; ***************************************************************************
; copyright   : (C) 2023 by Martin Preuss
; email       : martin@libchipcard.de
;
; ***************************************************************************
; * This file is part of the project "AqHome".                              *
; * Please see toplevel file COPYING of that project for license details.   *
; ***************************************************************************



; ***************************************************************************
; defines

.equ COM2_BUFFER_SIZE = 24

.equ COM2_MAXWAIT_US       = 100  ; maximum wait time in microseconds when waiting for rising/falling clock
.equ COM2_MAINTENANCE_ADDR = 0xc1
.equ COM2_CRC_POLYNOMIAL   = 0x97


; flags for variable payload enqueue function
.equ COM2_PAYLOAD_FLAGS_SECONDS    = 0x01
.equ COM2_PAYLOAD_FLAGS_UID        = 0x02
.equ COM2_PAYLOAD_FLAGS_RESERVED1  = 0x04
.equ COM2_PAYLOAD_FLAGS_NUM0       = 0x08
.equ COM2_PAYLOAD_FLAGS_NUM1       = 0x10
.equ COM2_PAYLOAD_FLAGS_NUM2       = 0x20
.equ COM2_PAYLOAD_FLAGS_NUM3       = 0x40
.equ COM2_PAYLOAD_FLAGS_NUM4       = 0x80
.equ COM2_PAYLOAD_FLAGS_SHIFT_NUM  = 3



.equ COM2_MSG_OFFS_DESTADDR        = 0
.equ COM2_MSG_OFFS_MSGLEN          = 1
.equ COM2_MSG_OFFS_MSGDATA         = 2
.equ COM2_MSG_OFFS_CMD             = 2 ; first at COM2_MSG_OFFS_MSGDATA
.equ COM2_MSG_OFFS_SRCADDR         = 3
.equ COM2_MSG_OFFS_PAYLOAD         = 4 ; payload for the cmd follows here



; ***************************************************************************
; data

.dseg


com2DataBegin:
  com2Address:             .byte 1

  com2Interrupts:          .byte 2

  com2StatsPacketsIn:      .byte 2
  com2StatsPacketsOut:     .byte 2
  
  com2StatsIoError:        .byte 2
  com2StatsContentError:   .byte 2
  com2StatsNoBufferError:  .byte 2

  com2StatsBusyError:      .byte 2
  com2StatsCollisions:     .byte 2
  
  com2RecvBuffer:          .byte COM2_BUFFER_SIZE
  com2SendBuffer:          .byte COM2_BUFFER_SIZE
com2DataEnd:



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

.cseg

COM2_BEGIN:




; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Module interface
; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++



; ---------------------------------------------------------------------------
; Com2_Init
;
; IN:
; OUT:
; - CFLAG: set if okay, clear on error
; USED: R16, R17, X, Y

Com2_Init:
  ; preset SRAM data area
  ldi xh, HIGH(com2DataBegin)
  ldi xl, LOW(com2DataBegin)
  clr r16
  ldi r17, (com2DataEnd-com2DataBegin)
  rcall Utils_FillSram

  ; set address to 0 (will be updated later)
  clr r16
  sts com2Address, r16

  ; setup pins and interrupts
  cbi COM_PORT_DATA, COM_PINNUM_DATA      ; disable internal pullup for DATA
  cbi COM_DDR_DATA, COM_PINNUM_DATA       ; set DATA port as input

  cbi COM_PORT_ATTN, COM_PINNUM_ATTN      ; disable internal pullup for ATTN
  cbi COM_DDR_ATTN, COM_PINNUM_ATTN       ; set ATTN port as input

  sbi COM_IRQ_ADDR_ATTN, COM_IRQ_BIT_ATTN ; enable pin change irq for ATTN line
  in r16, GIMSK                           ; enable pin change irq PCIE0 or PCIE1
  ori r16, (1<<COM_IRQ_GIMSK_ATTN)
  out GIMSK, R16
  
  ldi r16, (1<<COM_IRQ_GIFR_ATTN)         ; clear pending irq by writing 1 to ATTN bit
  out GIFR, r16
  
  sec
  ret



; ---------------------------------------------------------------------------
; Com_Run
;
; IN:
; - nothing
; OUT:
; - CFLAG: set if something done, can be called again immediately (otherwise: wait for timer interrupt and retry)
; REGS: (R1, R3, R16, R17, R18, R19, R22, X)

COM2_Run:
  lds r16, com2RecvBuffer
  tst r16
  brne COM2_Run_packetReceived
  clc
  ret

