I'm using a Rapsberry Pi Zero 2 to control an FPGA, the Lattice iCE40, via SPI (which I have accordingly enabled via raspi-config
). In order to upload the image on the FPGA, I need to follow the procedure shown at page 30 of the technical manual of the board. As shown in Figure 13.2, that I pasted below, this means that I need to bring the SPI_SS
signal low, then high, then low again. This signal corresponds to the slave select pin, which on RPi is indicated as SPI_CE0_N
(if one uses the /dev/spidev0.0 SPI device, as I am currently doing) and corresponds to GPIO8
.
I tried to do this by treating the SPI_CE0_N
as a GPIO. Therefore, I thus tried to control the pin with the approach indicated here by simply copying and pasting the blink.c
file, selecting the GPIO8
pin and making some other minor modifications:
/* blink.c
*
* Raspberry Pi GPIO example using sysfs interface.
* Guillermo A. Amaral B. <g@maral.me>
*
* This file blinks GPIO 4 (P1-07) while reading GPIO 24 (P1_18).
*/
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define IN 0
#define OUT 1
#define LOW 0
#define HIGH 1
#define PIN 24 /* P1-18 */
#define POUT 7 /* P1-24 */
static int
GPIOExport(int pin)
{
#define BUFFER_MAX 3
char buffer[BUFFER_MAX];
ssize_t bytes_written;
int fd;
fd = open("/sys/class/gpio/export", O_WRONLY);
if (-1 == fd) {
fprintf(stderr, "Failed to open export for writing!\n");
return(-1);
}
bytes_written = snprintf(buffer, BUFFER_MAX, "%d", pin);
write(fd, buffer, bytes_written);
close(fd);
return(0);
}
static int
GPIOUnexport(int pin)
{
char buffer[BUFFER_MAX];
ssize_t bytes_written;
int fd;
fd = open("/sys/class/gpio/unexport", O_WRONLY);
if (-1 == fd) {
fprintf(stderr, "Failed to open unexport for writing!\n");
return(-1);
}
bytes_written = snprintf(buffer, BUFFER_MAX, "%d", pin);
write(fd, buffer, bytes_written);
close(fd);
return(0);
}
static int
GPIODirection(int pin, int dir)
{
static const char s_directions_str[] = "in\0out";
#define DIRECTION_MAX 35
char path[DIRECTION_MAX];
int fd;
snprintf(path, DIRECTION_MAX, "/sys/class/gpio/gpio%d/direction", pin);
fd = open(path, O_WRONLY);
if (-1 == fd) {
fprintf(stderr, "Failed to open gpio direction for writing!\n");
return(-1);
}
if (-1 == write(fd, &s_directions_str[IN == dir ? 0 : 3], IN == dir ? 2 : 3)) {
fprintf(stderr, "Failed to set direction!\n");
return(-1);
}
close(fd);
return(0);
}
static int
GPIORead(int pin)
{
#define VALUE_MAX 30
char path[VALUE_MAX];
char value_str[3];
int fd;
snprintf(path, VALUE_MAX, "/sys/class/gpio/gpio%d/value", pin);
fd = open(path, O_RDONLY);
if (-1 == fd) {
fprintf(stderr, "Failed to open gpio value for reading!\n");
return(-1);
}
if (-1 == read(fd, value_str, 3)) {
fprintf(stderr, "Failed to read value!\n");
return(-1);
}
close(fd);
return(atoi(value_str));
}
static int
GPIOWrite(int pin, int value)
{
static const char s_values_str[] = "01";
char path[VALUE_MAX];
int fd;
snprintf(path, VALUE_MAX, "/sys/class/gpio/gpio%d/value", pin);
fd = open(path, O_WRONLY);
if (-1 == fd) {
fprintf(stderr, "Failed to open gpio value for writing!\n");
return(-1);
}
if (1 != write(fd, &s_values_str[LOW == value ? 0 : 1], 1)) {
fprintf(stderr, "Failed to write value!\n");
return(-1);
}
close(fd);
return(0);
}
int
main(int argc, char *argv[])
{
int repeat = 10;
/*
* Enable GPIO pins
*/
if (-1 == GPIOExport(POUT) || -1 == GPIOExport(PIN))
{
return(1);
}
/*
* Set GPIO directions
*/
if (-1 == GPIODirection(POUT, OUT) || -1 == GPIODirection(PIN, IN))
{
return(2);
}
do {
/*
* Write GPIO value
*/
if (-1 == GPIOWrite(POUT, HIGH))
{
return(3);
}
usleep(100);
if (-1 == GPIOWrite(POUT, LOW))
{
return(3);
}
usleep(100);
}
while (repeat--);
if (-1 == GPIOWrite(POUT, LOW))
{
return(3);
}
/*
* Disable GPIO pins
*/
if (-1 == GPIOUnexport(POUT) || -1 == GPIOUnexport(PIN))
{
return(4);
}
return(0);
}
However, when I try to compile and run this script with g++
, I get the error "Failed to open gpio direction for writing!"
, which means that fd = open(path, O_WRONLY);
didn't work. I tried to run the same script with other GPIOs, and this happens only with the two SPI select ones, i.e. SPI_CE0_N
and SPI_CE1_N
(and not, for example, with the SPI_MOSI
(GPIO10
) one).
Why does this happen? Is there a way to solve this? The only one that comes to my mind is to use another GPIO pin as SPI_SS
, but I guess that that would make the process slower, as the signal wouldn't be synchronized with the clock anymore.