How to achieve stable sampling rate (1 kHz) when reading IMU data and sending over Serial on ESP32?
Posted: Thu Oct 02, 2025 7:45 pm
Hello everyone,
I am working with an ESP32 and an ICM-45686 IMU.
My goal is to acquire accelerometer and gyroscope data at 1.024 kHz and send it to a PC via serial. On the PC side, I have a Python script that receives the data and saves it.
I would like to evaluate whether the sampling rate of the ESP32 is actually stable. To check this, I compared the ESP32 data with a commercial data acquisition system, both synchronized by a trigger.
In the figure attached:
Black: measurement from the commercial system
Red: measurement from the ESP32
Blue: difference between the two signals
The signal is a measurement of a periodic movement of an object with 10 Hz of frequency.
We can observe that the difference (blue) is not just a fixed offset, but varies over time: the ESP32 signal starts slightly delayed and then seems to get ahead of the reference signal.
How should I interpret this behavior?
Does this variation in the blue residual indicate that the ESP32 sampling rate is not stable (jitter), or could there be another explanation?
I accept any suggestions.
THanks!
The esp32 code is below (using arduino IDE):
I am working with an ESP32 and an ICM-45686 IMU.
My goal is to acquire accelerometer and gyroscope data at 1.024 kHz and send it to a PC via serial. On the PC side, I have a Python script that receives the data and saves it.
I would like to evaluate whether the sampling rate of the ESP32 is actually stable. To check this, I compared the ESP32 data with a commercial data acquisition system, both synchronized by a trigger.
In the figure attached:
Black: measurement from the commercial system
Red: measurement from the ESP32
Blue: difference between the two signals
The signal is a measurement of a periodic movement of an object with 10 Hz of frequency.
We can observe that the difference (blue) is not just a fixed offset, but varies over time: the ESP32 signal starts slightly delayed and then seems to get ahead of the reference signal.
How should I interpret this behavior? Does this variation in the blue residual indicate that the ESP32 sampling rate is not stable (jitter), or could there be another explanation?I accept any suggestions.
THanks!
The esp32 code is below (using arduino IDE):
Code: Select all
/*******************************************************************************************
* ICM-45686 – 1 kHz Acquisition + Laser Trigger *
* *
* FEATURES: *
* • Reads accelerometer and gyroscope signals from the ICM-45686 IMU at 1 kHz *
* • Sends the data over serial in tabular format *
* • Activates the laser (GPIO 17) immediately after button detection (GPIO 33) *
* *
* Author: Lucian Ribeiro da Silva – Jun/2025 *
*******************************************************************************************/
#include <Arduino.h>
#include <SPI.h>
#include "ICM45686.h"
#include "soc/gpio_reg.h" // For direct register manipulation (if needed)
#include "driver/gpio.h"
#include "soc/io_mux_reg.h"
#include "soc/soc.h"
/* ─────────────────────────────────────
* Application pin definitions
* ───────────────────────────────────── */
constexpr int CS_PIN = 5; // IMU Chip Select on the SPI bus
constexpr int INT1_PIN = 4; // IMU interrupt pin (not used)
// constexpr int PINO_BOTAO = 17; // <-- REMOVED
constexpr int PINO_TRIGGER = 33; // Digital output to activate the laser
constexpr int PINO_TRIGGER_SCADAS = 27; // Digital trigger output for SCADAS
constexpr int DEBUG_PIN = 2; // On-board LED for debugging (optional)
constexpr int tempoHIGH = 15000;
