Debug @ 9600 on 4Mhz 16F676

[sougata]

Hi,

I am working with a project on a 16f676. It is running on its internal 4MHz RC oscillator. It is using extensive interrutps. (Tmr0, Tmr1, INTF, Comparator). All of the interrupt routines are in ASM. Almost all pins are used. For debugging prupose I need to view a few registers. Only option is to use debug on one of the LED indicator.

Now my question is what is the Maximum Baud Rate usable ? PBP manual says that for SEROUT2 oscillators greater than 4MHz may be required for more than 4800bps.

[Darrel Taylor]

From Bruce,

Serial Baudrate 115200
http://www.picbasic.co.uk/forum/show...64&postcount=2
<br>

[sougata]

Hi,

Thank you very much Darrel. I have already successfully used it at 9600. This gives me enough courage and confidence to use it at 115200.

[Bruce]

That post was in error, but there was no way to change it. You can get up
to 19200 bps with a 4MHz osc with PBP's DEBUG.

Someone mentioned the error, but it wouldn't allow me to change it.

However, you definitely can get up to 115200 bps with a 4MHz osc, but you'll
need to use assembler. Try this;

	processor	16F876a
	#include	<P16F876a.inc>
	errorlevel -302 ; suppress "not in bank 0" message

	; Defines
	#define TxPin	PORTC,6		;define serial TX pin
	#define	bank1	banksel 0x80	;select bank 1
	#define	bank0	banksel 0x00	;select bank 0

	; variable allocations
	cblock 0x20	;start at RAM location 20h
	  Counter		;bit counter
	  TxByte		;holds byte to transmit
	  MsgPtr		;message character pointer offset
	  CharCnt		;number of characters in text message
	  dlyLSD		;for delay loop
	endc

	org	0x00	;reset vector
	goto	Main	

	org 	0x05	;required for MCS loader
Main
	call 	Init	;init port
	
MsgLoop
	movlw	.17	;17 characters in the message
	movwf	CharCnt	;load character count
	movlw	.0	;pointer offset to 1st char in table
	movwf	MsgPtr	;load character pointer
	
NextChar
	movf	MsgPtr,w   ;load char pointer into W reg
	call	Message	   ;fetch character from table
	call	TxRoutine  ;send it
	incf	MsgPtr,f   ;increment char pointer
	decfsz	CharCnt,f  ;decrement char count, skip if done
	goto	NextChar   ;not finished, then fetch next char
	call	Delay_50uS ;short delay between messages
	goto	MsgLoop    ;continuous loop

Message 			;prints Hello World....!<CR>
	addwf	PCL,f	;load character offset
	retlw	'H'	;return with ASCII character in W reg
	retlw	'e'	 
	retlw	'l'	
	retlw	'l'	
	retlw	'o'	
	retlw	' '	
	retlw	'W'
	retlw	'o'	
	retlw	'r'	
	retlw	'l'	
	retlw	'd'	
	retlw	'.'	
	retlw	'.'	
	retlw	'.'	
	retlw	'.'	
	retlw	'!'
	retlw	'\r'		;carriage return tagged onto end of message

TxRoutine
	movwf	TxByte		;load byte to send into TxByte
	movlw	.8		;preload bit counter with 8
	movwf	Counter

StartBit
	bcf	TxPin	;true mode / low for START bit
	goto	$+.1  ; 2uS
	goto	$+.1  ; 2uS
	nop

TxLoop
	rrf	TxByte,f	            ;rotate into carry / send LSB first
	btfsc	STATUS,C	;if bit = 1 then
	goto	SetBit		;send a 1
	bcf	TxPin		;else send a 0
	goto	$+.1		;2uS
	decfsz	Counter,f	;decrement bit count
	goto	TxLoop		;if not 0, then continue
	goto	StopBit+.2	;else send stop bit

SetBit
	bsf	TxPin		;send 1
	nop		
	decfsz	Counter,f
	goto	TxLoop
	
StopBit
	nop
	goto	$+.1		;2uS
	goto	$+.1		;2uS
	bsf	TxPin		;begin stop bit period
	goto	$+.1		;2uS
	goto	$+.1		;2uS
	return			;2uS rest of program handles the rest 

Init
	bank0
	movlw	b'01000000' ;TX pin idles high (true mode)
	movwf	PORTC		;set port latches
	bank1			;bank 1
	movlw	b'10000000' ;RX in, rest outs
	movwf	TRISC		;set port directions
	bank0			;back to bank 0
	return
        
Delay_50uS			;call + delay + return = 50uS
	movlw	.15		;at 4MHz
             movwf	dlyLSD
             decfsz	dlyLSD, f
             goto	$ - .1
             return

	end

[sougata]

Hi,

Thank you Bruce for your ASM routine. This would make my life a lot more easier. Since I have interrupts running around I was worried that if my serial transmission gets interrupted too often then I would have large timing error. Now with your ASM routine I can stuff it in inside my INT Handler since I am already having a timebase running.

One more question ?

Since I am using hyperterm I am restricted to ASCII transmission. For example say I need to watch a variable. Using DEBUG with the # modifier sends an "ASCII" representation. This increases the tx overhead. Is there anyway I can view the Binary value of the Tx Data. All I need is an automatically linefeed display of the incoming byte on my PC.

[Bruce]

Hi Sougata,

I'm not sure how to do that with HyperTerm. Displaying a single byte value in
binary sends an ASCII 1 or 0 byte-per-bit. You would probably need to write
your own serial terminal program for something like that.

[sougata]

Hi,

I am not that experienced on the PC side programming but then again your site provides excellent startup pointers. http://www.rentron.com/VisualBasic.htm

So thanks again for the insight.