ASM help

[Jerson]

You need to consider the interrupt latency along with the bit time and adjust the timer accordingly. Look at the generated listing to know how much it is. Another improvement is here

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
      bsf   STATUS,C   ; move bit into carry
      btfss PORTB,0   ; check status of portB.0
      bcf   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

[Charles Linquis]

I understand your improvement, and I realized that I hadn't considered latency, but at 40Mhz, it is only a few uSec.

Even with a small timing error, it should get the first few bits right - but doesn't!

[Darrel Taylor]

In Donebyte, before re-enabling the INT0 interrupt, it clears the Timer4 flag.
INT0IF will still be set and you'll get a false start when there's no data.

      bcf  PIR3,3       ; clear the int flag
      bsf INTCON,4   ; turn on the INT0 interrupt for next byte

And I think the 1-1/2 bit time is off.
97 would be right if it were decimal, but it's hex.
Try it with 0x61.


Knowing you, this is probably part of a much bigger program with lots of variables ...
You'll probably want to put the variables used in ASM into BANK0

         bit_cntr  VAR BYTE BANK0
         rcv_byte  VAR BYTE BANK0
         rcv_buffer   VAR BYTE BANK0

There's really no need to stop timer4 when reloading since it's an 8-bit timer.
But adding the reload value to TMR4 instead of just loading it will cancel out any latencies.
<br>

[Charles Linquis]

Yes, this *WILL* be part of a much bigger program!

I made some changes -

I cleared INTCON.1 before I re-enabled it in the
Donebyte routine

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
      bcf INTCON,1
      bsf INTCON,4   ; turn on the INT0 interrupt for next byte
      INT_RETURN

But that didn't help. It shouldn't have been set anyway, since I clear it
after I shut off INT0 in the
GetStart section.

I changed the TMR4 reload value to 0x61

I put the variables in BANK0

I made the change recommend by Jerson to take one line out of the setb/clrb
sequence.

Still, when I hit ANY character, all the LEDs on PORTD light up (high).

Something small still isn't right.

[Darrel Taylor]

I think you need to disable the TMR4 interrupts in Donebyte, instead of clearing the flag.

I'll try to run it here on a 4520.

No Timer4, but Timer2 should do the same thing for me.
<br>

[Darrel Taylor]

OK, got it working here ...

You do need to disable T4 ints (last post).

And I haven't figured out why yet, but it was missing a bit in each byte, so I increased _bit_cntr to 9 and now it works.

I'll play with it some more and post the final results.
<br>

[Darrel Taylor]

Here ya go ...

The 9-bit issue was due to flags not being cleared first before enabling.

I've changed it to put the bit periods in PR4 instead of TMR4, this eliminates the latencies without having to reload the timer.

Since I was using an 18F4520 (no TMR4), I made it so you can use any of the 8-bit timers that count up to PR?, TMR2, 4 or 6.

I've also changed several other things, so look carefully if you're trying to pick them out.

My debug method was sending the received byte out the USART, instead of PORTD.

DEFINE OSC 40

DEFINE RX_TMR  4          ; can be 2, 4 or 6
DEFINE TMR_INT TMR4_INT   ; int must match timer selection

;RCSTA = $90 ' Enable serial port & continuous receive
;TXSTA = $20 ' Enable transmit, BRGH = 0
;SPBRG = 64  ' 9600 Baud @ 40MHz, 0.16%


               
 bit_cntr    VAR BYTE BANK0
 rcv_byte    VAR BYTE BANK0
 byteready   VAR BYTE BANK0
 rcv_buffer  VAR BYTE BANK0
;-----------------------------------------------------------------------

        INCLUDE "DT_INTS-18.bas"         ; Base Interrupt System

  
ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
              INT_Handler    INT0_INT,   GetStart,   ASM,  yes 
              INT_Handler     TMR_INT,   BitTimer,   ASM,  yes

          endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM
                         
Goto OverInt

'---[INT - interrupt handler]---------------------------------------------------
ASM
#define TMR_VAL TMR#v(RX_TMR)
#define TMR_CON T#v(RX_TMR)CON
#define TMR_PR  PR#v(RX_TMR)
ENDASM
   
ASM
GetStart  
      movlw 0x02             ; T? OFF, /16 prescale, no postscalers
      movwf TMR_CON          
      movlw 0x61             ; 156uSec dly @ 40Mhz = 1.5 bit times
      movwf   TMR_PR 
      clrf    TMR_VAL
      bsf     TMR_CON,2      ; Turn timer on
      INT_ENABLE  TMR_INT    ; turn on the Timer INT
      movlw   0x08           ; load the bit counter  
      movwf   _bit_cntr
      INT_DISABLE INT0_INT   ; shut off the INT0 interrupt
      INT_RETURN
ENDASM     
     
ASM
BitTimer
      movlw 0x41             ; load PR2 for 1 bit period
      movwf TMR_PR 
      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
      INT_RETURN
      
Donebyte      
      movff _rcv_byte,_rcv_buffer
;TXwait
;      btfss  PIR1, TXIF
;      bra  TXwait
;      movff _rcv_buffer, TXREG
      
      movff _rcv_buffer,PORTD  ; this is a debug, just to see rec'd byte on LEDS
      bcf  TMR_CON,2
      INT_DISABLE  TMR_INT
      INT_CLEAR    INT0_INT
      INT_ENABLE   INT0_INT      ; turn on the INT0 interrupt for next byte
      INT_RETURN
ENDASM      
      
          
OVERINT:

    TRISD = 0
    PORTD = 0
    TRISE = 0
    INTCON2.6 = 0            ; int0 on falling edge
         
    PAUSE 100

@   INT_CLEAR  INT0_INT  
@   INT_ENABLE INT0_INT  
  
  
WAITT:

    GOTO WAITT

HTH,