Group Assignment on Electronics design

Group members

🖨️ Week 6 - Electronics design (Group Work)

In this week's group assignment, our task was to pulse-width modulate (PWM) an output of a microcontroller and measure it. The second task was to send data over a serial connection and measure the waveforms using a mixed-signal oscilloscope.

🟢 Pulse width modulation

To generate a PWM signal with an ESP32-C3 microcontroller, we used an example program fade.ino found in Arduino IDE and modified the variable int led to set the PWM output to pin 0. We also connected an LED in series with a 50 ohm current-limiting resistor to observe the PWM-modulated output.

Due to the high frequency (1 kHz) of the PWM signal, the human eye cannot perceive the rapid PWM modulation. Instead, it perceives the average value of the PWM signal: At 100% duty cycle, the LED appears the brightest. At 50% duty cycle, the LED appears half as bright. The LED is shown in the right side of the video below.

We measured the PWM signal using an analog probe of the oscilloscope (yellow curve in the video below) and a digital probe (light blue signal). The key difference between them is that the analog signal reveals details such as noise and the finite rise time of the signal whereas the digital probe provides a simplified, symbolic representation of the waveform.

DC voltages of the microcontroller are convenient to check with a digital voltage meter (DVM).

🟢 Serial data transmission

To test sending data over a serial connection, we used the function Serial.write(). The TX data transmission follows the UART (Universal Asynchronous Receiver Transmitter) protocol.

We tested transmission by sending two symbols, capital letter A (ASCII code 10000001), and capital letter B (ASCII code 10000010).

The measured output signals from the TX pin of the ESP32-C3 are shown below. According to Wikipedia, UART "sends data bits one by one, from the least significant to the most significant, framed by start and stop bits." This is seen in our measured signals:

  • Start Bit: When data transmission begins, a LOW start bit is sent.
  • Data Bits: The LSB (least significant bit) is sent first.
  • Stop Bit: The transmission concludes with a stop bit, and the TX pin returns to HIGH.

ASCII A

ASCII B

We also tried to send an ASCII code for the number 9 and first we got strange results. Quotation marks needed to be used, i.e. Serial.write("9"); or else just the number 9 was sent in binary format 00001001.

🎯 Summary

  • PWM is a convenient way to control many devices such as the brightness of an LED.
  • Mixed signal oscilloscopes help to visualize both analog and digital aspects of signals.
  • UART transmits data LSB first surrounded by start and stop bits.