Max SPI transfers / sec speed using WiringPi

Question

I'm trying to interface with MCP3002 ADC using SPI and it's working nicely except for the amount of the samples I'm able to collect per second. I'm using WiringPi and the code is very simple:

wiringPiSPISetup(0, 2 * 1000 * 1000);

unsigned char buf[2];

for (;;) {
  buf[0] = 104; // x start_bit single_mode ch1 msbf xxx
  buf[1] = 0;

  wiringPiSPIDataRW(0, buf, 2);

  // convert buf to an int and print to stdout
}

The problem is that regardless of the clock speed set, the wiringPiSPIDataRW call takes about 70us which limits the speed of the whole system to 14k samples / sec.

Has anyone had better luck with SPI on Raspberry Pi with very short messages?

The MCP3002 can conservatively handle 80k samples at 3.3V which then assuming some extra clock cycles for asserting the CS line should run at 1.6Mhz clock and perform a RW call in 12.5 usec. Is then the speed limited by WiringPi overhead?

EDIT: I tried using spidev directly using SPI_IOC_MESSAGE and the speed is the same so it's not WiringPi. I forgot to mention I'm using a slightly older Raspberry board, v1 rev.B if I'm not mistaken. lsmod shows that spi_bcm2835 is loaded, so that should be fine. Using SPI_IOC_MESSAGE with an array of 128 messages gets the speed up to 45k samples / sec, but that's about as high as it will go.

I appreciate any pointers. Thanks.

Answer 1

My pigpio and the bcm2835 library will both be faster as they don't use the Linux SPI driver.

I think the Linux SPI driver tops out at about 20k calls per second on all but the Pi3 where it reaches about 70k calls per second. I don't understand why the Pi3 seems so much better.

pigpio

#include <stdio.h>
#include <stdlib.h>

#include <pigpio.h>

/*
   gcc -pthread -o spi-speed spi-speed.c -lpigpio
*/

int main(int argc, char *argv[])
{
   int i;
   int h;
   int loops;
   int speed;
   double start, diff, sps;
   unsigned char buf[2];

   if (argc > 1) loops = atoi(argv[1]);
   else loops = 1000000;

   if (argc > 2) speed = atoi(argv[2]);
   else speed = 1000000;

   if (gpioInitialise() < 0) return 1;

   h = spiOpen(0, speed, 0);

   if (h < 0) return 2;

   start = time_time();

   for (i=0; i<loops; i++)
   {
      buf[0] = 104; // x start_bit single_mode ch1 msbf xxx
      buf[1] = 0;

      spiXfer(h, buf, buf, 2);
   }

   diff = time_time() - start;

   printf("sps=%.1f @ %d bps (%d/%.1f)\n",
      (double)loops / diff, speed, loops, diff);

   spiClose(h);

   gpioTerminate();

   return 0;
}

pigpio results at various SPI bit rates (run on a Pi B+).

$ sudo ./spi-speed  1000000 500000
sps=24184.0 @ 500000 bps (1000000/41.3)

$ sudo ./spi-speed  1000000 1000000
sps=47018.5 @ 1000000 bps (1000000/21.3)

$ sudo ./spi-speed  1000000 2000000
sps=89840.5 @ 2000000 bps (1000000/11.1)

$ sudo ./spi-speed  1000000 3000000
sps=128846.8 @ 3000000 bps (1000000/7.8)

$ sudo ./spi-speed  1000000 4000000
sps=164745.8 @ 4000000 bps (1000000/6.1)