📚 Assignment 3 CMPE2250: Measuring Period, Frequency, and Duty Cycle Using Timer Input Capture

📋 Overview

In this assignment, you will measure the period, frequency, and duty cycle of a digital signal using two different timer configurations on the STM32G0B1RE:

• Part A: Single‑channel input capture
• Part B: Input PWM mode (dual‑channel capture)
• Part C: Detecting and handling timer overflow (16‑bit wraparound)

You will feed a known signal from a function generator into the microcontroller and verify your results using both UART output and an oscilloscope.

1️⃣ Preparatory Work:

• Select a 16-bit timer that has at least 2 channels (TIM15 is suggested) and configure it as input capture.

• Configure the proper alternate functions for the GPIO to use as required.

• Compute the period and positive width, then calculate frquency and duty cycle.

2️⃣ Part A - Single channel input capture

The objective of this part is to use one capture channel to:

• Measure the period
• Measure the High-time (positive pulse width)
• Calculate frequency and duty cycle

2.1 - Configure GPIO

• Select a timer channel and configure the alternate function fot channel 1 (e.g., TIM15_CH1 on PC1).

2.2 - Configure the Timer

• Prescaler so the timer ticks at 1 MHz (1[µs] resolution).

• Channel 1 as input capture on both rising and falling edges.

• The pulse with is the time difference between a rising edge and a falling edge

• The period can be obtained by the time difference between two falling edges or two rising edges.

2.3 - Compute frequency and duty cycle

Perform these calculations four times per second (every 250[ms]). You may use SysTick for this.

• Frequency:

\[f = \frac{1}{T_{\text{period}} \times 10^{-6}}\]

• Duty cycle:

\[\text{Duty Cycle} = \frac{T_{\text{HIGH}}}{T_{\text{PERIOD}}} \times 100\]

Where:

• $T_{\mathrm{HIGH}}$ = positive pulse width

• $T_{\mathrm{PERIOD}}$ = rising‑to‑rising or falling-to-falling period

2.4 - Print the results on the serial terminal (USART 2)

• Print period, frequency, positive width, and duty cycle over USART 2 and display them on Tera Term.

• Try different frequencies and duty cycles and verify that the capture works as intended.

3️⃣ Part B – Input PWM mode (Dual-channel Capture)

Repeat the procedure from part A, this time using two channels (CHANNEL 1 and CHANNEL 2) in Input PWM Mode

3.1 - Configure GPIO alternate function

• Configure only the CH1 pin (e.g., TIM15_CH1).

• CH2 does not need a GPIO configuration

3.2 - Configure the Timer

• Prescaler so the timer ticks at 1 MHz (1[µs] resolution), same as in Part A.

• Configure

  • Channel 1 → TI1, rising edge
  • Channel 2 → TI1, falling edge

• Enable Both Capture Channels, but only enable interrupt for Channel 2 (falling edge),

• The pulse with can be obtained as PW = CCR2 - CCR1

• The period can be obtained by the time distance between two falling edges.

3.3 - Compute frequency and duty cycle

Same formulas as in Part A. Perform these calculations every 250[ms].

• Frequency:

\[f = \frac{1}{T_{\text{period}} \times 10^{-6}}\]

• Duty cycle:

\[\text{Duty Cycle} = \frac{T_{\text{HIGH}}}{T_{\text{PERIOD}}} \times 100\]

Where:

• $T_{\mathrm{HIGH}}$ = positive pulse width

• $T_{\mathrm{PERIOD}}$ = rising‑to‑rising or falling-to-falling period

3.4 - Print the results on the serial terminal (USART 2)

• Print period, frequency, positive width, and duty cycle over USART 2 and display on Tera Term.

• Try different frequencies and duty cycles and verify the capture works as intended.

3.5 Questions

• How is the implementation of Part B more or less efficient than the implementation of Part A? Explain.

• How can you verify that the timer is only interrupting on falling‑edge captures?

• Could the program still interrupt on both rising and falling edges? What difference would it make?

4️⃣ Part C - Challenge

• Using the configuration from part B, what happens when you feed a 10[Hz] signal?. Can the period be detected properly? Explain.

• What is the minimum and maximum frequencies that the input capture could detect using a 16-bit timer and 1[us] ticks?

• How can we add a fix so the timer could detect frequecnies lower than the minimum capabl;e of detecting at 1[us] ticks?