urm37 0.12.0

Platform-agnostic no_std driver for the DFRobot URM37 V4.0 ultrasonic distance sensor (UART, PWM, Analog)
Documentation

urm37

crates.io docs.rs License

no_std embedded driver for the DFRobot URM37 V4.0 ultrasonic distance sensor.

An industrial-grade ultrasonic sensor offering advanced capabilities with improved accuracy, temperature correction, and versatile output modes. Supports all interface modes: synchronous/asynchronous UART, PWM trigger, and analog (DAC).

Key Features (V4.0)

  • Serial Level Selection — Onboard button to switch between RS232 and TTL modes (takes effect after reboot)
  • Improved Algorithm — Reduced dead zone and enhanced accuracy
  • Analog Voltage Output — DAC output directly proportional to measured distance (6.8 mV/cm)
  • Wide Voltage Support — Operating range 3.3 V to 5.0 V
  • Hardware Safety — Integrated power reverse protection
  • Configurable Timing — Automatic measurement interval customizable via EEPROM
  • Servo Control — 0–180° angle mapping (compatible with standard servos)
  • no_std — works on any microcontroller
  • Synchronous UART via embedded-io
  • Asynchronous UART via embedded-io-async (Embassy, RTIC…)
  • PWM conversion: ECHO pulse → distance in cm
  • Analog conversion: raw DAC voltage → distance in cm
  • Temperature reading with 0.1 °C resolution (UART mode)
  • Internal EEPROM configuration (thresholds, mode, timing interval)
  • Optional defmt support for embedded logging
  • Zero dynamic allocation (heapless-free)

Specifications

Parameter Value
Power Supply 3.3 V – 5.0 V
Operating Current < 20 mA
Operating Temperature −10 °C to +70 °C
Detecting Range 5 cm – 500 cm
Resolution 1 cm
Communication RS232 / TTL (selectable), PWM, DAC
Dimensions 22 mm × 51 mm
Weight 25 g

Accuracy & Timing

  • PWM Mode (ECHO): 50 µs per 1 cm (0–25000 µs pulse width)
  • Analog Mode (DAC): 6.8 mV per 1 cm
  • Default Auto Interval: 25 ms
  • Temperature Coefficient: Automatic correction via on-chip sensor

Integration with STM32 & Embassy

See examples/STM32_EXAMPLES.md for:

  • 5 real-world integration patterns (UART, PWM, ADC, configuration)
  • Hardware wiring diagrams for each mode
  • Build and flash instructions
  • Troubleshooting guide
  • Performance characteristics
  • Board-specific setup examples

The documentation includes patterns for:

  • Async UART - Simple distance & temperature reading
  • PWM mode - High-precision input capture measurements
  • Analog/ADC - Voltage-to-distance conversion
  • EEPROM config - Sensor threshold and mode setup
  • Production code - Error handling, retries, statistics

Pin Configuration

Pin Label Description
1 VCC Power input (reference +5 V, accepts 3.3 V – 5.0 V)
2 GND Ground
3 NRST Reset (active low)
4 ECHO PWM output: pulse width ∝ distance (50 µs = 1 cm, range 0–25000 µs)
5 MOTO Servo motor control output (0–180° angle mapping)
6 COMP/TRIG COMP: Pulls low when distance < threshold (comparator mode)
TRIG: PWM trigger input for single measurements
7 DAC Analog voltage output (6.8 mV per 1 cm)
8 RXD Serial data receive (RS232 / TTL level, configurable)
9 TXD Serial data transmit (RS232 / TTL level, configurable)

WARNING: Select RS232 or TTL mode via the on-board button before wiring.
Never connect a TTL MCU while the sensor is in RS232 mode — permanent damage will result.
Default: TTL level (LED flashes: 1 long + 1 short). Press button 1 second (until LED off), then cycle power.

Communication Protocol

Serial Settings

  • Baud Rate: 9600 bps
  • Parity: None
  • Stop Bits: 1
  • Data Bits: 8

Frame Format

All commands consist of 4 bytes: [Command] [Data0] [Data1] [SUM]

SUM = low 8 bits of the sum of the first 3 bytes (checksum).

