ASM help

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- 18th October 2009, 17:32
Charles Linquis

ASM help

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
- 18th October 2009, 18:03
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

Code:

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
- 18th October 2009, 18:41
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!
- 18th October 2009, 21:19
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.

Code:

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

Code:

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.
- 18th October 2009, 22:07
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

Code:

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.
- 18th October 2009, 22:40
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.
- 18th October 2009, 23:22
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.
- 19th October 2009, 00:26
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.

Code:

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,
- 19th October 2009, 00:31
Charles Linquis

Darrel,

Thanks a million! While I don't know for sure, I'd guess that your little program is the start of something that will be really useful to all those people who post "how can I receive bytes while....... with my 16Fxxx". Tomorrow, I'll work on the SEND side and the buffering.
- 19th October 2009, 00:37
Darrel Taylor

Sure, no problem Charles,

Quote:

Originally Posted by Charles Linquis

Tomorrow, I'll work on the SEND side ...

Already done ...

Tmr_tx-18
http://www.picbasic.co.uk/forum/show...1&postcount=28

Or, at least it's a good start.
- 19th October 2009, 02:20
Mike, K8LH

Charles,

Just for an additional example and reference, I posted a 9600 baud half-duplex example on PICLIST several years ago that uses Timer 2 and IOC. I also have a full-duplex demo and another half-duplex demo that uses Timer 0 that shows how to reload a free-running 8 bit timer.

Hope the examples help. Have a great day.

Regards, Mike

Code:

;****************************************************************** ;* * ;* Filename: 12F683 Half-Duplex 9600 Demo.asm * ;* Author: Mike McLaren, K8LH (k8lh_at_arrl.net) * ;* Date: 02-Jun-05 (last revision 02-Dec-05) * ;* * ;* Half Duplex Bit-Banged 9600 Baud Serial I/O Demo * ;* (with 16-byte circular receive character buffer) * ;* * ;* ·Uses 12F683 INTOSC running at 8-MHz * ;* ·Bit rate error 0.16% plus or minus 1.0% for INTOSC * ;* ·Bit-banged 9600 baud serial I/O * ;* ·Half Duplex (should not TX and RX simultaneously) * ;* ·TMR2 interrupts at 104-usec intervals (every 208 * ;* instruction cycles) and IOC (interrupt on change) * ;* for RX start bit leading edge detection on RXPIN * ;* ·Circular 16-byte RX character buffer * ;* ·Inverted TX and RX signals (MAX232A or similar * ;* inverting RS-232 interface required) * ;* ·Relatively small - the ISR and the support routines * ;* Init232, Put232, and Get232 use 102 words of code * ;* space at locations 0004 through 0069 * ;* ·Worst case 23% ISR 'overhead' (24-usecs) when a * ;* complete RX character is added to the circular * ;* buffer once every 1.04-msecs while receiving * ;* * ;* MPLab: 7.21 (tabs=8) * ;* MPAsm: 4.02 * ;* * ;******************************************************************
- 19th October 2009, 02:29
Darrel Taylor

That is by far, the smallest program I've ever seen. :D
- 19th October 2009, 03:20
Mike, K8LH

2 Attachment(s)

Yeah, that didn't quite work out right Darrel (LOL). I'll go back and add the file...

Regards, Mike

Oops! Too late. I'll add it here (below). Sorry Gentlemen.
- 19th October 2009, 04:53
Charles Linquis

Thanks Mike,

I thought that I was the first guy who dreamed up (what I think is) a relatively elegant way to save processor cycles. I should have known better!
- 19th October 2009, 12:42
Mike, K8LH

Quote:

Originally Posted by Charles Linquis

Thanks Mike,

I thought that I was the first guy who dreamed up (what I think is) a relatively elegant way to save processor cycles. I should have known better!

You're welcome Sir. And btw, just about everything I ever ended up patting myself on the back for inventing seems to have been done before too (LOL)...

Regards, Mike
- 19th October 2009, 15:20
Melanie

But at least you have the satisfaction of INDEPENDANTLY coming up with the idea, which makes you as much the owner of that idea as anyone else who thought of it earlier or may do so later than you. Did Mr Bell or Mr Gray invent the telephone first? Personally I thought it was my kindergarden teacher with two cans of coffee and a length of string...
- 21st October 2009, 03:08
Charles Linquis

Thank you, Melanie!

You know that I will always consider you the "Princess of the PICs"!