Another variation is to use a single 125 style buffer device (eg
74LVC1G125) to charge and discharge a capacitor (in reality an RC
network when the ADC input is taken into account) via a series resistor.
The input to the buffer is driven by the input to a conventional
synchroniser whilst the buffer output enable is driven by the
synchroniser output.
The buffer output being enabled whilst the synchroniser output is low
(for a 0-> 1 PPS input transition) and disabled whenever the
synchroniser output is high.
This ensures that the capacitor network is discharged to zero between
PPS events without requiring an additional device (or a resistor) to
discharge the RC network.
The capcitor reset level sensitivity to leakage currents is greatly
reduced over that when a 1M discharge resistor is used.
The nonlinearity can be calibrated by using a statistical fill the
buckets technique.
This requires a relatively noisy test signal generator (RC oscillator??)
to drive the synchroniser input.
However its essential to ensure that this oscillator isnt injection
locked to the synchroniser clock.
Bruce
Lars Walenius wrote:
Hi Bruce
You are absolute right that it is wise to put some time in the estimation of
such effects as asynchronous Clocks. An iteration between thinking and
building seems always to be necessary but we all have different capabilities
for that. For the Arduino I came to an end with the interrupts as I am not good
at uP´s.
The Arduino GPSDO has two interrupts. One is synchronous with the 10MHz and
comes from timer1 overflows. The other is synchronous with the 1PPS. So it is
three asynchronous clocks right now in the GPSDO controller.
As I understand my problem it is that an interrupt takes some time to execute
and if you get the two interrupts to close you will have a problem with timing
as you can´t execute both at the same time?
Of course the easy solution could be to have the needed resoulution higher than
the time it takes to execute the interrupts but in the GPSDO I want a
resolution of 200ns (5MHz Clock) and the shortest interrupt is 3us.
I would be glad if somebody (Chris?) could have a look in the Aduino GPSDO code
to see if it possible to get rid of the uncertainty due to the interrupts from
the timer1 overflow.
Another question: Does a PIC not need overflow interrupts to count say 5000000
counts as I do in the Arduino?
Lars
From: Bruce Griffiths
Sent: söndag den 16 februari 2014 20:14
To: [email protected]
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<[email protected]> wrote:
On Sat, Feb 15, 2014 at 7:10 PM, Tom Van Baak<[email protected]> 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
[email protected]
+1.916.877.5067
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