Command Reference

Live Measurement Commands

Operation Frame Response Notes
Read Distance 0x22 Deg 0x00 SUM 0x22 High Low SUM Distance (cm) = (High × 256) + Low. Returns 0xFF 0xFF if invalid. Deg drives servo (0x00 if unused).
Read Temperature 0x11 0x00 0x00 0x11 0x11 High Low SUM 0.1 °C resolution. High byte bits [7:4]: if 0 → positive, if 1 (0xF0) → negative. Returns 0xFF 0xFF if invalid.

EEPROM Access Commands

Operation Frame Response Notes
Read EEPROM 0x33 Add 0x00 SUM 0x33 Add Data SUM Reads configuration value at address Add.
Write EEPROM 0x44 Add Data SUM 0x44 Add Data SUM Sensor echoes the frame to confirm successful write.

EEPROM Memory Map (Configuration Registers)

Address Name Values Purpose
0x00 Low Threshold 0x00–0xFF (cm) COMP pin triggers low if distance ≥ this value
0x01 High Threshold 0x00–0xFF (cm) COMP pin triggers low if distance ≤ this value
0x02 Operating Mode 0xAA = Autonomous, other = Passive PWM Controls measurement behavior
0x03 Serial Level 0x00 = TTL, 0x01 = RS232 Selects UART signal voltage
0x04 Time Interval 25–255 ms (hex value) Polling delay in Autonomous mode; 0x64 = 100 ms

Default factory values: All registers initialized to 0x00.

Measurement Modes

  1. PWM Triggered Mode
    Host sends a low pulse (> 1 µs) on COMP/TRIG pin. Sensor responds with ECHO pulse width encoding distance.

  2. Autonomous (Automatic) Mode
    Sensor automatically measures at user-defined intervals (register 0x04). If measured distance ≤ High Threshold or ≥ Low Threshold, COMP pin pulls low (ultrasonic switch behavior).

  3. Serial Passive Mode
    Host MCU queries sensor via UART commands (0x22 for distance, 0x11 for temperature).

Servo Rotation Mapping

The MOTO pin accepts angle codes (0x00–0x1E) that map to 0–176°:

Hex Deg Hex Deg Hex Deg Hex Deg
0x00 0° 0x01 6° 0x02 12° 0x03 18°
0x04 24° 0x05 29° 0x06 35° 0x07 41°
0x08 47° 0x09 53° 0x0A 59° 0x0B 65°
0x0C 70° 0x0D 76° 0x0E 82° 0x10 94°
0x11 100° 0x12 106° 0x13 112° 0x14 117°
0x15 123° 0x16 129° 0x17 135° 0x18 141°
0x19 147° 0x1A 153° 0x1B 159° 0x1C 164°
0x1D 170° 0x1E 176° — — — —

Examples

This crate includes three ready-to-run examples for STM32F767ZI with Embassy:

1. Async UART Mode (examples/uart_async_stm32.rs)

Demonstrates synchronous distance and temperature reading via UART, plus EEPROM configuration.

Hardware:

  • STM32F767ZI (Nucleo F767ZI)
  • UART5: RX=PD2, TX=PC12

Run:

cargo run --example uart_async_stm32 --features uart-async --release

Features:

  • Read distance in cm
  • Read temperature (0.1 °C resolution)
  • Configure COMP thresholds via EEPROM
  • Passive mode (MCU queries sensor on demand)

2. PWM Passive Mode (examples/pwm_stm32.rs)

Demonstrates high-precision distance measurement using GPIO trigger + InputCapture ECHO pin.

Hardware:

  • STM32F767ZI (Nucleo F767ZI)
  • GPIO PA0: TRIG output (active LOW)
  • TIM2 CH1 PA5: ECHO input (InputCapture)
  • Timer: 1 MHz (1 tick = 1 µs)

Run:

cargo run --example pwm_stm32 --features pwm --release

Features:

  • Trigger pulse generation (10 ms default)
  • Echo pulse measurement with microsecond precision
  • Direct distance calculation from pulse width

3. Analog/ADC Mode (examples/analog_stm32.rs)

Demonstrates simple voltage-to-distance conversion using ADC.

