[time-nuts] Pictic II mods

Bob Camp lists at rtty.us
Sat Jul 3 00:39:11 UTC 2010


Hi

The gotcha with the tune register is that it's not much help for those without a PIC programmer...

Bob


On Jul 2, 2010, at 8:15 PM, Richard H McCorkle wrote:

> Bob,
> Another option with the 16F688 is to "tweak" the internal oscillator
> rate slightly to give the proper baud rate. The code includes the
> following two lines (normally commented out) to do this.
> 
> ;	movlw	0x00	;put the cal value in OSCTUNE
> ;	movwf 	OSCTUNE 	;to calibrate oscillator
> 
>> From the Spec Sheet:
> 
> The HFINTOSC is factory calibrated but can be adjusted in software
> by writing to the OSCTUNE register. The default value of the OSCTUNE
> register is ‘0’. The value is a 5-bit two’s complement number. When
> the OSCTUNE register is modified, the HFINTOSC frequency will begin
> shifting to the new frequency. Code execution continues during this
> shift.
> 
> bit 4-0 TUN<4:0>: Frequency Tuning bits
> 
> 01111 = Maximum frequency
> 00001 = Increasing Frequency
> 00000 = Oscillator module is running at the calibrated frequency.
> 11111 = Decreasing Frequency
> 10000 = Minimum frequency
> 
> Richard
> 
> 
>> Hi
>> 
>> Yup, and as you go to higher baud rates the +/- one makes for a bigger "hop" in the
>> baud rate.
>> 
>> One cute way to attack the problem:
>> 
>> Set the baud rate so it *should* be fast
>> Set the PIC transmitter to send two stop bits (or marking parity and two stop bits
>> if you can).
>> Set the PC receiver to look for one stop bit and no parity.
>> 
>> Net result is that you can tolerate a much higher baud rate error in a "transmit
>> mostly" application like this one.
>> 
>> Bob
>> 
>> 
>> On Jul 2, 2010, at 5:44 PM, Stanley Reynolds wrote:
>> 
>>> yes, internal clock, may need to tune the osc or try +- one number, external osc
>>> pins are in use.
>>> 
>>> Stanley
>>> 
>>> 
>>> 
>>> ----- Original Message ----
>>> From: Bob Camp <lists at rtty.us>
>>> To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
>>> Sent: Fri, July 2, 2010 4:11:11 PM
>>> Subject: Re: [time-nuts] Pictic II mods
>>> 
>>> Hi
>>> 
>>> I believe he's using the internal clock on the PIC. It's not super accurate,
>>> so running a lot of data can be an issue.
>>> 
>>> Bob
>>> 
>>> -----Original Message-----
>>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
>>> Behalf Of Stanley Reynolds
>>> Sent: Friday, July 02, 2010 3:41 PM
>>> To: time-nuts at febo.com
>>> Subject: [time-nuts] Pictic II mods
>>> 
>>> Looking at Richard's code in PICTICII.ASM :
>>> 
>>>   bsf     TXSTA,BRGH            ;set USART hi speed mode
>>>   movlw D'51'                         ;set async rate at 9600 baud (51. for
>>> 8
>>> MHz int, BRGH=1)
>>>   movwf SPBRG
>>> 
>>> Then '25' would be 19,200 baud
>>> and '12' would be 38,400 baud
>>> 
>>> Need to test this but could be a way to get more data out.
>>> 
>>> Stanley
>>> 
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>>> 
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>> 
>> 
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> 
> 
> 
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