Re: Embedded system control loops and reading from daisy-chained ADCs

[Nicholas Kinar <n.kinar-/KKvz3x1pcI@public.gmane.org>]

> That would help, but I measured interrupt latencies of 100-600usec
> (200us typical) on the PXA255, with occasional longer delays.  That
> was with the network driver busy also.
Thanks for confirming this, Ned.

>> If I have
>> the trigger pin of the daisy-chained ADCs tied to one of the ARM
>> processor's pulse width modulator (PWM) pins, might I be able to trigger
>> a series of samples with PWM, and then read the data over SPI?  I think
>> that a timer would be required, but how would I set up an interrupt to
>> read from SPI once the conversion has been triggered?  I would also have
>> to wait a predetermined time after triggering the conversion to be able
>> to read data from the ADCs.
> If the timer controls the period and pulse width of the trigger, you
> program the trailing edge for the correct acquisition delay, and use
> the trailing edge to generate the interrupt for the driver.
> Alternatively, you could interrupt on the leading edge and then loop
> until the trailing edge (reading a GPIO pin); that would help overlap
> some of the interrupt latency.  This latter idea is only acceptable if
> the acquisition time is short, or if the processor is not busy and
> thus you don't mind generating increased latency for other interrupts
> (normally not a good idea).
Okay, I think that I understand.

> It's been a while since I worked on my driver.  I recall that its
> maximum word size was 32-bits.  I don't recall if there is anything in
> the SPI core that limits that, or if it is simply a driver limit.  If
> the latter, you can write a driver to do anything you want.
I'm not familiar with the SPI core either, and I would also wonder if 
the word size is limited for the ARM architecture.

>
> It sounds like you have a lot of options. Good luck.
>

Yes - I am designing, building and testing an embedded system for my own 
research.  Once again, thank you for your help!

Nicholas




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