"The (SLAVE SELECT) pin must NOT be driven LOW for the SS pin to
function as an input."

> Setup
>      ; port initialization
>      bcf     STATUS,RP0          ; sets bank 0
>      movlw   b'00000000' <------ THIS IS DRIVING THE SS pin LOW!!!
>      movwf   PORTA

On 4 Jun 2002 at 17:50, Guillermo Rodriguez Garcia wrote:

> Hello,
>
> I am using a PIC in slave mode with SS control enabled. I have
> an ARM processor working as the SPI master that always sends
> and receives 16 bits per transfer. When connecting the two
> devices toghether, I have found that the first byte of each
> transfer is always read by the PIC, but the second one is
> ignored (it always reads $ff for the second byte).
>
> I thought I might need to cycle SS between bytes in a multi-byte
> transfer but then I found out in the list that this was not
> necessary.
>
> The SPI mode I'm using is:
>
> - PIC in slave mode, SS control enabled.
> - CKP = 1, CKE = 1
>
> I'm looking at the signals with a scope and they all look right.
> However the PIC doesn't see the second byte in each transfer.
>
> I don't know what I may be doing wrong. Any suggestions?
>
> I'm attaching the code I use on the PIC, in case someone wants
> to look at it.
>
> Thanks!
>
>
> === Cut ===
> Startup
>      call    Setup
>
>      clrf    SPICount
>      clrf    SPIByte
>
> Main
>      ; SPI cycle
>      movlw   b'10100101'         ; write this byte ($a5)
>      movwf   SSPBUF              ; place data in buffer to send
>
>      bsf     STATUS, RP0         ; specify bank 1
> loop
>      btfss   SSPSTAT, BF         ; has data been received (xmit done)?
>      goto    loop
>
>      bcf     STATUS, RP0         ; specify bank 0
>      movf    SSPBUF, W           ; empty receive buffer
>      movwf   SPIByte
>      incf    SPICount, f
>
> next_1
>      ; reset SPI just to be on the safe side
>      bcf     SSPCON, SSPEN
>      bsf     SSPCON, SSPEN
>
>      goto    Main                ; loop forever
>
>
> ;---------------------------------------------------------------------
> ; Setup
> ;
> ; Initializes program variables and peripheral registers.
> ;---------------------------------------------------------------------
>
> Setup
>      ; port initialization
>      bcf     STATUS,RP0          ; sets bank 0
>      movlw   b'00000000'
>      movwf   PORTA
>      movlw   b'00000000'
>      movwf   PORTB
>
>      ; SPI setup
>      bsf     STATUS,RP0          ; sets bank 1
>      movlw   0x18                ; all bits are outputs except SDI and SCK
>      movwf   TRISC
>
>      bsf     TRISA, 5            ; SS = input
>
>      movlw   b'01000000'         ; SMP=0, CKE=1
>      movwf   SSPSTAT
>
>      movlw   b'00001110'
>      movwf   ADCON1              ; set RA5 as digital I/O
>
>      bcf     STATUS,RP0          ; sets bank 0
>      movlw   0x34                ; SPI enable, CKP=1, slave & SS enable
>      movwf   SSPCON
>
> Setup_end
>      return
> === Cut ===
>
>
> TIA,
> G.
>
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>


  *
  |  __O    Thomas C. Sefranek  tcs@cmcorp.com
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