; ***************************************************************************
; code

.cseg



; ---------------------------------------------------------------------------
; comSendByte
;
; Send a byte.
; We only set the data pin to low at the beginning for the startbit. After that
; we only change the pin direction (e.g. input vs output):
; - for 0 bit: set DDR to output, forcing the data line low
; - for 1 bit: set DDR to input, letting the external pullup R pull the data line to HIGH
;              since the output pin is still set to 0 the internal pullup is disabled
; IN:
; - R16: byte to send
; OUT:
; - CFLAG: set if okay, clear otherwise
; MODIFIED REGS: R16, R21, R22

comSendByte:
  ldi r21, 8                                     ; +1 bits left
  ; send startbit
  cbi COM_PORT_DATA, COM_PINNUM_DATA             ; +2 set DATA low
  sbi COM_DDR_DATA, COM_PINNUM_DATA              ; +2 set DATA as output
  Utils_WaitNanoSecs COM_BIT_LENGTH, 5, r22      ; wait for one bit duration
  ; send data bits
comSendByte_loop:                                ; 9 for low bit
    lsr r16                                      ; 1+ bit to send -> CARRY
    brcs comSendByte_setHigh                     ; HI: +2, LO: +1
comSendByte_setLow:
    sbi COM_DDR_DATA, COM_PINNUM_DATA            ; +2 set DATA as output
    Utils_WaitNanoSecs COM_BIT_LENGTH, 9, r22
    rjmp comSendByte_loopEnd                     ; +2
comSendByte_setHigh:
    cbi COM_DDR_DATA, COM_PINNUM_DATA            ; +2 set DATA as input, pullup R makes it ONE
    nop                                          ; +1 (to make pin change available)
    Utils_WaitNanoSecs COM_BIT_LENGTH/2, 11, r22 ; wait for half a bit length for line to safely settle
    sbis COM_PIN_DATA, COM_PINNUM_DATA           ; +1 if no skip, +2 if skipped
      rjmp comSendByte_error                     ; +2 if error (collision: we wanted line to be high but it is low)
    Utils_WaitNanoSecs COM_BIT_LENGTH/2, 0, r22
comSendByte_loopEnd:
    dec r21                                      ; +1
  brne comSendByte_loop                          ; +2, sum per loop: 10 cycles
  ; send stopbit
  cbi COM_DDR_DATA, COM_PINNUM_DATA              ; +2 set DATA as input, pullup R makes it ONE
  Utils_WaitNanoSecs COM_BIT_LENGTH, 4, r22      ; wait for one bit length
  sec
  ret

comSendByte_error:
  clc
  ret



; ---------------------------------------------------------------------------
; comReceiveByte
;
; Receive a byte.
;
; IN:
; - nothing
; OUT:
; - CFLAG: set if okay, clear otherwise
; - R16: byte read (if CFLAG set)
; MODIFIED REGS: R16, R20, R21, R22 (R17)

comReceiveByte:
  cbi COM_DDR_DATA, COM_PINNUM_DATA            ; set DATA port as input
  cbi COM_PORT_DATA, COM_PINNUM_DATA           ; disable internal pullup for DATA
  
  ldi r21, 8                                   ; bits left
  clr r20                                      ; byte currently receiving
  ; wait for startbit
  rcall comWaitForDataLow                      ; (R17)
  brcc comReceiveByte_error
  Utils_WaitNanoSecs COM_BIT_LENGTH/2, 5, r22  ; goto middle of startbit to maximize sync stability
comReceiveByte_loop:
    Utils_WaitNanoSecs COM_BIT_LENGTH, 8, r22  ; 8 cycles used in the complete loop between waits
    sec                                        ; +1
    sbic COM_PIN_DATA, COM_PINNUM_DATA         ; LOW: +2, HIGH: +1
      rjmp comReceiveByte_shiftIn              ; HIGH: +2, rjmp, use set CFLAG
    clc                                        ; LOW: +1
comReceiveByte_shiftIn:
    ror r20                                    ; +1
    dec r21                                    ; +1
  brne comReceiveByte_loop                     ; +2, sum per loop: 8 cycles
  rcall comWaitForDataHigh                     ; wait for start of stopbit
  brcc comReceiveByte_error
  mov r16, r20
  sec
  ret
comReceiveByte_error:
  clc
  ret



; ---------------------------------------------------------------------------
; comWaitForDataLow
;
; Waits up to COM_MAXWAIT loops for low data line
; IN:
; OUT:
; - CFLAG: set if okay, clear otherwise
; MODIFIED REGISTERS: r17, r22

comWaitForDataLow:
  ldi r17, COM_MAXWAIT

comWaitForDataLow_loop:
    sbis COM_PIN_DATA, COM_PINNUM_DATA
      rjmp comWaitForDataLow_done
    Utils_WaitNanoSecs 100, 0, r22 ; wait for 100 nanosecs
    dec r17
  brne comWaitForDataLow_loop
  clc   ; timeout
  ret

comWaitForDataLow_done:
  sec   ; ok
  ret



; ---------------------------------------------------------------------------
; comWaitForDataHigh
;
; Waits up to COM_MAXWAIT loops for high data line
; IN:
; OUT:
; - CFLAG: set if okay, clear otherwise
; MODIFIED REGISTERS: r17, r22, X

comWaitForDataHigh:
  ldi r17, COM_MAXWAIT

comWaitForDataHigh_loop:
    sbic COM_PIN_DATA, COM_PINNUM_DATA
      rjmp comWaitForDataHigh_done
    Utils_WaitNanoSecs 100, 0, r22 ; wait for 100 nanosecs
    dec r17
  brne comWaitForDataHigh_loop
  clc   ; timeout
  ret

comWaitForDataHigh_done:
  sec   ; ok
  ret



; ---------------------------------------------------------------------------
; comWaitForAttnHigh
;
; Waits up to COM_MAXWAIT loops for high ATTN line
; IN:
; OUT:
; - CFLAG: set if okay, clear otherwise
; MODIFIED REGISTERS: r17, r22, X

comWaitForAttnHigh:
  ldi r17, COM_MAXWAIT

comWaitForAttnHigh_loop:
    sbic COM_PIN_ATTN, COM_PINNUM_ATTN
      rjmp comWaitForAttnHigh_done
    Utils_WaitNanoSecs 100, 0, r22 ; wait for 100 nanosecs
    dec r17
  brne comWaitForAttnHigh_loop
  clc   ; timeout
  ret

comWaitForAttnHigh_done:
  sec   ; ok
  ret