[time-nuts] Another "atomic" clock question

Jim Lux jimlux at earthlink.net
Fri Mar 7 15:47:24 EST 2014

On 3/7/14 12:31 PM, Chris Albertson wrote:
> On Fri, Mar 7, 2014 at 10:28 AM, Lars Walenius
> <lars.walenius at hotmail.com> wrote:
>> Chris, about using one Arduino for two GPSDO controllers:
>> Even if a microcontroller has lots of capacity I would recommend to use separate controllers for each oscillator. One of the reasons is what Tom van Baak said about using only one interrupt to avoid jitter and even if you trigger both channels from the same PPS and have just one interrupt you will have a problem that you can´t read two ADC´s at the same time.
> You don't have to read both at the same time.  All you need is to have
> a constant time between the interrupt and when you read the ADC. That
> constant can be any reasonable number so long as it remains constant

there are plenty of Arduino-like boards out there that have ADCs 
triggered by the timer, which also fires the interrupt, but you don't 
have to worry about reading the ADC late.

The teensy3.1 (new version of the teensy3 from PJRC) has a dual ADC, 
which can simulataneously sample.  I've run the teensy3 at 300ksps+ (48 
MHz processor clock).  Right now, I've got software that is interrupt 
driven at 50 kHz that does two adc reads in a row and then feeds a 2 
stage CIC decimator chain.  It consumes about 60% of the processor, the 
bulk of which is the actual ADC read and the first integrators.

for <$20, it's hard to beat.. the only downside is that you can't go 
down to radio shack and buy one on the spur of the moment.

They'll also do a not very optimized fixed point 128 point FFT in about 
0.9 milliseconds:
N pts	μs
128	897
64	402
32	175
16	82

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