I'm trying to re-create a hardware serial port with pure software. I need a non-blocking RECEIVE port because I need to add a 3rd port to my 8723.
Most of the interrupt-driven routines I have seen have a very large processor overhead because they sample all the time and because they sample often so as to get the bit timing right.
I'm trying an approach that uses INT0. It watches for the first falling edge of the START bit, then starts a TMR4 interrupt to time the rest of the bits. Using this approach should use zero processing power when a byte isn't coming in, and not too many cycles when one is. INT0 is turned on only to detect the falling edge of the START bit. TMR4 int is turned on only when receiving bits.
The trouble is - the following code always returns 0xff whenever I send it a character. Can someone tell me what I'm doing wrong? RxD is coming through a MAX232 and is fed to PORTB.0
Code:
T4CON = %00000110 ; T4 ON, /16 prescale, no postscalers
INTCON = %11000000 ; Global Int enable, peripheral INT enable
INTCON2 = %00000000
PIE3.3 = 0 ;Turn off Int on tmr 4
IPR3.3 =1 ; High priority for INT4
bit_cntr VAR BYTE
rcv_byte VAR BYTE
byteready VAR BYTE
rcv_buffer VAR BYTE
;-----------------------------------------------------------------------
INCLUDE "DT_INTS-18.bas" ; Base Interrupt System
ASM
INT_LIST macro ; IntSource, Label, Type, ResetFlag?
INT_Handler INT0_INT, GetStart, ASM, no
INT_Handler TMR4_INT, BitTimer, ASM, no
endm
INT_CREATE ; Creates the interrupt processor
ENDASM
Goto OverInt
'---[INT - interrupt handler]---------------------------------------------------
ASM
GetStart
bcf T4CON,2 ; Turn off Timer 4
movlw 0x97 ; w /16 prescaler gives 156uSec dly @ 40Mhz = 1.5 bit times
movwf TMR4
bsf T4CON,2 ; Turn timer 4 on
bsf PIE3,3 ; turn on the Timer4 INT
movlw 0x08 ; load the bit counter
movwf _bit_cntr
bcf INTCON,4 ; shut off the INT0 interrupt
bcf INTCON,1 ; clear INT0 int flag bit
INT_RETURN
ENDASM
ASM
BitTimer
bcf T4CON,2 ; Turn Timer4 off
movlw 0xBF ; 104uSec / 16, 40 Mhz 1 bit time
movwf TMR4 ; load it
bsf T4CON,2 ; Turn it back on
btfss PORTB,0 ; check status of portB.0
bcf STATUS,C ; move bit into carry
btfsc PORTB,0 ; check status of portb.0
bsf STATUS,C ; move bit into carry
rrcf _rcv_byte,f ; shift carry into rec'd byte
decfsz _bit_cntr,f ; decrement the bit counter
bra Donebit ; get out and wait for next int
bra Donebyte ; got 8 bits
Donebit
bcf PIR3,3 ; clear the int flag
bsf PIE3,3 ; start the TMR4 int again
INT_RETURN
Donebyte
movff _rcv_byte,_rcv_buffer
movff _rcv_buffer,PORTD ; this is a debug, just to see rec'd byte on LEDS
bcf PIR3,3 ; clear the int flag
bsf INTCON,4 ; turn on the INT0 interrupt for next byte
INT_RETURN
ENDASM
OVERINT:
TRISD = 0
PORTD = 0
TRISE = 0
INTCON.1 = 0
PIE3.3 = 0
PIR3.3 = 0
PAUSE 100
@ INT_ENABLE INT0_INT
WAITT:
GOTO WAITT
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