Hi, =


Were do you clear RCIF?

/Ruben

> 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=3D16MHz)
>      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
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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
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