enter image description hereI'm trying to get started interfacing my Pi 3 (running Stretch) with an ADC via SPI, but all I'm ever reading from SPI are zeros, including when I wire GPIO pin 19 to wire 21 for a loopback test (and keep all other pins disconnected, so the ADC obviously isn't a factor).

I enabled SPI via the Rasperry Pi Configuration tool (and rebooted for good measure); lsmod now shows spidev and spi_bcm2835, and in /dev/ I have /dev/spidev0.0 and /dev/spidev0.1.

In order to make sure I had the right connections (all the way from the GPIO pins through the breakout kit to the breadboard), I've used the filesystem interface in /sys/class/gpio to successfully set "pin 19" and "pin 21" on the breadboard to both low and high according to my multimeter (and then unexported them before testing SPI again just to be safe).

I've tried two different test programs, both of which fail the loopback test by returning all zeros. One such test program is the one linked to on https://importgeek.wordpress.com/2017/09/11/raspberry-pi-spi-loopback-testing/ (namely, the actual code is at https://raw.githubusercontent.com/raspberrypi/linux/rpi-3.10.y/Documentation/spi/spidev_test.c), which I've compiled and run with ./spidev_test -D /dev/spidev0.0 as suggested. Mucking with the speed (say, lowering it to 50kHz) or adding a delay seems to have no effect.

Any ideas what might be going wrong, or how to further debug this?

Here's the test code:

 * SPI testing utility (using spidev driver)
 * Copyright (c) 2007  MontaVista Software, Inc.
 * Copyright (c) 2007  Anton Vorontsov <avorontsov@ru.mvista.com>
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License.
 * Cross-compile with cross-gcc -I/path/to/cross-kernel/include

#include <stdint.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/types.h>
#include <linux/spi/spidev.h>

#define ARRAY_SIZE(a) (sizeof(a) / sizeof((a)[0]))

static void pabort(const char *s)

static const char *device = "/dev/spidev1.1";
static uint8_t mode;
static uint8_t bits = 8;
static uint32_t speed = 500000;
static uint16_t delay;

static void transfer(int fd)
    int ret;
    uint8_t tx[] = {
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0x40, 0x00, 0x00, 0x00, 0x00, 0x95,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xDE, 0xAD, 0xBE, 0xEF, 0xBA, 0xAD,
        0xF0, 0x0D,
    uint8_t rx[ARRAY_SIZE(tx)] = {0, };
    struct spi_ioc_transfer tr = {
        .tx_buf = (unsigned long)tx,
        .rx_buf = (unsigned long)rx,
        .len = ARRAY_SIZE(tx),
        .delay_usecs = delay,
        .speed_hz = speed,
        .bits_per_word = bits,

    ret = ioctl(fd, SPI_IOC_MESSAGE(1), &tr);
    if (ret < 1)
        pabort("can't send spi message");

    for (ret = 0; ret < ARRAY_SIZE(tx); ret++) {
        if (!(ret % 6))
        printf("%.2X ", rx[ret]);

static void print_usage(const char *prog)
    printf("Usage: %s [-DsbdlHOLC3]\n", prog);
    puts("  -D --device   device to use (default /dev/spidev1.1)\n"
         "  -s --speed    max speed (Hz)\n"
         "  -d --delay    delay (usec)\n"
         "  -b --bpw      bits per word \n"
         "  -l --loop     loopback\n"
         "  -H --cpha     clock phase\n"
         "  -O --cpol     clock polarity\n"
         "  -L --lsb      least significant bit first\n"
         "  -C --cs-high  chip select active high\n"
         "  -3 --3wire    SI/SO signals shared\n");

