How are pins defined for sensors in embedded C? - c

Question

I have a Nordic nRF52840DK board that needs to be connected to an LSM6DSL accelerometer. I am using the Zephyr RTOS and the sample code from Zephyr is shown below:
/*
* Copyright (c) 2018 STMicroelectronics
*
* SPDX-License-Identifier: Apache-2.0
*/
#include <zephyr.h>
#include <device.h>
#include <drivers/sensor.h>
#include <stdio.h>
#include <sys/util.h>
static inline float out_ev(struct sensor_value *val)
{
return (val->val1 + (float)val->val2 / 1000000);
}
static int print_samples;
static int lsm6dsl_trig_cnt;
static struct sensor_value accel_x_out, accel_y_out, accel_z_out;
static struct sensor_value gyro_x_out, gyro_y_out, gyro_z_out;
#if defined(CONFIG_LSM6DSL_EXT0_LIS2MDL)
static struct sensor_value magn_x_out, magn_y_out, magn_z_out;
#endif
#if defined(CONFIG_LSM6DSL_EXT0_LPS22HB)
static struct sensor_value press_out, temp_out;
#endif
#ifdef CONFIG_LSM6DSL_TRIGGER
static void lsm6dsl_trigger_handler(const struct device *dev,
struct sensor_trigger *trig)
{
static struct sensor_value accel_x, accel_y, accel_z;
static struct sensor_value gyro_x, gyro_y, gyro_z;
#if defined(CONFIG_LSM6DSL_EXT0_LIS2MDL)
static struct sensor_value magn_x, magn_y, magn_z;
#endif
#if defined(CONFIG_LSM6DSL_EXT0_LPS22HB)
static struct sensor_value press, temp;
#endif
lsm6dsl_trig_cnt++;
sensor_sample_fetch_chan(dev, SENSOR_CHAN_ACCEL_XYZ);
sensor_channel_get(dev, SENSOR_CHAN_ACCEL_X, &accel_x);
sensor_channel_get(dev, SENSOR_CHAN_ACCEL_Y, &accel_y);
sensor_channel_get(dev, SENSOR_CHAN_ACCEL_Z, &accel_z);
/* lsm6dsl gyro */
sensor_sample_fetch_chan(dev, SENSOR_CHAN_GYRO_XYZ);
sensor_channel_get(dev, SENSOR_CHAN_GYRO_X, &gyro_x);
sensor_channel_get(dev, SENSOR_CHAN_GYRO_Y, &gyro_y);
sensor_channel_get(dev, SENSOR_CHAN_GYRO_Z, &gyro_z);
#if defined(CONFIG_LSM6DSL_EXT0_LIS2MDL)
/* lsm6dsl external magn */
sensor_sample_fetch_chan(dev, SENSOR_CHAN_MAGN_XYZ);
sensor_channel_get(dev, SENSOR_CHAN_MAGN_X, &magn_x);
sensor_channel_get(dev, SENSOR_CHAN_MAGN_Y, &magn_y);
sensor_channel_get(dev, SENSOR_CHAN_MAGN_Z, &magn_z);
#endif
#if defined(CONFIG_LSM6DSL_EXT0_LPS22HB)
/* lsm6dsl external press/temp */
sensor_sample_fetch_chan(dev, SENSOR_CHAN_PRESS);
sensor_channel_get(dev, SENSOR_CHAN_PRESS, &press);
sensor_sample_fetch_chan(dev, SENSOR_CHAN_AMBIENT_TEMP);
sensor_channel_get(dev, SENSOR_CHAN_AMBIENT_TEMP, &temp);
#endif
if (print_samples) {
print_samples = 0;
accel_x_out = accel_x;
accel_y_out = accel_y;
accel_z_out = accel_z;
gyro_x_out = gyro_x;
gyro_y_out = gyro_y;
gyro_z_out = gyro_z;
#if defined(CONFIG_LSM6DSL_EXT0_LIS2MDL)
magn_x_out = magn_x;
magn_y_out = magn_y;
magn_z_out = magn_z;
#endif
#if defined(CONFIG_LSM6DSL_EXT0_LPS22HB)
press_out = press;
temp_out = temp;
#endif
}
}
#endif
void main(void)
{
int cnt = 0;
char out_str[64];
struct sensor_value odr_attr;
const struct device *lsm6dsl_dev = device_get_binding(DT_LABEL(DT_INST(0, st_lsm6dsl)));
if (lsm6dsl_dev == NULL) {
printk("Could not get LSM6DSL device\n");