COM2_Run_packetReceived:
  ldi xl, LOW(com2RecvBuffer)
  ldi xh, HIGH(com2RecvBuffer)
  rcall onPacketReceived
  clr r16
  sts com2RecvBuffer, r16  ; unlock buffer
  sec
  ret






; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Preparing messages
; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++



; ---------------------------------------------------------------------------
; Write a simple packet into the given buffer with payload being only CMD and source address.
;
; IN:
; - R16: destination address
; - R18: command (e.g. CPRO_CMD_PING or CPRO_CMD_PONG)
; - X  : pointer to buffer to write to
; OUT:
; - CFLAG: set if okay, clear otherwise
; MODIFIED REGS: R16, R17, X, Y (R3, R4, R15, R16, R17, R18, R19, R20, R21)

COM2_WriteMsgWithCmdAndSrcAddr:
  ldi r17, COM2_PAYLOAD_FLAGS_SECONDS
  push xh
    push xl
      rcall COM2_BeginMsgWithVariablePayload     ; (R3, R4, R16, R17, R18, R19, R20, R21, X)
    pop xl
  pop xh
  rcall com2CalcAndAddChecksumByte              ; (R16, R17, R18, R19, X)
  sec
  ret
COM2_WriteMsgWithCmdAndSrcAddr_error:
  clc
  ret



; ---------------------------------------------------------------------------
; begin packet with variable payload.
;
; IN:
; - R16: destination address
; - R17: flags
; - R18: command (e.g. CPRO_CMD_PING)
; - X  : pointer to packet buffer
; OUT:
; - X  : points to end of packet as it was written so far
; MODIFIED REGS: R3, R16, R17, R18, R19, R20, R21, X (R4)

COM2_BeginMsgWithVariablePayload:
  ; write header (dest address, msg length)
  st X+, r16                               ; destination address
  mov r16, r17                             ; calculate payload size
  mov r3, r17
  rcall com2CalcPayloadSize                ; (R4, R16, R17)
  inc r16                                  ; add CMD byte
  inc r16                                  ; add source address byte
  st X+, r16
  ; write payload
  st X+, r18                               ; 0: CMD
  lds r16, com2Address                     ; 1: source address
  st X+, r16
  lsr r3                                   ; shift out COM2_PAYLOAD_FLAGS_SECONDS
  brcc COM2_BeginMsgWithVariablePayload_l1
  ; write seconds
  lds r16, timerModuleCounterSecs          ; adding of current seconds counter requested
  st X+, r16
  lds r16, timerModuleCounterSecs+1
  st X+, r16
  lds r16, timerModuleCounterSecs+2
  st X+, r16
  lds r16, timerModuleCounterSecs+3
  st X+, r16
COM2_BeginMsgWithVariablePayload_l1:
  lsr r3                                   ; shift out shift out COM2_PAYLOAD_FLAGS_UID
  brcc COM2_BeginMsgWithVariablePayload_l2
  ; write uid
  push xh
    push xl
      rcall Utils_ReadUid                  ; (R16, X)
    pop xl
  pop xh
  st X+, r18
  st X+, r19
  st X+, r20
  st X+, r21
COM2_BeginMsgWithVariablePayload_l2:
  ret



; ---------------------------------------------------------------------------
; com2CalcPayloadSize
;
; Calculate payload size from given flags
;
; IN:
; - R16: flags
; OUT:
; - R16: payload size
; MODIFIED REGS: R4, R16, R17

com2CalcPayloadSize:
  clr r4
  ldi r17, 4
  lsr r16                       ; shift out COM2_PAYLOAD_FLAGS_SECONDS
  brcc com2CalcPayloadSize_l1
  add r4, r17                   ; add 4 bytes
com2CalcPayloadSize_l1:
  lsr r16                       ; shift out COM2_PAYLOAD_FLAGS_UID
  brcc com2CalcPayloadSize_l2
  add r4, r17                   ; add 4 bytes
com2CalcPayloadSize_l2:
  lsr r16                       ; shift out reserved1, after this R16 contains COM2_PAYLOAD_FLAGS_NUM0-4
  add r16, r4                   ; add previous bytes to R16
  ret







; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Sending and receiving messages
; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


; ---------------------------------------------------------------------------
; receive packet.
;
; OUT:
; - CFLAG: set if okay (packet received), cleared on error
; REGS: r16, r17, x (r18, r19, r20, r21, r22)

com2ReceivePacket:
  ldi xl, LOW(com2RecvBuffer)
  ldi xh, HIGH(com2RecvBuffer)
  
  ld r16, X                              ; check: buffer in use?
  tst r16
  breq com2ReceivePacket_bufferAvailable

  ldi xl, LOW(com2StatsNoBufferError)    ; buffer in use, don't release
  ldi xh, HIGH(com2StatsNoBufferError)   ; just increment error counter
  rcall Utils_IncrementCounter16         ; (r18, r19, 22)
  clc
  ret

com2ReceivePacket_bufferAvailable:
  ; read destination address
  rcall com2ReceiveByte                   ; read byte (R16, R17, R20, R21, R22)
  brcc com2ReceivePacket_ioError
  ; compare destination address (accept "FF" and own address)
  cpi r16, 0xff
  breq com2ReceivePacket_acceptAddr
  lds r17, com2Address
  cp r16, r17
  breq com2ReceivePacket_acceptAddr
  clc                                    ; not for me
  ret

com2ReceivePacket_acceptAddr:
  st X+, r16                             ; store dest address, lock buffer
  ; read msg length
  rcall com2ReceiveByte                   ; read packet length (R16, R17, R20, R21, R22)
  brcc com2ReceivePacket_ioError
  st X+, r16
  cpi r16, (COM2_BUFFER_SIZE-3)
  brcc com2ReceivePacket_contentError    ; packet too long
  inc r16                                ; account for checksum byte
  mov r17, r16
com2ReceivePacket_loop:
    push r17
      rcall com2ReceiveByte               ; read byte (R16, R17, R20, R21, R22)
    pop r17
    brcc com2ReceivePacket_ioError
    st X+, r16
    dec r17
  brne com2ReceivePacket_loop

  ldi xl, LOW(com2RecvBuffer)
  ldi xh, HIGH(com2RecvBuffer)
  rcall com2CheckMessageInBuffer         ; (R16, R17, R18, R19, R20, X)
  brcc com2ReceivePacket_contentError
  ; done, increment packet counter, set flags
  ldi xl, LOW(com2StatsPacketsIn)
  ldi xh, HIGH(com2StatsPacketsIn)
  rcall Utils_IncrementCounter16         ; (r18, r19, 22)
  sec
  ret

com2ReceivePacket_ioError:
  ldi xl, LOW(com2StatsIoError)
  ldi xh, HIGH(com2StatsIoError)
  rjmp com2ReceivePacket_incCounter

com2ReceivePacket_contentError:
  ldi xl, LOW(com2StatsContentError)
  ldi xh, HIGH(com2StatsContentError)
  rjmp com2ReceivePacket_incCounter

com2ReceivePacket_incCounter:
  rcall Utils_IncrementCounter16                ; (r18, r19, 22)
  clr r16
  sts com2RecvBuffer, r16                       ; unlock buffer
  clc
  ret



; ---------------------------------------------------------------------------
; send packet from sendBuffer, if line free.
;
; IN:
; - nothing
; OUT:
; - CFLAG: set if okay (packet sent), cleared on error
; REGS: r22, x (r18, r19, r22)

COM2_SendPacket:
  ldi xl, LOW(com2SendBuffer)
  ldi xh, HIGH(com2SendBuffer)

COM2_SendPacketAtX:
  in r15, SREG
  push r15
    cli
    ; check for ATTN line: busy?
    cbi COM_PORT_ATTN, COM_PINNUM_ATTN             ; disable pullup on ATTN
    cbi COM_DDR_ATTN, COM_PINNUM_ATTN              ; set ATTN as input
    nop                                            ; needed to sample current input
    sbis COM_PIN_ATTN, COM_PINNUM_ATTN             ; ATTN low?
      rjmp COM2_SendPacket_lineBusyError           ; jump if it is
  
    cbi COM_PORT_ATTN, COM_PINNUM_ATTN             ; set ATTN low
    sbi COM_DDR_ATTN, COM_PINNUM_ATTN              ; set ATTN as output
    Utils_WaitNanoSecs COM_BIT_LENGTH, 0, r22      ; wait for one bit duration

    adiw xh:xl, COM2_MSG_OFFS_MSGLEN
    ld r16, X
    sbiw xh:xl, COM2_MSG_OFFS_MSGLEN
    inc r16                                        ; account for dest addr
    inc r16                                        ; account for msglen byte
    inc r16                                        ; account for crc byte
    rcall com2SendPacketRaw
    
    cbi COM_DDR_ATTN, COM_PINNUM_ATTN              ; release ATTN line (by setting direction to IN)
    brcc COM2_SendPacket_ioError
    ; packet successfully sent
    ldi xl, LOW(com2StatsPacketsOut)
    ldi xh, HIGH(com2StatsPacketsOut)
    rcall Utils_IncrementCounter16                 ; (r18, r19, r22)
  pop r15
  out SREG, r15
  sec
  ret

COM2_SendPacket_ioError:
    ldi xl, LOW(com2StatsCollisions)
    ldi xh, HIGH(com2StatsCollisions)
    rjmp COM2_SendPacket_incCounter

COM2_SendPacket_lineBusyError:
    ldi xl, LOW(com2StatsBusyError)
    ldi xh, HIGH(com2StatsBusyError)
    rjmp COM2_SendPacket_incCounter

COM2_SendPacket_incCounter:
    rcall Utils_IncrementCounter16                ; (r18, r19, r22)
  pop r15
  out SREG, r15
  clc
  ret



; ---------------------------------------------------------------------------
; send packet over wire, expects ATTN to be low.
;
; IN:
; - r16: number of bytes to send
; - x  : ptr to buffer to send
; OUT:
; - nothing
; REGS: r16, r17, x (r21, r22)

com2SendPacketRaw:
  mov r17, r16
com2SendPacket_loop:
    ld r16, X+
    rcall com2SendByte                 ; send byte (R16, R21, R22)
    brcc com2SendPacket_ioError
    dec r17
  brne com2SendPacket_loop
  sec
  ret
com2SendPacket_ioError:
  clc
  ret






; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; ISR
; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++



com2IsrPcint0:
  push r15
    in r15, SREG
    sbic COM_PIN_ATTN, COM_PINNUM_ATTN
      rjmp com2IsrPcint0_end
    ; low, read packet
    push r1
      push r16
        push r17
          push r18
            push r19
              push r20
                push r21
                  push r22
                    push xh
                     push xl
                       rcall com2ReceivePacket ; (r16, r17, r18, r19, r20, r21, r22, x)
                       lds xl, com2Interrupts
                       lds xh, com2Interrupts+1
                       adiw xh:xl, 1
                       sts com2Interrupts, xl
                       sts com2Interrupts+1, xh
                     pop xl
                    pop xh
                  pop r22
                pop r21
              pop r20
            pop r19
          pop r18
        pop r17
      pop r16
    pop r1
    
com2IsrPcint0_end:
  pop r15
  reti







; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; Lowlevel IO
; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++



; ---------------------------------------------------------------------------
; com2SendByte
;
; 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

com2SendByte:
  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
com2SendByte_loop:                                ; 9 for low bit
    lsr r16                                      ; 1+ bit to send -> CARRY
    brcs com2SendByte_setHigh                     ; HI: +2, LO: +1
com2SendByte_setLow:
    sbi COM_DDR_DATA, COM_PINNUM_DATA            ; +2 set DATA as output
    Utils_WaitNanoSecs COM_BIT_LENGTH, 9, r22
    rjmp com2SendByte_loopEnd                     ; +2
com2SendByte_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 com2SendByte_error                     ; +2 if error (collision: we wanted line to be high but it is low)
    Utils_WaitNanoSecs COM_BIT_LENGTH/2, 0, r22
com2SendByte_loopEnd:
    dec r21                                      ; +1
  brne com2SendByte_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

com2SendByte_error:
  clc
  ret



; ---------------------------------------------------------------------------
; com2ReceiveByte
;
; 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)

com2ReceiveByte:
  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 com2WaitForDataLow                      ; (R17)
  brcc com2ReceiveByte_error
  Utils_WaitNanoSecs COM_BIT_LENGTH/2, 5, r22  ; goto middle of startbit to maximize sync stability
com2ReceiveByte_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 com2ReceiveByte_shiftIn              ; HIGH: +2, rjmp, use set CFLAG
    clc                                        ; LOW: +1
com2ReceiveByte_shiftIn:
    ror r20                                    ; +1
    dec r21                                    ; +1
  brne com2ReceiveByte_loop                     ; +2, sum per loop: 8 cycles
  rcall com2WaitForDataHigh                     ; wait for start of stopbit
  brcc com2ReceiveByte_error
  mov r16, r20
  sec
  ret
com2ReceiveByte_error:
  clc
  ret



; ---------------------------------------------------------------------------
; com2WaitForDataLow
;
; Waits up to COM2_MAXWAIT_US loops for low data line
; IN:
; OUT:
; - CFLAG: set if okay, clear otherwise
; MODIFIED REGISTERS: r17, r22

com2WaitForDataLow:
  ldi r17, COM2_MAXWAIT_US

com2WaitForDataLow_loop:
    sbis COM_PIN_DATA, COM_PINNUM_DATA
      rjmp com2WaitForDataLow_done
    rcall com2WaitFor1000ns
    dec r17
  brne com2WaitForDataLow_loop
  clc   ; timeout
  ret

