[time-nuts] What's the easiest way to divide by 10 for Thunderbolt reference output?

Richard H McCorkle mccorkle at ptialaska.net
Tue Jul 24 18:36:05 UTC 2012


Chris,

A PIC requires 4 clock cycles per instruction which limits the
maximum output rate a PIC can provide as partial instruction
times can't be used. With a 10 MHz input each instruction takes
400ns and if duty cycle isn't an issue nop instructions can be
added each loop to extend the cycle period giving the following
maximum PIC output rates with a 10 MHz clock.

2 instructions  1.25 MHz     50% duty cycle
3 instructions  833.333 KHz  33% duty cycle
4 instructions  625 KHz      25% duty cycle
5 instructions  500 KHz      20% duty cycle

While a PIC can produce almost any division ratio for slower
output rates the 4 clocks per instruction time limits the
maximum rate a PIC can produce and generating a 1 MHz output
with a 10 MHz clock is not an option.

Richard


> John,
>
> That's interesting to me. What exactly are the actual structural limitations of
> [that] pic?
>
> -CH
>
> On Jul 24, 2012, at 7:55, John Ackermann N8UR <jra at febo.com> wrote:
>
>> Unfortunately, the TAPR T2-Mini divider (http://www.tapr.org/kits_t2-mini.html)
>> can't quite get to 1 MHz from 10 MHz with the PIC divider chip due to limitations
>> in the chip architecture.
>>
>> However, nothing says you couldn't "dead bug" in a decade divider chip in place
>> of the PIC, and let the T2-Mini provide the input conditioning, output driver,
>> voltage regulation, connectorization, etc. for you, making it much a smaller
>> project.
>>
>> John
>> ----
>>
>> On 7/24/2012 8:18 AM, Chris Wilson wrote:
>>>
>>>
>>>   24/07/2012 13:14
>>>
>>> My Racal Dana 9908 can take a 1 Mhz external reference. Inputting my
>>> Thunderbolt at 10 MHz works, but shifts the decimal point over. I am
>>> not sure if this has any other detrimental effects as to accuracy or
>>> other? What's the easiest way to have a 1 MHz reference from the
>>> Thunderbolt for this timer / counter please, yet retaining 10 Mhz for
>>> my other devices that want a 10MHz reference signal?  Thanks.
>>>
>>
>>
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