-- serial routine from Axelk

--
-- ------------------------------------------------------------------------
-- bit lengths:
--     9600 baud  =   104 cycles
--    19200 baud  =    52 cycles
--    38400 baud  =    26 cycles
--    76800 baud  =    13 cycles
--
--
-- ------------------------------------------------------------------------
-- const asynch_baudrate = 19200  -- hardcoded

var volatile bit asynch_in_pin         is rx
var volatile bit asynch_out_pin        is tx

--
-- -------------------------------------------------------------------------
-- some easy delays
--
-- -------------------------------------------------------------------------

procedure delay10 is -- 10 usec including call
  -- call=2 + return=2 + 3*(goto=2)
  assembler
    local L1, L2, L3
        goto L1
    L1: goto L2
    L2: goto L3
    L3:
  end assembler
end procedure

--
-- ------------------------------------------------------------------------

procedure delay44 is -- 44 usec including call
  -- call=2 + return=2 + 4*10
  delay10
  delay10
  delay10
  delay10
end procedure

--
-- -------------------------------------------------------------------------
-- send a byte
--
-- -------------------------------------------------------------------------

procedure asynch_send( byte in x ) is

   var bit current_bit at x : 0
   var byte times

   assembler
     local sendloop, L1, L2, by0, by1

     -- wait 2 bits = 104 clocks
     call delay44
     call delay44
     call delay10
     call delay10   -- 108 is ok here

     -- the start bit
     bcf   asynch_out_pin    -- low
     call  delay44

     -- the 8 data bits
     movlw   8               -- 8 times
     movwf   times

                                                   -- 46
     goto    L1                                    -- 48
L1:  goto    L2                                    -- 50
L2:

sendloop:
     -- asynch_out_pin = current_bit
     btfss   current_bit
     goto    by0
     bsf     asynch_out_pin
     goto    by1
by0:
     bcf     asynch_out_pin
     nop
by1:                                               -- 5

     -- x = x >> 1
     rrf     x, f                                  -- 6
     call    delay44                               -- 50

      -- end loop
     decfsz times, f                               -- 51
     goto   sendloop                               -- 53

     -- the stop bit(s)
     bsf   asynch_out_pin                          -- high
     call  delay44
     call  delay10

     clrwdt
  end assembler

end procedure

--
-- -------------------------------------------------------------------------
-- receive a byte into lastreceived
--
-- -------------------------------------------------------------------------

var byte lastreceived

procedure asynch_receive is

   var byte times

   assembler
     local WaitIdle, WaitStart, RecvLoop

        clrwdt

     -- wait for idle
     WaitIdle:
        btfss asynch_in_pin  -- active low
        goto  WaitIdle

     -- wait for start bit
     WaitStart:
        btfsc asynch_in_pin  -- active low
        goto  WaitStart

     -- wait for half a bit time (26) until recvloop
        call    delay10
        call    delay10
        nop

     -- now repeat 8 times:
     -- wait for the next interval and sample

        movlw   8               -- 8 times
        movwf   times
        goto    recvloop        -- 2 usec

      recvloop:
        call    delay44         -- 44
        nop                     -- 45

        -- x = x >> 1
        clrc                                    --   1
        rrf   lastreceived, f                   --   2

        -- bit x:7 = asynch_in_pin
        btfsc  asynch_in_pin                    --   3
        bsf    lastreceived, 7                  --   4

        -- end loop
        decfsz times, f                         --   5
        goto   recvloop                         --   7

        -- wait for the (first) stop bit
        call    delay44

        clrwdt

   end assembler
end procedure -- asynch_receive

--
-- -------------------------------------------------------------------------

    Source: geocities.com/vsurducan/electro/PIC

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