I noticed it's set up for 9 bit recieve. Do you need to also read the 9th bit to prevent overrun? ----- Original Message ----- From: "Josh Koffman" To: "Microcontroller discussion list - Public." Sent: Monday, May 03, 2010 5:46 AM Subject: [PIC] UART Receive Overrun Error > Hi all. I'm having a problem using the EUSART in receive mode. Basic > info is that I'm using a PIC18f4321. It has the EUSART module. Data is > coming in at 250 Kbps via an SN75176 line driver (yes, this is for > DMX512, a lighting control protocol). The chip is clocked at 16MHz. > > I'm currently rewriting my stock DMX512 routines to use them on the > PIC18 series and in general update them a bit. Transmit is finished > and works fine. Receive is being really stubborn though. My current > problem seems to be that I'm getting a ton of overrun errors. I think > this is throwing off all of my other receive routines resulting in > data being received incorrectly in general. > > I thought the problem might be the length of my ISR, so I've modified > it so that it basically consists of: > 1. Context save > 2. Check UART receive flag > 3. Read RCREG > 4. Check OERR > 4a. If set, clear CREN, nop, then set CREN. Then toggle a pic (so > I can get an idea of how often this is happening) > 5. Exit ISR > > My main program loop is empty. So basically the code should just be > receiving bytes as fast as possible and discarding them. However I'm > still getting about 10 or more overrun errors per DMX packet. I feel > like I must be missing something as it just doesn't seem right that > I'm overruning that much. > > Here is a snippet of code where I set up the serial port: > ----- > ; Setup serial port > movlw b'00000100' ; Disable transmitter, high speed mode > movwf TXSTA > movlw b'10010000' ; Serial port enabled, reception enabled > ; movlw b'11010000' ; Serial port enabled, reception enabled, 9 bit > RX > movwf RCSTA > bsf BAUDCON,BRG16 ; select EUART 16-bit Asynchronous mode > operation > > ; bsf BAUDCON, RXDTP ; received data is inverted > > movlw d'15' ; init baud rate generator for 250k baud (assume > Fosc=16MHz) > movwf SPBRG > clrf SPBRGH > ----- > There are a couple of lines commented where where I had tried > different options in case that was my problem. I am still unsure if I > should be using 9 bit receive and/or inverted idle. I believe I've > seen valid data when in a watch window when I pause (I'm running in > debug). My reasoning is also that if I did have a setup that was > totally wrong I'd be getting more framing errors than overruns. > > For the record, I've also tried setting up the BRG in non high speed > mode, but I got the same results. > > The part of my ISR that deals with the UART receive looks like this: > ----- > DMXReceive ; Handle DMX Receiving > movf RCREG, w ; Read the receive register and discard > > btfss RCSTA, OERR ; If we have an overrun error, disable the > goto NoOERR ; port then re-enable to clear it > bcf RCSTA, CREN > nop > bsf RCSTA, CREN > btg pLED3 ; Toggle LED to advise of problem > > NoOERR > clrf DMXLED ; Clear the counter to keep LED lit > bsf pLED2 ; LED on > goto _END_DMXReceive > ----- > This is all code I wrote just to try to figure out where the overruns > are coming from. The above routine is called just after context save > and interrupt flag check, and _END_DMXReceive takes us to context > restore and interrupt exit. The DMXLED register and pLED2 are a sort > of receive indicator. > > I must admit I'm pretty stuck on this one. By my calculations I > shouldn't have trouble unloading the UART FIFO, and I think I'm > unloading it pretty fast. Yet I still overrun. I must be missing > something. I checked the part's errata but there isn't any mention of > something that might cause this. I used the same chip to write my > transmit routine on. > > Can anyone shed any light on what I've missed? > > Thanks! > > Josh > -- > A common mistake that people make when trying to design something > completely foolproof is to underestimate the ingenuity of complete > fools. > -Douglas Adams > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist