📄 part3.lst
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00207 ; if that interrupt is disabled.
00208
00209 ; Check first for a timer overflow interrupt. The overflow bit gets
MPASM 03.20 Released PART3.ASM 10-24-2002 12:35:27 PAGE 5
LOC OBJECT CODE LINE SOURCE TEXT
VALUE
00210 ; set even if the interrupt is disabled.
00211
0007 1A8B 00212 btfsc INTCON,T0IE
0008 282D 00213 goto DoBit ;we're in the middle of sending or
00214 ;receiving
00215
00216 ; 10 cycles executed on entry to DoBit
00217
00218 ; If not a timer overflow interrupt, check for external interrupt:
00219
0009 1A0B 00220 btfsc INTCON,INTE ;RB0 is our receive line and it
000A 281D 00221 goto StartRX ;generates an interrupt on a high-
00222 ;to-low transition
00223
00224 ; 12 cycles executed on entry to StartRX
00225
00226 ; Else, must be something we don't care about:
00227
00228 ;do nothing for now
00229
00230 ; Restore the W and STATUS registers:
00231
000B 0E12 00232 Restore swapf SSave,W
000C 0083 00233 movwf STATUS
000D 0E91 00234 swapf WSave,F
000E 0E11 00235 swapf WSave,W
00236
000F 0009 00237 retfie
00238
00239 ;------end Main Interrupt Routine------------------------------------
00240
00241
00242 ;------Subroutine SerSetup-------------------------------------------
00243
0010 00244 SerSetup
00245
00246 ; set up the option register for internal counting, WDT disabled,
00247 ; no prescaler.
00248
0010 0181 00249 clrf TMR0
0011 1683 00250 bsf STATUS,RP0
0012 0064 00251 clrwdt ;set bits in OPTION_REG to
0013 3088 00252 movlw b'10001000' ;enable internal clock counting,
Message[302]: Register in operand not in bank 0. Ensure that bank bits are correct.
0014 0081 00253 movwf OPTION_REG ;disable watchdog timer.
0015 1283 00254 bcf STATUS,RP0 ;switch to bank 0
00255
00256 ; set the output line to idle (high)
00257
0016 1485 00258 bsf PORTA,_SER_OUT ;set the output line to idle
00259 ;(high) state
00260
0017 3001 00261 movlw b'00000001'
MPASM 03.20 Released PART3.ASM 10-24-2002 12:35:27 PAGE 6
LOC OBJECT CODE LINE SOURCE TEXT
VALUE
0018 0086 00262 movwf PORTB
00263
00264 ; enable the external interrupt via RB0
00265
0019 3090 00266 movlw b'10010000' ;set bits in INTCON to enable
001A 008B 00267 movwf INTCON ;external interrupt
00268
00269 ; initialize the SerialReg:
00270
001B 0190 00271 clrf SerialReg
00272
001C 0008 00273 return
00274
00275 ;------end SetSetup--------------------------------------------------
00276
00277 ;------Subroutine StartRX--------------------------------------------
00278 ;
00279 ; This subroutine is called by the main interrupt routine when an
00280 ; external interrupt on RB0 occurs. This means we're receiving the
00281 ; start bit for a character. We want to enable the external TMR0
00282 ; interrupt and prepare to receive the character.
00283
001D 00284 StartRX
00285
00286 ; wait halfway through the bit to see if it's real:
00287
001D 108B 00288 bcf INTCON,INTF ;clear the interrupt
001E 300C 00289 movlw _StartRxDelay
001F 008C 00290 movwf BitCount ;this is the 15th instruction since
00291 ;the interrupt. Note--we're using
00292 ;BitCount for this loop purely for
00293 ;convenience. Usually it's used to
00294 ;actually count the bits we TX/RX.
00295
0020 0B8C 00296 RXWait decfsz BitCount,F ;this loop takes 3 times the initial
0021 2820 00297 goto RXWait ;value of BitCount clock cycles
00298
00299 ; now we should be at the middle of the start bit. Is the input still
00300 ; low? If not, goto Restore and ignore this interrupt.
00301
0022 1806 00302 btfsc PORTB,_SER_IN
0023 280B 00303 goto Restore
00304
00305 ; if we get to here it must really be the start bit. Load TMR0,
00306 ; disable the external interrupt, and enable the TMR0 interrupt.
00307
00308 ; load up the appropriate delay to get us to the middle of the
00309 ; first bit:
00310
0024 30AD 00311 movlw _BitRxDelay
0025 0081 00312 movwf TMR0 ;4 cycles from read of PORTB
0026 3020 00313 movlw b'00100000'
0027 008B 00314 movwf INTCON
MPASM 03.20 Released PART3.ASM 10-24-2002 12:35:27 PAGE 7
LOC OBJECT CODE LINE SOURCE TEXT
VALUE
00315
00316 ; set the SerialReg to indicate that the routines are busy getting
00317 ; a character:
00318
0028 3002 00319 movlw b'00000010'
0029 0090 00320 movwf SerialReg
00321
00322 ; initialize BitCount:
00323
002A 3008 00324 movlw 8
002B 008C 00325 movwf BitCount
00326
00327 ; okay, now we return.
00328
002C 280B 00329 goto Restore
00330
00331 ;------end StartRX---------------------------------------------------
00332
00333
00334 ;------DoBit---------------------------------------------------------
00335 ;
00336 ; sends or receives the next bit. Bits are sent/received from least
00337 ; to most significant bit.
00338
002D 00339 DoBit
00340
00341 ; clear the TMR0 overflow interrupt flag:
00342
002D 110B 00343 bcf INTCON,T0IF
00344
00345 ; Are we receiving?
00346
002E 1910 00347 btfsc SerialReg,2
002F 2840 00348 goto Sending
00349
00350 ; check to see if we're receiving the stop bit:
00351
0030 088C 00352 movf BitCount,F
0031 1903 00353 btfsc STATUS,Z
0032 2839 00354 goto GetStopBit
00355
00356 ; if we get to here, we're in the middle of receiving. Get the next
00357 ; bit: (16 cycles to get to the next instruction from the start of
00358 ; the interrupt).
00359
0033 0C06 00360 rrf PORTB,W ;rrf PORTB into W. This sets
00361 ;the carry bit if RB0 was high.
0034 0C8D 00362 rrf RXChar,F ;doing a rrf on RXChar brings
00363 ;in the carry bit to the MSB.
00364
00365 ; Decrement the bit counter.
00366
0035 038C 00367 decf BitCount,F
MPASM 03.20 Released PART3.ASM 10-24-2002 12:35:27 PAGE 8
LOC OBJECT CODE LINE SOURCE TEXT
VALUE
00368
00369 ; reload TMR0 for the next interrupt, and
00370 ; go to the end of the interrupt routine.
00371
0036 30AD 00372 movlw _BitRxDelay
0037 0081 00373 movwf TMR0 ;21 cycles from start of interrupt
0038 280B 00374 goto Restore
00375
00376 ; if we get to here it's because we need to check for the stop bit.
00377
0039 00378 GetStopBit
0039 1C06 00379 btfss PORTB,_SER_IN ;is the RX line low? If so, it's not
003A 285A 00380 goto Done ;the stop bit. Otherwise, set the
003B 3001 00381 movlw b'00000001' ;SerialReg to show a character has
003C 0090 00382 movwf SerialReg ;been received
003D 080D 00383 movf RXChar,W ;copy the received character to RXBuff
003E 008E 00384 movwf RXBuff
003F 285A 00385 goto Done
00386
00387 ; We got here because we're sending.
00388 ; check to see if we're finished sending the stop bit:
00389
0040 00390 Sending
0040 1990 00391 btfsc SerialReg,3
0041 285A 00392 goto Done
00393
00394 ; check to see if we need to send the stop bit:
00395
0042 088C 00396 movf BitCount,F
0043 1903 00397 btfsc STATUS,Z ;18th cycle
0044 2852 00398 goto SendStopBit
00399
00400 ; if we get to here, we're in the middle of sending. Send the next
00401 ; bit: (16 cycles to get to the next instruction from the start of
00402 ; the interrupt).
00403
0045 0C8F 00404 rrf TXChar,F ;doing rrf on TXChar puts the
0046 1C03 00405 btfss STATUS,C ;least significant bit in the
0047 284B 00406 goto SendZero ;carry flag.
0048 0000 00407 nop
0049 1485 00408 bsf PORTA,_SER_OUT ;if carry is set, send a one.
004A 284E 00409 goto EndDoBit ;PORTA,1 is set on the 24th cycle
00410
004B 00411 SendZero
004B 1085 00412 bcf PORTA,_SER_OUT ;otherwise, send a zero. (24th cycle)
004C 0000 00413 nop ;nop's are for taking the same time
004D 0000 00414 nop ;to get to reloading TMR0 as for when
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