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  ;;
  ;;  spreadspace pic utils
  ;;
  ;;
  ;;  Copyright (C) 2011-2013 Christian Pointner <equinox@spreadspace.org>
  ;;
  ;;  This file is part of spreadspace pic utils.
  ;;
  ;;  spreadspace pic utils is free software: you can redistribute it and/or modify
  ;;  it under the terms of the GNU General Public License as published by
  ;;  the Free Software Foundation, either version 3 of the License, or
  ;;  any later version.
  ;;
  ;;  spreadspace pic utils is distributed in the hope that it will be useful,
  ;;  but WITHOUT ANY WARRANTY; without even the implied warranty of
  ;;  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  ;;  GNU General Public License for more details.
  ;;
  ;;  You should have received a copy of the GNU General Public License
  ;;  along with spreadspace pic utils. If not, see <http://www.gnu.org/licenses/>.
  ;;

  ;; --- transmit byte and add it to csum
com_tx_byte
  movlb     .4
  movwf     SSP1BUF
  movlb     .0
  bcf       INTPIN
  xorwf     csum,f
  movlb     .4
com_tx_byte_wait1
  btfss     SSPSTAT,BF
  goto      com_tx_byte_wait1
  movf      SSP1BUF,w
  movlb     .0
  bsf       INTPIN
com_tx_byte_wait2               ; also wait for SS to go up again
  btfss     PORTB,5             ; this lowers the risk for raise conditions
  goto      com_tx_byte_wait2   ; with next byte (INT pin)
  return

  ;; ---- wait for byte to be received
com_rx_byte
  movlb     .4
  btfsc     SSP1STAT,BF
  goto      com_rx_got_byte
  btfss     SSP1CON1,SSPOV
  goto      com_rx_byte
  movf      SSP1BUF,w
  bcf       SSP1CON1,SSPOV
  goto      com_rx_byte

com_rx_got_byte
  movf      SSP1BUF,w
  movlb     .0
  return

  ;; ----- initialize com (this is called by bootmacro, it's not a subroutine - no return at the end)
com_init
  movlb     .3
  bcf       ANSELB,ANSB1        ; SDI as digital input
  bcf       ANSELB,ANSB4        ; SCK as digital input

  movlb     .1
  movlw     b'01110000'         ; set internal OSC to 8MHz
  movwf     OSCCON
  bcf       TRISB,2             ; RB2(SDO) = out
  bcf       INTPIN              ; interrupt line to master

  movlb     .4
  movlw     b'01000000'         ; SMP=0,CKE=1
  movwf     SSP1STAT
  movlw     b'00100100'         ; SSPEN=1,CKP=0,SSPM=0100(SPI Slave with SS)
  movwf     SSP1CON1
  bcf       SSP1CON3,BOEN

  movlb     .0
  bsf       INTPIN