Hi all,
I wonder if anyone can point me in the right direction, I'm trying to read to scio pin of a tms3705, the protocal is one start bit high, one stop bit low and no parity. Im using a pic 16f818 with 4mhz internal osc, tmr1 with a 1/2 prescale. Firstly does anyone know if the internal osc is accurate enough for what Im trying to do, has anyone used the tms3705?
The transponder I'm trying to read is iso 11784/11785 compliant and should send a start byte of 01111110 followed by the data bytes.
Here is my bit of code, any comments/help would be welcome;


ISR BCF PORTB,3 ;TURN OFF U2270B
BCF PORTB,2 ;wake up tms3705
MOVLW 37H ;LOAD FSR
MOVWF FSR
CALL TXCT ;4MS DELAY
BSF PORTB,2
CLRF STFLAG

CLRF D1

CLRF TMR0
RUF CLRF DATA1
MOVLW .25
SUBWF TMR0,W
BTFSC STATUS,C
GOTO BACK
BTFSS PORTB,1 ;WAIT FOR START BIT
GOTO RUF

CLRF TMR1L
CLRF TMR1H
RUG MOVLW .13 ;32 PUTS CHECK IN MIDDLE OF BIT
SUBWF TMR1L,W
BTFSS STATUS,C
GOTO RUG
CLRF TMR1L ;START TIMER AGAIN
BTFSS PORTB,1 ;SHOULD STILL BE SET
GOTO BACK ;IF NOT BE ERROR


RUH MOVLW .28 ;BIT LENGH 64uS
SUBWF TMR1L,W
BTFSS STATUS,C
GOTO RUH
CLRF TMR1L
BCF STATUS,C
BTFSS PORTB,1 ;IS BIT SET OR CLEAR?
BSF DATA1,7 ;SET MSB
RRF DATA1 ;ROTATE DATA
INCF D1

MOVLW .8
SUBWF D1,W ;HAVE 8 BITS BEEN RECIVED?
BTFSS STATUS,2
GOTO RUH
BTFSS STFLAG,0 ;HAVE STARTBITS BEEN RECIVED
GOTO CHECKS

MOVF DATA1,W
MOVWF INDF
INCF FSR
CLRF DATA1
CLRF D1
MOVF FSR,W
SUBLW 41H ;HAS ALL BEEN READ?
BTFSS STATUS,2
GOTO RUF ;NO CYCLE AGAIN

CALL HOME
CALL DFILE
CALL DISPLAY
CALL DISP

BACK BSF PORTB,3

BCF INTCON,2
BCF INTCON,1

RETFIE
CHECKS MOVLW B'01111110'
SUBWF DATA1,W
BTFSS STATUS,2
GOTO BACK
BSF STFLAG,0
GOTO RUF

 

Well I got it working! Had alot of things wrong, which I've now sorted out. The interal osc seems to work at room temp, but I think its wise to use a crystall.
I know its frustrating to find a unresolved post so here is my code, any comments welcome;

ISR BCF PORTB,3 ;TURN OFF U2270B
BCF PORTB,2 ;txct line on tms3750
BANKSEL T1CON
MOVLW B'00000001'
MOVWF T1CON ;CHANGE TMR1 PRESCALE
BANKSEL 0
MOVLW 37H ;LOAD FSR
MOVWF FSR0L

CALL TXCT ;4MS DELAY
BSF PORTB,2
CLRF STFLAG

CLRF D1

BOB CLRF TMR0L
RUF CLRF DATA1
MOVF TMR0L,W
SUBLW .50
BTFSC STATUS,2
GOTO BACK
BTFSS PORTB,1 ;WAIT FOR START BIT
GOTO RUF

CLRF TMR1L
CLRF TMR1H

RUG MOVLW .21 ;32 PUTS CHECK IN MIDDLE OF BIT
NOP
NOP
SUBWF TMR1L,W
BTFSS STATUS,C
GOTO RUG
CLRF TMR1L ;START TIMER AGAIN
BTFSS PORTB,1
GOTO BACK ;IF NOT BE ERROR


RUH MOVLW .57 ;BIT LENGH 64uS
NOP
SUBWF TMR1L,W
BTFSS STATUS,C
GOTO RUH
CLRF TMR1L
BCF STATUS,C
RRCF DATA1 ;ROTATE DATA
BTFSS PORTB,1 ;IS BIT SET OR CLEAR?
BSF DATA1,7 ;SET MSB

INCF D1


MOVLW .8
SUBWF D1,W ;HAVE 8 BITS BEEN RECIVED?
BTFSS STATUS,2
GOTO RUH
CLRF D1
BTFSS STFLAG,0 ;HAVE STARTBITS BEEN RECIVED
GOTO CHECKS



MOVF DATA1,W
MOVWF INDF0
INCF FSR0L
CLRF DATA1
MOVF FSR0L,W
SUBLW 47H ;HAS ALL BEEN READ?
BTFSS STATUS,2
GOTO BOC ;NO CYCLE AGAIN
BSF DON,0 ;TAG HAS BEEN READ SO SET DONE BIT & RETURN FROM INTERRUPT

BACK BSF PORTB,3
BANKSEL T1CON
MOVLW B'00010001'
MOVWF T1CON
BANKSEL 0
CLRF TMR0L

BCF INTCON,2
BCF INTCON,1

RETFIE
CHECKS MOVLW B'01111110'
SUBWF DATA1,W
BTFSS STATUS,2
GOTO BOC
BSF STFLAG,0
BOC MOVLW .60 ;CHECK FOR STOP BIT LOW
SUBWF TMR1L,W
BTFSS STATUS,C
GOTO BOC
BTFSC PORTB,1
GOTO BACK ;IF HIGH BE ERROR
GOTO BOB ;RETURN & WAIT FOR START BIT

;*******************************************************************************
TXCT CLRF TMR0L ;PRESCALER IS SET TO /256 BEFORE
BUCK MOVF TMR0L,W ;INTERUPT IS ENABLED, SO 15 GIVE
SUBLW .200 ;4MS DELAY (ISH)
BTFSS STATUS,2
GOTO BUCK
RETLW 0

 

Yes, you have to use a XTAL for reliable operation.