constexpr int Fs_loop = 1024; // frequency of signal transmission via serial (not the sensor ODR, be careful)
int Ts = 0;
/* ─────────────────────────────────────
* Global variables
* ───────────────────────────────────── */
int32_t off_ax, off_ay, off_az; // Acceleration offsets
int32_t off_gx, off_gy, off_gz; // Gyroscope offsets
ICM456xx IMU(SPI, CS_PIN); // IMU object connected via SPI
volatile int triggerAtivado = 0; // <-- MODIFIED: renamed from 'botaoPressionado'
unsigned long tempoAcionamento = 0; // Moment when the button was pressed
char buffer[512]; // Buffer for serial output formatting
/* ─────────────────────────────────────
* Button interrupt // <-- REMOVED
* ───────────────────────────────────── */
// void handleBotao() {
// if (botaoPressionado == 0) { // Ensures only the first click is registered
// botaoPressionado = 1;
// tempoAcionamento = millis(); // Saves the activation timestamp
// }
// }
/* ─────────────────────────────────────
* System initial setup
* ───────────────────────────────────── */
void setup() {
Serial.begin(1500000);
while (!Serial) {}
// pinMode(PINO_BOTAO, INPUT_PULLDOWN); // <-- REMOVED
// attachInterrupt(digitalPinToInterrupt(PINO_BOTAO), handleBotao, RISING); // <-- REMOVED
pinMode(PINO_TRIGGER, OUTPUT);
digitalWrite(PINO_TRIGGER, LOW);
pinMode(PINO_TRIGGER_SCADAS, OUTPUT);
digitalWrite(PINO_TRIGGER_SCADAS, LOW);
pinMode(DEBUG_PIN, OUTPUT);
digitalWrite(DEBUG_PIN, LOW);
SPI.begin(18, 19, 23, CS_PIN);
pinMode(CS_PIN, OUTPUT);
digitalWrite(CS_PIN, HIGH);
pinMode(INT1_PIN, INPUT_PULLUP);
if (IMU.begin() != 0) {
Serial.println("# ERROR: IMU did not initialize");
while (true) delay(1000);
}
IMU.startAccel(1600, 2); // ±2g
IMU.startGyro (1600, 15); // ±15.625 °/s
delay(50);
int64_t sum[6] = {0};
inv_imu_sensor_data_t d;
for (int i = 0; i < 500; ++i) {
IMU.getDataFromRegisters(d);
sum[0] += d.accel_data[0];
sum[1] += d.accel_data[1];
sum[2] += d.accel_data[2];
sum[3] += d.gyro_data[0];
sum[4] += d.gyro_data[1];
sum[5] += d.gyro_data[2];
delayMicroseconds(1000);
}
off_ax = sum[0] / 500;
off_ay = sum[1] / 500;
off_az = sum[2] / 500;
off_gx = sum[3] / 500;
off_gy = sum[4] / 500;
off_gz = sum[5] / 500;
Serial.println("trigger\tax\tay\taz\tgx\tgy\tgz"); // <-- MODIFIED: updated header
Ts = (1000000UL / Fs_loop);
}
/* ─────────────────────────────────────
* Main loop – runs at 1 kHz
* ───────────────────────────────────── */
void loop() {
static uint32_t nextSampleTime = micros() + Ts;
// Busy-wait until the exact sampling moment
while (micros() < nextSampleTime) {
// Small pause to reduce CPU usage
__asm__ volatile("nop");
}
// Update the next sample time WITHOUT accumulating error
nextSampleTime += Ts;
// Check if we are behind schedule (drift protection)
if (micros() > nextSampleTime) {
// If we are too late, resynchronize
nextSampleTime = micros() + Ts;
}
// --- ADDED: Python command "listener" ---
if (Serial.available() > 0 && triggerAtivado == 0) {
int comando = Serial.parseInt();
if (comando == 1) {
triggerAtivado = 1;
digitalWrite(PINO_TRIGGER, HIGH);
digitalWrite(PINO_TRIGGER_SCADAS, HIGH);
//REG_WRITE(GPIO_OUT_W1TS_REG, (1ULL << PINO_TRIGGER) | (1ULL << PINO_TRIGGER_SCADAS));
}
}
// IMU reading
inv_imu_sensor_data_t d;
IMU.getDataFromRegisters(d);
int32_t ax = d.accel_data[0] - off_ax;
int32_t ay = d.accel_data[1] - off_ay;
int32_t az = d.accel_data[2] - off_az;
int32_t gx = d.gyro_data[0] - off_gx;
int32_t gy = d.gyro_data[1] - off_gy;
int32_t gz = d.gyro_data[2] - off_gz;
snprintf(buffer, sizeof(buffer),
"%d\t%ld\t%ld\t%ld\t%ld\t%ld\t%ld",
triggerAtivado, ax, ay, az, gx, gy, gz);
Serial.println(buffer);
}