[time-nuts] Pictic II mods

Richard H McCorkle mccorkle at ptialaska.net
Fri Jul 2 18:31:23 EDT 2010


Time-Nuts,
Thanks for your interest in checking out the PICTIC II and working
together to simplify ordering the boards and parts. My hope is by
making the code available others will find ways to improve on the
basic design and will share their modifications to make this a
better project. As Stanley and Bob have mentioned the baud rate
can be modified in the code and I have had good success at serial
rates up to 57.6K even though the internal PIC 8M osc is used.
  Many users will want to use an external timebase so 10M was
selected as the default, but the timebase rate can be increased
for greater resolution with appropriate selection of the sample
capacitor values and XO rate during construction as desired. The
board was designed so either a half or full can XO can be used
and for top adjust trimmers with either staggered or in-line
center pins for flexibility.
  The PICTIC II was a spin-off of a 1ns front-end for a GPSDO
controller and is not intended to replace a good commercial
time interval counter. But it is a simple TIC that could be
useful in many amateur applications. The accuracy will depend
on the timebase and interpolation gain used, with faster
timebase rates and lower interpolation gains increasing the
accuracy. Since commercial TIC designs use a fixed gain and
the PICTIC II interpolation gain is variable, you can decide
the best compromize between resolution and accuracy for your
application. This flexibility makes it difficult to publish
any meaningful specifications for the PICTIC II other than
to say the interpolators with both doubler stages installed
will provide a hardware gain of 800 when properly adjusted.
The software gain can then be reduced to say 100 so an 8
count variation in the interpolation hardware is reduced
to a single count variance in the combined 1ns TIC data for
improved accuracy.

Richard


> 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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