read()/ioctl disturbs GPIO signal? - c

Question

I connect a Linux embedded board(based on imx233) and a MSP430 MCU. They are connected via 4 pin SPI, but I use a GPIO for the chip select purpose on the Linux board. What I do is to use poll to detect falling edge of the GPIO(nr 52) then perform SPI reading either ioctl or read()
int main(void)
{
/********************************LINUX SCHEDULING**********************************/
sp.sched_priority = sched_get_priority_max(SCHED_FIFO); //scheduling
sched_setscheduler(0, SCHED_FIFO, &sp); //scheduling
/********************************LINUX SCHEDULING_END******************************/
struct pollfd fdset[2]; //declare the poll to be used in interrupt catching
int nfds = 2;
int gpio_fd, timeout, rc;
char *buf[MAX_BUF]; //max=64byte
int len;
initialize(); //gpio's are set to SPI_SLAVE
// spi_init();
gpio_fd = gpio_fd_open(CHIP_SELECT_PIN); //the CS(SS) pin is opened
timeout = POLL_TIMEOUT; //timeout 3 sec is set
// uint8_t voidFirstDetection = 1;
while (1) {
memset((void*)fdset, 0, sizeof(fdset));
fdset[0].fd = NULL;
fdset[0].events = POLLIN;
fdset[1].fd = gpio_fd;
fdset[1].events = POLLPRI;
/*** POLL starts to detect chipselects****/
rc = poll(fdset, nfds, timeout);
if (rc < 0) {
printf("\npoll() failed!\n");
return -1;
}
if (rc == 0) {
printf(".");
}
if (fdset[1].revents & POLLPRI ) { //HERE I need to run SPI_read
len = read(fdset[1].fd, buf, MAX_BUF);
/* if(voidFirstDetection){
voidFirstDetection = 0;
}else{*/
printf("\npoll() GPIO %d interrupt occurred\n", CHIP_SELECT_PIN);
int fd = open(device, O_RDWR);
if (fd < 0){
// snprintf(systemlogmsg, sizeof(systemlogmsg), "[1181]: errno:%s Cannot open /dev/spidev ", strerror(errno));
// error_logging(systemlogmsg, LOGLEVEL_ERROR);
printf("error spi recive\n");
}
//spi_transfer(fd);
do_read(fd);
close(fd);
// }
}
}
gpio_fd_close(gpio_fd);
return 0;
}
Above code works fine that it generates an interrupt only at the falling edge of the signal. I use the either of the below code when the interrupt is detected to read the /dev/spidev1-0
static void do_read(int fd)
{
unsigned char buf[1], *bp;
int status;
int len = 1;
/* read at least 2 bytes, no more than 32 */
memset(buf, 0, sizeof buf);
status = read(fd, buf, len);
if (status < 0) {
perror("read");
return;
}
if (status != len) {
fprintf(stderr, "short read\n");
return;
}
printf("read(%2d, %2d): %02x %02x,", len, status,
buf[0], buf[1]);
status -= 2;
bp = buf + 2;
while (status-- > 0)
printf(" %02x", *bp++);
printf("\n");
}
static void spi_transfer(int fd)
{
int ret;
uint8_t tx[2];
uint8_t rx[3] = {0 };
struct spi_ioc_transfer tr = {
.tx_buf = 0,
.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){
printf("can't send spi message");
exit(1);
}
for (ret = 0; ret < ARRAY_SIZE(tx); ret++) {
if (!(ret % 6))
puts("");
printf("%.2X ", rx[ret]);
}
puts("");
}
Whenever the either ret = ioctl(fd, SPI_IOC_MESSAGE(1), &tr); line on spi_transfer() or status = read(fd, buf, len); on do_read() is executed, I see an infinite loop that detects an interrupt on the GPIO52 (chipselect). I try the observe the GPIO via oscilloscope but I could not see any signal change (it might be a spike that my oscilloscope cannot detect), however, when I connect the chipselect to the Vcc, it does not get the infinite loop. As I am on the early stage, I set one of GPIO of the MCU as an output and a constant logic high. I use GPIO52 (Chip select) as an input because my aim is to transfer data from MCU to the linux board.
I guess, the read() and ioctl somehow effects the GPIO to sink more current than the GPIO can provide. If it is the problem, what can I do that ioctl or read() would not disturb GPIO. Or do you think something else could be a problem?

Answer 1

I was lucky that I found the problem quick. I tied the grounds of both boards and now it works fine. I will keep the post as someone else might have the same problem. But I am still curious how ioctl or read disturbs the GPIO signal level