Hardware:

  • STM32F767ZI (Nucleo F767ZI)
  • ADC1 PA4: Analog voltage from URM37 (6.8 mV/cm)

Run:

cargo run --example analog_stm32 --features analog --release

Features:

  • 12-bit ADC reading
  • Direct ADC-to-distance conversion
  • No UART or timing logic required (simplest option)

Installation

[dependencies]
# Choose the features you need:
urm37 = { version = "0.6", features = ["uart-async"] }
# or
urm37 = { version = "0.6", features = ["uart", "pwm", "analog"] }

Usage

Asynchronous UART (Embassy)

use urm37::uart_async::Urm37UartAsync;

let mut sensor = Urm37UartAsync::new(uart);

// Distance in centimetres
let dist_cm = sensor.read_distance().await?;

// Temperature in tenths of °C (235 = 23.5 °C)
let temp = sensor.read_temperature().await?;
let temp_c = temp as f32 / 10.0;

Synchronous UART

use urm37::uart::Urm37Uart;

let mut sensor = Urm37Uart::new(uart);
let dist_cm = sensor.read_distance()?;

PWM mode

use urm37::pwm::pulse_to_distance_cm;

// Trigger the measurement:
// 1. Pull COMP/TRIG low (> 1 µs)
// 2. Release high
// 3. Measure the ECHO pulse width with a timer

let pulse_us: u32 = measure_echo_us(); // your implementation
match pulse_to_distance_cm(pulse_us) {
    Some(cm) => println!("Distance: {} cm", cm),
    None     => println!("Out of range"),
}

Analog mode

use urm37::analog::adc_to_distance_cm;

// 12-bit ADC (STM32, RP2040…)
let raw: u16 = adc.read(&mut dac_pin)?;
let cm = adc_to_distance_cm(raw, 4095);

EEPROM configuration

use urm37::{uart_async::Urm37UartAsync, EepromRegister};

let mut sensor = Urm37UartAsync::new(uart);

// Set COMP/Switch threshold to 50 cm
sensor.set_comp_threshold(50).await?;

// Auto-measure every second (40 × 25 ms)
sensor.set_auto_mode(40).await?;

// Return to passive mode
sensor.set_passive_mode().await?;

Choosing the Right Mode

Mode Pros Cons Best For
UART Full sensor control, temperature, EEPROM config Requires serial setup, 9600 bps Configurable systems, monitoring
PWM/ECHO High precision (1 cm), no additional pins Manual triggering, timer required Applications needing accuracy
Analog/ADC Simplest, no UART or special timing Fixed 6.8 mV/cm mapping, lower precision Cost-sensitive, simple trigger systems

Cargo features

Feature Default Description
uart no Synchronous UART driver (embedded-io)
uart-async no Async UART driver (embedded-io-async)
pwm no PWM mode utilities (embedded-hal)
analog no Analog/ADC mode utilities (embedded-hal)
defmt no defmt logging on error types

embedded-hal compatibility

Crate Version
embedded-hal 1.0
embedded-io 0.6
embedded-io-async 0.6

Troubleshooting

Communication Failures

  • Check serial level mode: Verify the sensor's physical serial level mode (TTL vs. RS232) matches your microcontroller interface.
  • Button configuration: Press the on-board button for 1 second (LED turns off), then cycle power to activate mode changes.
  • Baud rate: Ensure communication at 9600 bps, 8 data bits, no parity, 1 stop bit.

Measurement Issues

Unstable or invalid readings (0xFFFF returned)

  • Ultrasonic signals attenuate as 1/d² in open environments.
  • Ensure good surface alignment and target orientation.
  • Soft surfaces or narrow objects (e.g., pens) may not reflect ultrasound effectively.

ECHO pulse out of range

  • Check power supply voltage (3.3 V – 5.0 V).
  • Verify ECHO pin is not floating or damaged.
  • Ensure pull-up resistor on ECHO if needed by your MCU.

COMP threshold not triggering

  • Read EEPROM registers 0x00 (low) and 0x01 (high) to confirm threshold values.
  • Verify the sensor is in Autonomous mode (0x02 = 0xAA).
  • Check the logic: COMP pulls low when distance ≤ high threshold OR ≥ low threshold.

Servo Control

  • Angle mapping uses hex codes 0x00 (0°) to 0x1E (176°).
  • MOTO output is 5 V logic; ensure servo is compatible.
  • Non-standard servo models may require PWM conditioning.

License

Dual-licensed under MIT and Apache 2.0 — your choice.