MSP430 Ultrasonic Range Finder

A standalone ultrasonic distance meter built around the MSP430G2553, an HC-SR04 ultrasonic sensor, and an I2C 16x2 LCD, with a potentiometer-controlled, PWM-dimmed LED brightness indicator. Designed and built as a PCB for EEL3926 Junior Design.

Overview

The board triggers the HC-SR04 sensor, times the returned echo pulse using a hardware timer/interrupt combo, converts that time into a distance, and prints the live reading to a 16x2 character LCD over I2C. A potentiometer feeds the on-board ADC and sets the duty cycle of a software-PWM signal that drives an indicator LED, while a second LED (driven through an N-channel MOSFET) blinks once per measurement cycle as a heartbeat indicator.

Every ~600 ms the device:

1. Fires a 10 µs trigger pulse on the HC-SR04.
2. Waits for the sensor's echo pulse and times its width using a Timer A interrupt (10 µs resolution) plus a Port 2 rising/falling-edge interrupt.
3. Converts the elapsed time into distance (mm and inches).
4. Samples the potentiometer with the 10-bit ADC and updates the LED PWM duty cycle.
5. Formats and writes "<mm>mm <in>.<in>in <duty>%" to the LCD's second line.

Features

• Real-time distance readout in millimeters and inches, updated twice a second
• Duty cycle 0–100% brightness control of an indicator LED via a potentiometer
• I2C-driven 16x2 LCD (PCF8574-style backpack, default address 0x27, HD44780 4-bit protocol)
• Interrupt-driven echo timing for consistent, accurate measurements (no busy-wait polling of the echo line)
• Custom boost-converter regulator board to supply a clean, stable rail from battery power
• Fully custom two-board PCB design (main sensor/MCU board + boost regulator board)

Hardware

Component	Part / Notes
MCU	Texas Instruments MSP430G2553 (20-pin TSSOP), 8 MHz DCO
Ultrasonic sensor	HC-SR04 (4-pin: VCC, TRIG, ECHO, GND)
Display	16x2 character LCD with I2C backpack (HD44780 controller, default I2C address 0x27)
Brightness control	Potentiometer into ADC10 channel A1 (INCH_1)
Indicators	2x LED — one PWM-dimmed via ADC/duty cycle, one MOSFET-switched heartbeat blink (BS170 N-channel MOSFET)
Power	Battery input through two TPS61322 boost regulators — one fixed at 3.3V (MCU/logic rail), one fixed at 5V (LCD/HC-SR04 rail)
Misc	Power switch, pull-up/pull-down resistors, decoupling caps

Pin Mapping (from Junior_Design.h)

Signal	MSP430 Pin	Definition
I2C SCL (to LCD)	P1.6	LCD_SCL
I2C SDA (to LCD)	P1.7	LCD_SDA
HC-SR04 Trigger	P2.1	USENSOR_TRIGGER
HC-SR04 Echo (interrupt)	P2.2	ECHO_INTERUPT
Potentiometer input (ADC)	P1.1 / INCH_1	ADC_INPUTPIN / ADC_INPUT
PWM LED output	P1.2	ADC_LED
Heartbeat LED (MOSFET gate driver)	P1.5	BIT5

Note: P1.6 has a default on-board LED on many MSP430 LaunchPad/experimenter boards — it must be removed/disconnected if that pin is reused for I2C SCL.

Circuit Design

Boards were designed in Autodesk Fusion 360 (Electronics workspace).

Main board schematic — MSP430G2553, HC-SR04 header, I2C LCD header, potentiometer, LEDs, and power switch/battery input:

Main board PCB layout:

Boost regulator schematic — TPS61322 boost converter stepping battery voltage up to a regulated output rail:

Boost regulator PCB layout:

Firmware Architecture

• Init_HW() — stops the watchdog, sets the DCO to 8 MHz, configures Port 1/2 directions, and brings up I2C, both timers, and the ADC.
• Timer A0 (10 µs tick) — drives the microsecond-resolution delay function (Delay_Timer) and increments the free-running counter used to time the ultrasonic echo pulse.
• Timer A1 (100 µs tick) — implements a software PWM with a 10 ms period, toggling the indicator LED on/off based on the Duty variable (0–100).
• Port 2 ISR — captures the timestamp of the echo pulse's rising edge; the falling edge is detected in the Timer A0 ISR, and the difference between the two gives the pulse width.
• Ultrasonic_Trigger() / Ultrasonic_Echo_Read() — fire the 10 µs trigger pulse and return the measured echo time (in 10 µs units), which main() converts to mm/inches using the speed of sound (340 m/s, double path).
• LCD driver — bit-bangs the HD44780 4-bit protocol over the hardware I2C module (UCB0), toggling the RS/RW/E/backlight lines through the I2C expander.

Timing resolution is configurable via TIMER_MAX_COUNT_A in the header — the current setting (10 µs/tick) gives roughly 3.4 mm double-path resolution (~1.7 mm single path).

Building & Flashing

This project targets Code Composer Studio (CCS) with the MSP430 toolchain.

1. Create a new MSP430 project in CCS targeting the MSP430G2553.
2. Add main.c and Junior_Design.h to the project.
3. Build and flash over Spy-Bi-Wire (SBWTDIO/SBWTCK, pins 16/17) using a LaunchPad or standalone programmer.
4. Power the board (battery + boost regulator, or a regulated 3.3V/5V bench supply) and observe the LCD.

Usage

• Power on — the LCD displays Junior Design :P on line 1.
• Line 2 updates continuously with the live reading, e.g.:

  152mm 5.9in 63%
  

• Turning the potentiometer changes the LED brightness (duty cycle shown as a percentage on the LCD).
• The heartbeat LED on P1.5 toggles once per measurement cycle to confirm the loop is running.

Acknowledgments

• Written for EEL3926 Junior Design.
• The I2C LCD driver portion is adapted from a public MSP430F5529 I2C/LCD1602 interface example (credited in Junior_Design.h); used here for educational purposes.
• Claude 5 Sonnet for drafting the README.md file

License

This project is MIT-licensed — see LICENSE — with one exception: firmware/Junior_Design.h was written by Dr. R. Weeks for educational use only and is not licensed for commercial use. See THIRD_PARTY_NOTICES.md for details.