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joan2937 / pigpio

pigpio is a C library for the Raspberry which allows control of the General Purpose Input Outputs (GPIO).
The Unlicense
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Question example "SPI bit bang MCP3008" #300

Closed u0078867 closed 4 years ago

u0078867 commented 5 years ago

I am going to use this great library to read simultaneously from some MCP3008 ADCs. I don't have to do much more than what is already in the example "SPI bit bang MCP3008", so I was studying it in detail. My target acquisition frequency is 2ksps, so much below the nominal max 25ksps.

I have the following questions:

1) REPEAT_MICROS is the inverse of my target sampling frequency. But what is the BUFFER for? I don't understand its meaning from: We need to ensure that the buffer is big enough to cope with any reasonable rescehdule. In practice make the buffer as big as you can.

2) usleep(1000); this should wait for 1ms. But if the sampling frequency of the example is 25 ksps (#define REPEAT_MICROS 40), doesn't it create an excessive delay in each sample iteration?

3) Would you suggest to run the example on a isolated RPi core, perhaps even excluding it from the Linux scheduling?

Thanks in advance!!!

joan2937 commented 5 years ago

The example uses DMA to transfer the SPI data. In effect this isolates it from anything happening in Linux (which is why the samples are read a accurately timed intervals).

DMA writes the samples to a cyclic buffer. As long as you empty the buffer faster than it is filled you will get good data.

Using this technique there is no advantage in isolating a core.

u0078867 commented 5 years ago

Thanks, this helped me getting the essential parts of your code.