;;
  ;;  mur.sat
  ;;
  ;;  Somewhen in the year 2011, mur.at will have a nano satellite launched
  ;;  into a low earth orbit (310 km above the surface of our planet). The
  ;;  satellite itself is a TubeSat personal satellite kit, developed and
  ;;  launched by interorbital systems. mur.sat is a joint venture of mur.at,
  ;;  ESC im Labor and realraum.
  ;;
  ;;  Please visit the project hompage at sat.mur.at for further information.
  ;; 
  ;;
  ;;  Copyright (C) 2011 Christian Pointner <equinox@mur.at>
  ;;                         
  ;;  This file is part of mur.sat.
  ;;
  ;;  mur.sat 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.
  ;;
  ;;  mur.sat 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 mur.sat. If not, see <http://www.gnu.org/licenses/>.
  ;;

  ;; -------------------------------------
ttx_send_byte
  bsf       STATUS,RP0
  bcf       TRISD,TTX_SDA
  bcf       STATUS,RP0
  movwf     TTX_BYTE
  xorwf     TTX_CSUM,f
  movlw     .8
  movwf     TTX_BIT_CNT
ttx_send_next_bit
  btfsc     TTX_BYTE,7
  bsf       PORTD,TTX_SDA
  btfss     TTX_BYTE,7
  bcf       PORTD,TTX_SDA
  bsf       PORTD,TTX_SCL
  nop
  nop
  nop
  bcf       PORTD,TTX_SCL
  nop
  nop
  nop
  nop
  rlf       TTX_BYTE,f
  decfsz    TTX_BIT_CNT,f
  goto      ttx_send_next_bit
  bsf       STATUS,RP0
  bsf       TRISD,TTX_SDA
  bcf       STATUS,RP0
  return

  ;; -------------------------------------
ttx_recv_byte
  movlw     .8
  movwf     TTX_BIT_CNT
ttx_recv_next_bit
  bsf       PORTD,TTX_SCL
  nop
  nop
  nop
  bcf       PORTD,TTX_SCL
  nop
  nop
  nop
  nop
  nop
  rlf       TTX_BYTE,f
  btfsc     PORTD,TTX_SDA
  bsf       TTX_BYTE,0
  btfss     PORTD,TTX_SDA
  bcf       TTX_BYTE,0
  decfsz    TTX_BIT_CNT,f
  goto      ttx_recv_next_bit
  return
    
  ;; -------------------------------------
ttx_break
  movlw     .13
  movwf     TTX_BIT_CNT
ttx_wait_octet
  decfsz    TTX_BIT_CNT,f
  goto      ttx_wait_octet
  return
  
  ;; -------------------------------------
ttx_send_frame
  movwf     TTX_BYTE
  andlw     b'00001111'
  movwf     TTX_BYTE_CNT
  bsf       PORTD,TTX_SCL
  movlw     .3
  bcf       PORTD,TTX_SCL
  call      wait_ms

  clrf      TTX_CSUM
  movf      TTX_BYTE,w
  call      ttx_send_byte

  movf      TTX_BYTE_CNT,f
  btfsc     STATUS,Z
  return

ttx_next_byte
  call      ttx_break
  movf      INDF,w
  call      ttx_send_byte

  incf      FSR,f
  decfsz    TTX_BYTE_CNT,f
  goto      ttx_next_byte

  call      ttx_break
  call      ttx_recv_byte
  movf      TTX_BYTE,w
  xorwf     TTX_CSUM,w  
  btfss     STATUS,Z
  goto      ttx_csum_error
  movlw     '.'
  movwf     TXREG
  return    

ttx_csum_error
  movlw     'E'
  movwf     TXREG
  return

  ;; -------------------------------------