'm using 9600baud, no parity, and the baud rate error is low (0.16%).

My question: Am I obliged to enforce a delay between bytes in my code?

UltraMagnus

Wed Jul 07 2010, 04:39PM

Registered Member #2875 Joined: Mon May 24 2010, 08:28AM
Location: England
Posts: 42

how accurate is your clock? if it is off the baud rate will be off too

Bjørn

Wed Jul 07 2010, 10:47PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

You do not need any pause between bytes, after receiving the required number of bits the receiving UART will look for a valid start bit/stop bit sequence to find the next byte.

There is a slight problem that if your data is noisy it may mistake the start of a byte and if you have no pause your data may have data that have the same transitions as the start bit/stop bit sequence. In that case it may not recover. So for the most reliable transmission you should have pause between bytes or send small packets with pause between. Then you are guaranteed that it will recover.

PC UARTs in general are very forgiving and will correctly receive even if the link is far outside specification.

The symptoms sounds quite like classical buffer overflow which can happen at either end and at any burst length or speed depending on the configrations and software at both ends.

AndrewM

Thu Jul 08 2010, 03:03PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Thanks Bjorn.

This portion of my application reports the values of 5 of the onboard ADC's via RS232. I changed the code so that rather than doing all 5 conversions and then sending the results all at once, it sends the data (4 bytes including my own spacing characters) immediately after each conversion finishes.

Its better, but not perfect. What do you suppose a good delay would be?

Second question - even when the link operates properly, sometimes the ADC's jump around (mostly when a source is not connected). I'm pulling them down with 33ks, should I be using a stronger pull-down?

big5824

Thu Jul 08 2010, 03:58PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

I had a similar problem that i somehow fixed by adding the #ERRORS tag (using CCS), you could maybes try that, although i did also introduce a delay between packets in the end.

GeordieBoy

Thu Jul 08 2010, 04:28PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

You are checking that the UART's transmit register is clear before trying to send another character?

It takes a finite amount of time to serially clock out a character when the UART is transmitting. You need to ensure that each byte has been fully transmitted before you try to ram another value into the transmit holding register. If you try to send another character before the UART has flagged that it is ready, it will either result in dropped characters or corrupted characters. I forget which one happens with PICs.

-Richie,

AndrewM

Thu Jul 08 2010, 08:47PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

GeordieBoy wrote ...

You are checking that the UART's transmit register is clear before trying to send another character?

It takes a finite amount of time to serially clock out a character when the UART is transmitting. You need to ensure that each byte has been fully transmitted before you try to ram another value into the transmit holding register. If you try to send another character before the UART has flagged that it is ready, it will either result in dropped characters or corrupted characters. I forget which one happens with PICs.

-Richie,

Yes. I move a character for sending into the w register and the call my sending function, which does not return until the send is complete. For scrutiny:

;*** Send character
send    	call 	errchk
  		  movwf 	TXREG           ; send data in W
TransWt 	bsf 	STATUS,	RP0	    ; RAM PAGE 1 
WtHere  	btfss 	TXSTA,	TRMT	    ; (1) transmission is complete if hi 
        	goto 	WtHere
        	bcf 	STATUS,	RP0	    ; RAM PAGE 0 
        	return

;*** Error check
errchk    btfss 	RCSTA, 	OERR
  		goto 	errdone
  		bcf 	RCSTA,	CREN ; reset USART RX logic, clear RCSTA,OERR
  		bcf 	RCSTA,	FERR ; reset framing error bit
  		bsf 	RCSTA,	CREN ; re-enable continuous reception
errdone 	return

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