accurate sampling in c++

Question

I want to sample values I get from a gpio 4000 times per second, currently I do something like that:

std::vector sample_a_chunk(unsigned int rate         


        

Answer 1

I think the best you can probably achieve is to use absolute timing so as to avoid drift.

Something like this:

std::vector<int> sample_a_chunk(unsigned int rate,
    unsigned int block_size_in_seconds)
{
    using clock = std::chrono::steady_clock;

    std::vector<int> data;

    const auto times = rate * block_size_in_seconds;
    const auto delay = std::chrono::microseconds{1000000 / rate};

    auto next_sample = clock::now() + delay;

    for(int j = 0; j < times; j++)
    {
        data.emplace_back(/* read the value from the gpio */);

        std::this_thread::sleep_until(next_sample);

        next_sample += delay; // don't refer back to clock, stay absolute
    }
    return data;
}

Answer 2

I would use boost::asio::deadline_timer.

#include <vector>

#define BOOST_ERROR_CODE_HEADER_ONLY 1
#include <boost/asio.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>

std::vector<int> sample_a_chunk(unsigned int rate, unsigned int block_size_in_seconds) {
  std::vector<int> data;
  const unsigned int times = rate * block_size_in_seconds;
  auto expiration_time = boost::posix_time::microsec_clock::local_time();
  const auto delay = boost::posix_time::microseconds(1000000/rate);
  boost::asio::io_service io;
  boost::asio::deadline_timer t(io);

  for (unsigned int j=0; j < times; j++) {
    expiration_time += delay;
    data.emplace_back(/* read the value from the gpio */);
    t.expires_at(expiration_time);
    t.wait();
  }
  return data;
}