Am Sonntag, 5. Juni 2016 13:45:59 UTC+2 schrieb Michal Suchanek:
> There is a comment in the SPI driver regarding CS polarity which boils
> down to 'changing CS polarity affects all CS lines'.
>
This may be the reason that I could see activities on both CS lines.
> done with an IOCTL. Even if you send a few bytes it might be
> challenging to measure the level change unless you are using an
> oscilloscope.
>
I have checked this with an oscilloscope (a cheap logic analyser is ordered
so that I could attach a better picture of the output of all pins hopefully
next week). I have written a small test program (attached to this message)
which sends repeatedly 4 bytes over the SPI bus using the ioctl. But on my
oscilloscope I could see only activities on the CS lines, but no change of
the CLK and MOSI outputs.
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#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>
int main(int argc, char *argv[]) {
struct spi_ioc_transfer xfer[2];
char buf[] = {0x11, 0x22, 0x44, 0x88};
char rx_buf[32];
int i;
int mode = 0;
__u32 speed=500;
int file = open("/dev/spidev0.0", O_RDWR);
if (file < 0)
{
perror("Failed to open the bus.");
exit(1);
}
if (ioctl(file, SPI_IOC_WR_MODE, &mode)<0) {
perror("can't set spi mode");
exit(1);
}
if (ioctl(file, SPI_IOC_RD_MODE, &mode) < 0)
{
perror("SPI rd_mode");
exit(1);
}
if (ioctl(file, SPI_IOC_RD_MAX_SPEED_HZ, &speed) < 0) {
perror("SPI max_speed_hz");
exit(1);
}
for (i = 0; i < 500000; i++) {
xfer[0].tx_buf = (unsigned long) buf;
xfer[0].len = 4; /* Length of command to write*/
xfer[0].cs_change = 0; /* Keep CS activated */
xfer[0].delay_usecs = 0, //delay in us
xfer[0].speed_hz = speed, //speed
xfer[0].bits_per_word = 8, // bites per word 8
xfer[1].rx_buf = (unsigned long) rx_buf;
xfer[1].len = 4; /* Length of Data to read */
xfer[1].cs_change = 0; /* Keep CS activated */
xfer[0].delay_usecs = 0;
xfer[0].speed_hz = speed;
xfer[0].bits_per_word = 8;
if (ioctl(file, SPI_IOC_MESSAGE(1), xfer) < 0) {
perror("SPI_IOC_MESSAGE");
exit(1);
}
usleep(1);
}
}