com2WaitForDataLow_done:
  sec   ; ok
  ret



; ---------------------------------------------------------------------------
; com2WaitForDataHigh
;
; Waits up to COM2_MAXWAIT_US loops for high data line
; IN:
; OUT:
; - CFLAG: set if okay, clear otherwise
; MODIFIED REGISTERS: r17, r22, X

com2WaitForDataHigh:
  ldi r17, COM2_MAXWAIT_US

com2WaitForDataHigh_loop:
    sbic COM_PIN_DATA, COM_PINNUM_DATA
      rjmp com2WaitForDataHigh_done
    rcall com2WaitFor1000ns
    dec r17
  brne com2WaitForDataHigh_loop
  clc   ; timeout
  ret

com2WaitForDataHigh_done:
  sec   ; ok
  ret



; ---------------------------------------------------------------------------
; com2WaitForAttnHigh
;
; Waits up to COM2_MAXWAIT_US loops for high ATTN line
; IN:
; OUT:
; - CFLAG: set if okay, clear otherwise
; MODIFIED REGISTERS: r17, r22, X

com2WaitForAttnHigh:
  ldi r17, COM2_MAXWAIT_US

com2WaitForAttnHigh_loop:
    sbic COM_PIN_ATTN, COM_PINNUM_ATTN
      rjmp com2WaitForAttnHigh_done
    rcall com2WaitFor1000ns
    dec r17
  brne com2WaitForAttnHigh_loop
  clc   ; timeout
  ret

com2WaitForAttnHigh_done:
  sec   ; ok
  ret



; ---------------------------------------------------------------------------
; comWaitFor100ns
;
; Waits for 100 nanoseconds.
; IN:
; OUT:
; - CFLAG: set if okay, clear otherwise
; REGS: r17, r22

com2WaitFor1000ns:
    Utils_WaitNanoSecs 1000, 7, r22 ; wait for 100 nanosecs (minus 3 cycles for rcall, 4 for ret)
    ret









; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
; CRC code
; +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


; ---------------------------------------------------------------------------
; add checksum byte to buffer
;
; IN:
; - X  : pointer to packet buffer
; OUT:
; - CFLAG: set if okay, clear otherwise
; MODIFIED REGS: R16, R17, R18, R19, X

com2CalcAndAddChecksumByte:
  adiw xh:xl, COM2_MSG_OFFS_MSGLEN
  ld r18, X                           ; read msg len
  sbiw xh:xl, COM2_MSG_OFFS_MSGLEN
  inc r18                             ; account for dest address
  inc r18                             ; account for msg len byte
  ldi r19, COM2_CRC_POLYNOMIAL
  rcall com2CalcCrc8                  ; (R16, R17, R18, R20, X)
  st X, r16                           ; add checksum byte
  sec
  ret



; ---------------------------------------------------------------------------
; check message in buffer
;
; IN:
; - X  : pointer to packet buffer
; OUT:
; - CFLAG: set if okay, clear otherwise
; MODIFIED REGS: R16, R17, R18, R19, R20, X

com2CheckMessageInBuffer:
  adiw xh:xl, COM2_MSG_OFFS_MSGLEN
  ld r18, X                           ; read msg len
  sbiw xh:xl, COM2_MSG_OFFS_MSGLEN
  inc r18                             ; account for dest address
  inc r18                             ; account for msg len byte
  ldi r19, COM2_CRC_POLYNOMIAL
  rcall com2CalcCrc8                  ; (R16, R17, R18, R20, X)
  ld r17, X
  cp r16, r17                         ; should be equal
  brne com2CheckMessageInBuffer_error
  sec
  ret
com2CheckMessageInBuffer_error:
  clc
  ret



; ---------------------------------------------------------------------------
; calc crc8 checksum using given polynomial
;
; IN:
; - X  : pointer to data to calc crc8 for
; - r18: number of bytes to calc crc8 for
; - r19: polynomial to use (e.g. 0x97: HD=4 up to 119 bytes, e.g. detects all 1 to 3 bit errors)
; OUT:
; - r16: crc8 checksum
; - X  : point directly behind the checked area
; MODIFIED REGS: R16, R17, R18, R20, X

com2CalcCrc8:
  ldi r16, 0xff              ; crc
  
com2CalcCrc8_loop1:
    ld r17, X+               ; running var
    eor r16, r17
    ldi r20, 8               ; counter for loop2
com2CalcCrc8_loop2:
      lsl r16
      brcc com2CalcCrc8_l1
      eor r16, r19
com2CalcCrc8_l1:
      dec r20
    brne com2CalcCrc8_loop2
    dec r18
  brne com2CalcCrc8_loop1
  ret







COM2_END:
.equ MODULE_SIZE_COM2 = COM2_END-COM2_BEGIN