[time-nuts] TIC model

[Bruce Griffiths]

The response time to an external asynchronous interrupt is never 
deterministic.
The external interrupt has to be synchronous with the uP clock to avoid 
the non deterministic synchronisation delay.
Even when the external event is synchronous with the clock input to the 
uP and the uP uses a divider to produce its internal clock then there is 
the issue of divider phase shift.
This phase shift can lead to sampling the waveform before the peak 
across the sampling cap. This is far from ideal, its better to sample at 
or slightly after the peak when the sensitivity to timing variations is 
far smaller.
To complicate the issue further the time of occurrence of the peak is 
temperature dependent and the sampling switch on resistance is nonlinear 
so that peak delay varies with temperature and input signal amplitude.

Its generally quicker and cheaper to estimate the magnitude of such 
effects and make appropriate choices than just build a sequence of 
breadboards each of  which then needs to be extensively characterised.

Bruce

Chris Albertson wrote:
> You all are "inventing problem".  Solve them AFTER you find a problem you
> can measure.   Interrupts are not an issue on a UP like the AVR because
> they are completely deterministic.  It don't matter the lenth of time as
> long as it is 100% deterministic and predictable.   On a multi-tasking OS
> running on a super scaler CPU you have unknowable latentcy but this is not
> the problem on a chip that does one machine cycle per clock cycle.
>
>
> On Sat, Feb 15, 2014 at 6:50 PM, Brian Lloyd<brian at lloyd.com>  wrote:
>
>    
>> On Sat, Feb 15, 2014 at 7:10 PM, Tom Van Baak<tvb at leapsecond.com>  wrote:
>>
>>      
>>> For Arduino and other less fortunate uC you can always use external chips
>>> to obtain optimal and jitter-free charge/discharge timing. I'm not that
>>> familiar with Atmel chips; could capture/compare be used instead of
>>> interrupts somehow?
>>>
>>>        
>> One should investigate the Propeller.
>>
>> --
>> Brian Lloyd, WB6RQN/J79BPL
>> 706 Flightline Drive
>> Spring Branch, TX 78070
>> brian at lloyd.com
>> +1.916.877.5067
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>>      
>
>
>