static void parse_opts(int argc, char *argv[])
    while (1) {
        static const struct option lopts[] = {
            { "device",  1, 0, 'D' },
            { "speed",   1, 0, 's' },
            { "delay",   1, 0, 'd' },
            { "bpw",     1, 0, 'b' },
            { "loop",    0, 0, 'l' },
            { "cpha",    0, 0, 'H' },
            { "cpol",    0, 0, 'O' },
            { "lsb",     0, 0, 'L' },
            { "cs-high", 0, 0, 'C' },
            { "3wire",   0, 0, '3' },
            { "no-cs",   0, 0, 'N' },
            { "ready",   0, 0, 'R' },
            { NULL, 0, 0, 0 },
        int c;

        c = getopt_long(argc, argv, "D:s:d:b:lHOLC3NR", lopts, NULL);

        if (c == -1)

        switch (c) {
        case 'D':
            device = optarg;
        case 's':
            speed = atoi(optarg);
        case 'd':
            delay = atoi(optarg);
        case 'b':
            bits = atoi(optarg);
        case 'l':
            mode |= SPI_LOOP;
        case 'H':
            mode |= SPI_CPHA;
        case 'O':
            mode |= SPI_CPOL;
        case 'L':
            mode |= SPI_LSB_FIRST;
        case 'C':
            mode |= SPI_CS_HIGH;
        case '3':
            mode |= SPI_3WIRE;
        case 'N':
            mode |= SPI_NO_CS;
        case 'R':
            mode |= SPI_READY;

int main(int argc, char *argv[])
    int ret = 0;
    int fd;

    parse_opts(argc, argv);

    fd = open(device, O_RDWR);
    if (fd < 0)
        pabort("can't open device");

     * spi mode
    ret = ioctl(fd, SPI_IOC_WR_MODE, &mode);
    if (ret == -1)
        pabort("can't set spi mode");

    ret = ioctl(fd, SPI_IOC_RD_MODE, &mode);
    if (ret == -1)
        pabort("can't get spi mode");

     * bits per word
    ret = ioctl(fd, SPI_IOC_WR_BITS_PER_WORD, &bits);
    if (ret == -1)
        pabort("can't set bits per word");

    ret = ioctl(fd, SPI_IOC_RD_BITS_PER_WORD, &bits);
    if (ret == -1)
        pabort("can't get bits per word");

     * max speed hz
    ret = ioctl(fd, SPI_IOC_WR_MAX_SPEED_HZ, &speed);
    if (ret == -1)
        pabort("can't set max speed hz");

    ret = ioctl(fd, SPI_IOC_RD_MAX_SPEED_HZ, &speed);
    if (ret == -1)
        pabort("can't get max speed hz");

    printf("spi mode: %d\n", mode);
    printf("bits per word: %d\n", bits);
    printf("max speed: %d Hz (%d KHz)\n", speed, speed/1000);



    return ret;

EDIT: added a picture of the oh-so-complicated wiring.

EDIT: after poking at this with an oscilloscope, I'm pretty sure SPI is never writing any non-zero value to either pin 19 or 21, despite my being able to make those pins produce pulses I can see just fine via /sys/class/gpio. Guessing this is some sort of software configuration SNAFU...

  • Did you set the SPI GPIO back to SPI mode (or reboot) after setting them high and low? Add a listing of the code you used and details of the ADC and your wiring. – joan Jan 23 '18 at 10:18
  • Yes, I rebooted after setting the pins high and low...doesn't help. The ADC is irrelevant, because I took it out for loopback testing; literally the only wire now in the whole project (other than the breakout kit to the breadboard) is a short between pin 19 and pin 21 (or rather their corresponding rows in the breadboard). – Scott Jan 23 '18 at 16:25
  • I can also use the /sys/class/gpio interface to successfully read 0 or 1 values from pins 19 and 21 when they're wired to ground or the 3.3V power line, for what that's worth. – Scott Jan 23 '18 at 16:56
  • You probably need to zero the spi_ioc_transfer tr struct before setting the other values, i.e. try memset(&tr, 0, sizeof(tr));. – joan Jan 23 '18 at 17:44
  • 1
    Pins 19/21 are GPIO 10/9. See abyz.me.uk/rpi/pigpio/index.html#Type_3 – joan Jan 24 '18 at 9:37

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