Re: how to write and read back GPIO pin states with lgpio

On 04/08/2024 21:13, Josef Möllers wrote:

Hi,
 I used to use the sysfs interface to the GPIO pins (/sys/class/gpio) but I understand that is deprecated nowadays. So I tried to switch to lgpio which looks OK. However, I have problems writing and reading back pin states from different programs.
 My setup is as follows:
I have a couple of relays (solid state and mechanical ones) that control various external devices.
I use one program to switch devices on and off and want to use another program to read back the state of the device.
 Doing that with sysfs is easy:
1) export the pin:
       echo $pin > /sys/class/gpio/export
       echo $direction > /sys/class/gpio/gpio$pin/direction
    this needs to be done only once.
2) write the state of the pin, thus switching the device on/off:
       echo $newstate > /sys/class/gpio/gpio$pin/value
    this is done every time this is required
3) read back the state of the pin
       value=$(</sys/class/gpio/gpio$pin/value
    this is done every time I want to check the state of the device
 Now I switch a device on/off with lgpio as follows:
1) open the GPIO chip:
       h = lgGpiochipOpen(0);
2) claim the pin as an output:
       lgGpioClaimOutput(h, LG_SET_PULL_NONE, pin, value);
    which, to my understanding, already changes the pin's state?!?
3) write the new state
       lgGpioWrite(h, pin, value);
4) close the chip
       lgGpiochipClose(h);
 Reading back the state of the pin requires me to
1) open the GPIO chip:
       h = lgGpiochipOpen(0);
2) claim the pin as an input:
       lgGpioClaimInput(h, LG_SET_PULL_NONE, pin);
3) read back the pin's state
       lgGpioRead(h, pin);
4) close the chip
       lgGpiochipClose(h);
 However ... When I set the pin's state to "1", I still read back "0"!
 What am I doing wrong? Thanks in advance for any pointers.
 Josef

