[time-nuts] Pictic II mods

[Peter Putnam]

Group Members,

Beginning in the days of the 1200 bps DPSK modem, clock crystals were 
specified to .01% to allow the demodulator at the receiving end to 
recover the clock in the data-stream and lock to it. The asynchronous 
serial port attached to the modem needed to be no more accurate than 1%, 
as the modem included an async-to-sync converter that would strip every 
eighth stop bit, if necessary, to reduce the incoming data rate to the 
modem rate. The sync-to-async converter in the receiving modem 
re-inserted the missing stop bits. The case of incoming data under-speed 
was handled in a similar manner.

The older FSK modems were much different. They would shift from mark to 
space tones at the data-rate you used. The filters and tone spacings 
were typically optimized for 300 bps, but across town, 450 bps was 
achievable.

Peter Putnam



On 7/2/2010 6:17 PM, Bob Camp wrote:
> Hi
>
> A lot depends on just what you worry about. Most clock specs for serial com are in the 0.01 to 0.1% range.
>
> Bob
>
>
> On Jul 2, 2010, at 9:02 PM, Chuck Harris wrote:
>
>    
>> The PIC's internal silicon clock is better than 1% over the entire industrial
>> voltage and temperature range.  They are accurate enough.
>>
>> -Chuck Harris
>>
>> Bob Camp wrote:
>>      
>>> 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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