return;
}
/* set accel/gyro sampling frequency to 104 Hz */
odr_attr.val1 = 104;
odr_attr.val2 = 0;
if (sensor_attr_set(lsm6dsl_dev, SENSOR_CHAN_ACCEL_XYZ,
SENSOR_ATTR_SAMPLING_FREQUENCY, &odr_attr) < 0) {
printk("Cannot set sampling frequency for accelerometer.\n");
return;
}
if (sensor_attr_set(lsm6dsl_dev, SENSOR_CHAN_GYRO_XYZ,
SENSOR_ATTR_SAMPLING_FREQUENCY, &odr_attr) < 0) {
printk("Cannot set sampling frequency for gyro.\n");
return;
}
#ifdef CONFIG_LSM6DSL_TRIGGER
struct sensor_trigger trig;
trig.type = SENSOR_TRIG_DATA_READY;
trig.chan = SENSOR_CHAN_ACCEL_XYZ;
if (sensor_trigger_set(lsm6dsl_dev, &trig, lsm6dsl_trigger_handler) != 0) {
printk("Could not set sensor type and channel\n");
return;
}
#endif
if (sensor_sample_fetch(lsm6dsl_dev) < 0) {
printk("Sensor sample update error\n");
return;
}
while (1) {
/* Erase previous */
printk("\0033\014");
printf("LSM6DSL sensor samples:\n\n");
/* lsm6dsl accel */
sprintf(out_str, "accel x:%f ms/2 y:%f ms/2 z:%f ms/2",
out_ev(&accel_x_out),
out_ev(&accel_y_out),
out_ev(&accel_z_out));
printk("%s\n", out_str);
/* lsm6dsl gyro */
sprintf(out_str, "gyro x:%f dps y:%f dps z:%f dps",
out_ev(&gyro_x_out),
out_ev(&gyro_y_out),
out_ev(&gyro_z_out));
printk("%s\n", out_str);
#if defined(CONFIG_LSM6DSL_EXT0_LIS2MDL)
/* lsm6dsl external magn */
sprintf(out_str, "magn x:%f gauss y:%f gauss z:%f gauss",
out_ev(&magn_x_out),
out_ev(&magn_y_out),
out_ev(&magn_z_out));
printk("%s\n", out_str);
#endif
#if defined(CONFIG_LSM6DSL_EXT0_LPS22HB)
/* lsm6dsl external press/temp */
sprintf(out_str, "press: %f kPa - temp: %f deg",
out_ev(&press_out), out_ev(&temp_out));
printk("%s\n", out_str);
#endif
printk("loop:%d trig_cnt:%d\n\n", ++cnt, lsm6dsl_trig_cnt);
print_samples = 1;
k_sleep(K_MSEC(2000));
}
}
From my previous experience working with Arduino, you need to define the pins before the main function, but there isn't any definition on pins in this code example. So my question is: How does the board know which pins to use when connected to the sensor?
Thanks in advance!

Answer 1

In embedded C (bare metal and some RTOS), the developer is expected to know what is connected where and which registers to use to access the connected hardware by driving the right pins.
In the specific case of Zephyr (as well as Das U-Boot and Linux on some architectures), the system configuration (among other things pins, but not only) is described by a device-tree. It is a "a data structure for describing hardware", which allows the firmware code to become independent of the actual hardware.
The device-tree will describe many things, the most important being:
the device's memory map
the list of CPUs and their properties (frequency, caches, ...)
the peripherals, as well as their configuration
Zephyr does have a very good introduction to device-tree.
In the code you posted, Zephyr does retrieve the device information from the device-tree when executing the following line:
const struct device *lsm6dsl_dev = device_get_binding(DT_LABEL(DT_INST(0, st_lsm6dsl)));