pico-timer

Non-blocking HC-SR04 ultrasonic ranging on the Raspberry Pi Pico 2, built around the SDK's hardware alarm/timer API and GPIO interrupts. Configurable via a simple serial CLI.

Overview

A distance-measurement system that drives an HC-SR04 sensor without ever blocking on the echo. Periodic readings are scheduled by add_alarm_in_ms, the echo pulse is timed using rising/falling-edge GPIO interrupts, and a watchdog alarm cancels the read if no echo arrives within 50 ms. The user can start, stop, and reconfigure the reading period at runtime over USB serial.

Features

• Non-blocking ultrasonic ranging: trigger pulse + edge-IRQ echo capture using time_us_64(), no pulse_in-style busy loops
• Watchdog alarm (50 ms) on each trigger to detect missing/lost echoes and flag a read failure
• Periodic reading scheduler via the Pico SDK alarm API (default 3 s, runtime-configurable)
• 1 Hz uptime counter maintained by a self-rescheduling alarm callback, used to timestamp readings
• Serial command interface with line buffering:
  • start — begin periodic readings
  • stop — pause periodic readings
  • period <seconds> — change the polling interval
• Glitch rejection: echo pulses longer than 30 ms are discarded as invalid
• Distance computed with the standard pulse_us * 0.0343 / 2 formula

Tech Stack

• Language: C (C11)
• SDK: Raspberry Pi Pico SDK 2.2.0 (hardware/timer, hardware/gpio)
• Hardware: Raspberry Pi Pico 2 (RP2350), HC-SR04 ultrasonic sensor
• Build system: CMake 3.12+, GNU Arm Embedded Toolchain 14.2

Architecture

All shared state lives in a single volatile-qualified struct that is written from ISRs (GPIO edges, alarm callbacks) and read from the main loop. The main loop is a small dispatcher: it polls the serial command buffer, fires triggers when the periodic alarm flags one, and prints results once per measurement.

main/
├── main.c           # ISRs, alarm callbacks, serial CLI, main loop
└── CMakeLists.txt
CMakeLists.txt       # Top-level project, SDK import

Pinout

Signal	GPIO
HC-SR04 TRIG	14
HC-SR04 ECHO	15

How to Build

Requires the Pico SDK and the ARM toolchain.

mkdir build && cd build
cmake ..
make

Flash the resulting .uf2 to the Pico 2 in BOOTSEL mode, then connect via USB serial at 115200 baud.

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Developed as part of the Embedded Systems course at Insper.