TLDR
Here is my C code to drive 4 relays from a Pi Zero W
Adpapted frim someone elses that also works
It works:
/* relays.h
2023-04-30
Public Domain */
/* zone 1-4, command ON or OFF */
/* All clever stuff is done in relayio, any call to this
  *  will initialise the hardware if it needs it. */
void relay(int zone, int command);
#define RELAY1 6
#define RELAY2 13
#define RELAY3 19
#define RELAY4 26
---------------------------------------------------------------------------------------
/*
    relayio.c
    2023-04-30
    Public Domain
*/
#include <stdio.h>
#include <unistd.h>
#include <stdint.h>
#include <string.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include "relays.h"
#define GPSET0 7
#define GPSET1 8
#define GPCLR0 10
#define GPCLR1 11
#define GPLEV0 13
#define GPLEV1 14
#define GPPUD     37
#define GPPUDCLK0 38
#define GPPUDCLK1 39
// port numbers for relays
unsigned piModel;
unsigned piRev;
static volatile uint32_t  *gpioReg = MAP_FAILED;
#define PI_BANK (gpio>>5)
#define PI_BIT  (1<<(gpio&0x1F))
/* gpio modes. */
#define PI_INPUT  0
#define PI_OUTPUT 1
#define PI_ALT0   4
#define PI_ALT1   5
#define PI_ALT2   6
#define PI_ALT3   7
#define PI_ALT4   3
#define PI_ALT5   2
static int initialized=0;
void gpioSetMode(unsigned gpio, unsigned mode)
{
int reg, shift;
reg=gpio/10;
shift=(gpio%10) * 3;
gpioReg[reg] = (gpioReg[reg] & ~(7<<shift)) | (mode<<shift);
}
int gpioGetMode(unsigned gpio)
{
int reg, shift;
reg   =  gpio/10;
shift = (gpio%10) * 3;
return (*(gpioReg + reg) >> shift) & 7;
}
/* Values for pull-ups/downs off, pull-down and pull-up. */
#define PI_PUD_OFF  0
#define PI_PUD_DOWN 1
#define PI_PUD_UP   2
void gpioSetPullUpDown(unsigned gpio, unsigned pud)
{
*(gpioReg + GPPUD) = pud;
usleep(20);
*(gpioReg + GPPUDCLK0 + PI_BANK) = PI_BIT;
usleep(20);
*(gpioReg + GPPUD) = 0;
*(gpioReg + GPPUDCLK0 + PI_BANK) = 0;
}
int gpioRead(unsigned gpio)
{
if ((*(gpioReg + GPLEV0 + PI_BANK) & PI_BIT) != 0)
return 1;
else
return 0;
}
void gpioWrite(unsigned gpio, unsigned level)
{
if (level == 0)
*(gpioReg + GPCLR0 + PI_BANK) = PI_BIT;
else
*(gpioReg + GPSET0 + PI_BANK) = PI_BIT;
}
void gpioTrigger(unsigned gpio, unsigned pulseLen, unsigned level)
{
if (level == 0)
*(gpioReg + GPCLR0 + PI_BANK) = PI_BIT;
else
*(gpioReg + GPSET0 + PI_BANK) = PI_BIT;
usleep(pulseLen);
if (level != 0) *(gpioReg + GPCLR0 + PI_BANK) = PI_BIT;
else
*(gpioReg + GPSET0 + PI_BANK) = PI_BIT;
}
/* Bit (1<<x) will be set if gpio x is high. */
uint32_t gpioReadBank1(void) { return (*(gpioReg + GPLEV0)); }
uint32_t gpioReadBank2(void) { return (*(gpioReg + GPLEV1)); }
/* To clear gpio x bit or in (1<<x). */
void gpioClearBank1(uint32_t bits) { *(gpioReg + GPCLR0) = bits; }
void gpioClearBank2(uint32_t bits) { *(gpioReg + GPCLR1) = bits; }
/* To set gpio x bit or in (1<<x). */
void gpioSetBank1(uint32_t bits) { *(gpioReg + GPSET0) = bits; }
void gpioSetBank2(uint32_t bits) { *(gpioReg + GPSET1) = bits; }
unsigned gpioHardwareRevision(void)
{
static unsigned rev = 0;
FILE * filp;
char buf[512];
char term;
int chars=4; /* number of chars in revision string */
if (rev)
return rev;
piModel = 0;
filp = fopen ("/proc/cpuinfo", "r");
if (filp != NULL)
{
while (fgets(buf, sizeof(buf), filp) != NULL)
{
if (piModel == 0)
{
if (!strncasecmp("model name", buf, 10))
{
if (strstr (buf, "ARMv6") != NULL)
{
piModel = 1;
chars = 4;
}
else if (strstr (buf, "ARMv7") != NULL)
{
piModel = 2;
chars = 6;
}
                else if (strstr (buf, "ARMv8") != NULL)
{
piModel = 2;
chars = 6;
}
}
}
          if (!strncasecmp("revision", buf, 8))
{
if (sscanf(buf+strlen(buf)-(chars+1),
                "%x%c", &rev, &term) == 2)
{
if (term != '\n') rev = 0;
}
}
}
fclose(filp);
}
return rev;
}
int gpioInitialise(void)
{
int fd;
piRev = gpioHardwareRevision(); /* sets piModel and piRev */
fd = open("/dev/gpiomem", O_RDWR | O_SYNC) ;
if (fd < 0)
{
fprintf(stderr, "failed to open /dev/gpiomem\n");
return -1;
}
gpioReg = (uint32_t *)mmap(NULL, 0xB4, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
close(fd);
if (gpioReg == MAP_FAILED)
{
fprintf(stderr, "Bad, mmap failed\n");
return -1;
}
return 0;
}
void initializeRelays()
{
if (gpioInitialise() < 0)
return;
gpioSetMode(RELAY1,PI_OUTPUT);
gpioSetMode(RELAY2,PI_OUTPUT);
gpioSetMode(RELAY3,PI_OUTPUT);
gpioSetMode(RELAY4,PI_OUTPUT);
initialized = 1;
}

void relay(int zone, int command)
{
int port;
if(!initialized)
initializeRelays();
switch (zone)
{
case 1: port=RELAY1;
break;
case 2: port=RELAY2;
break;
case 3: port=RELAY3;
break;
case 4: port=RELAY4;
break;
default: return;
}
gpioWrite(port, command);
}
--
"The most difficult subjects can be explained to the most slow witted man if he has not formed any idea of them already; but the simplest thing cannot be made clear to the most intelligent man if he is firmly persuaded that he knows already, without a shadow of doubt, what is laid before him."
    - Leo Tolstoy