ã 1997 Microchip Technology Inc.

DS00665A-page 11

AN665

00247 ;*

Output Variables:

00248 ;*

BUFFER0..7

00249 ;**********************************************************************

00250 ;

004A

00251 Receive

004A 0686

00252

btfsc

HCSIN

; wait for a falling edge

004B 0A4A

00253

goto

Receive

00254

00255 ; will accept sync pulses from

3.0 to 6.1 ms.

00256 ; more than 128 cycles but less than 256

00257 ; each cycle is 24 us @4MHz

00258 ;

00000038

00259 PREBIT

EQU

.56

00260

004C 006F

00261

clrf

BITCOUNT

; init counter

004D

00262 Rise

004D 0686

00263

btfsc

HCSIN

; wait rising edge

004E 0A54

00264

goto

Rise2

00265

004F 0913

00266

call

SubDelay10

; 24us per cycle

0050 0914

00267

call

SubDelay9

;

00268

0051 03EF

00269

incfsz

BITCOUNT,F

; more than 6,0ms timeout

0052 0A4D

00270

goto

Rise

; waiting loop

0053 0A4A

00271

goto

Receive

; timeout restart

00272

0054

00273 Rise2

0054 07EF

00274

btfss

BITCOUNT,7

; if bit7=1 ok

0055 0A4A

00275

goto

Receive

; else less than

3.0 ms timeout

00276

00277 ;-------------------------------------------------------------------

00278 ; read folowing 8 bytes (ignore last 2 bit)

00279 ;

0056 0C40

00280

movlw

.64

; 8 bit per byte

0057 002F

00281

movwf

BITCOUNT

;

00282

0058

00283 FirstPreload

0058 0C38

00284

movlw

PREBIT

; first bit needs no balance

0059 0031

00285

movwf

TIMEHI

005A 0A5D

00286

goto

WHL

00287

005B

00288 RNextBit

005B 0C36

00289

movlw

PREBIT-2

; preload counter

005C 0031

00290

movwf

TIMEHI

; balance extra rrf time

00291

005D

00292 WHL

005D 02B1

00293

incf

TIMEHI,F

; measure high period

005E 0643

00294

btfsc

STATUS,Z

005F 0A4A

00295

goto

Receive

; after 1.2ms (200*6) timeout

0060 0686

00296

btfsc

HCSIN

; loop while High

0061 0A5D

00297

goto

WHL

;

00298

0062 0C38

00299

movlw

PREBIT

; preload counter

0063 0032

00300

movwf

TIMELO

00301

0064

00302 WLL

0064 02B2

00303

incf

TIMELO,F

; measure low period

0065 0643

00304

btfsc

STATUS,Z

0066 0A4A

00305

goto

Receive

; after 1.2ms (200*6) timeout

0067 0786

00306

btfss

HCSIN

; loop while Low

0068 0A64

00307

goto

WLL

00308

00309 ; shift in the new bit

0069 0211

00310

movf

TIMEHI,W

006A 00B2

00311

subwf

TIMELO,F

; if TIMEHI > TIMELO Carry = 0

006B 032E

00312

rrf

BUFFER7,F

; insert bit in buffer