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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