PIC Forth Programming Tip

Dallas 1-Wire Communications

Dallas Semiconductor make a range of devices that communicate
on a network which may be only two wires. The devices take their
power from the data line when there is no communication taking place.

Below is code to talk to these devices. This assumes a PIC running at
10MHz and the 'Wire' is the open-collector pin RA4. The temperature
measurement shown here assumes a Vcc powered DS1820 and NOT
a parasitic powered device.

The code is mainly in assembler as this is necessary to get the best
speed. Forth is quite happy to have either.

REQUIRED FC!      \ Used during debug

XASM MACRO:  WIRE   PORTA  4  MACRO;   \ Define Wire drive
IN-META
|  VARIABLE CHECKSUM

: INIT-WIRE    $10 PORTA C!    $0F TRISA C!   ;

CODE RESET-WIRE ( - )
           BCF WIRE                    \ Wire LO
   N 2   DO                            \ Wait ~600uS
   N 1+ 250  DO  LOOP
           LOOP
           BSF WIRE                    \ Wire HI
   N 20 DO  LOOP
   BEGIN  WIRE -BIT UNTIL              \ Wait for Presence to
   BEGIN  WIRE  BIT UNTIL              \ come and go
           CLRF  CHECKSUM              \ Reset checksum
           RETURN  C;

CODE READ-WIRE ( - b )
           DECF FSR
           CLRF  (FSR)                 \ Zero stack item
   N 8  DO                             \ 8 Bits
           BCF WIRE                    \ Pulse >1uS
           NOP
           NOP
           BSF WIRE
   N 1+ 13 DO  LOOP                    \ ~ 10uS
           BSF STATUS CY
           BTFSS WIRE                  \ Read Wire
           BCF STATUS CY
           RRF (FSR)                   \ Add bit to stack
           RLF (FSR)  ,W               \ Get it back
           CLRW                        \ Checksum X8+X5+X4+1
           BTFSC STATUS CY
           MOVLW 1
           XORWF CHECKSUM  ,W
           ANDLW 1
           STATUS Z BIT
           IF        BCF STATUS  CY
           ELSE  BSF STATUS  CY
                      MOVLW $18
                      XORWF  CHECKSUM
           THEN RRF  CHECKSUM
  N 1+ 60 DO  LOOP                     \ ~ 45 uS
           LOOP
           RETURN   C;

CODE WRITE-WIRE ( b - )
   N 8      DO                         \ 8 Bits
               RRF  (FSR)              \ Get bit from stack
               BCF  WIRE               \ Wire LO
   STATUS CY BIT
               IF        NOP
                         NOP
                         BSF  WIRE          \ 1uS then Wire HI
                         N 1+ 75 DO LOOP    \ Delay till end ~ 60uS
               ELSE N 1+ 75 DO LOOP         \ Delay till end
                         BSF  WIRE          \ Wire HI
               THEN
                NOP
                NOP                         \ >1uS between bits
                LOOP
                INCF  FSR                   \ DROP stack value
                RETURN   C;

: ?TEMP  ( - n f )  \ Convert temp. read and calc to 0.01
      RESET-WIRE
      $CC WRITE-WIRE                        \ Skip ROM
      $44 WRITE-WIRE                        \ Convert
      BEGIN  READ-WIRE  UNTIL               \ Convertion over
      RESET-WIRE
      $CC WRITE-WIRE
      $BE WRITE-WIRE                        \ Read Scratchpad
      READ-WIRE
      READ-WIRE  SWAP  ABS  2/  100  UM*  20 0  W-
      3 FOR  READ-WIRE  DROP  NEXT          \ Not required
      READ-WIRE
      READ-WIRE DUP  ROT  -  100  UM*  ROT UM/MOD NIP  0  W+
      ROT IF  ||  NEGATE  |  THEN
      READ-WIRE DROP  CHECKSUM C@           \ = 0 If correct
      RESET-WIRE   C;

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