MCU architecture

Why this matters

A single drone contains several small computers: the flight controller, every ESC, the GPS and even some smart batteries each have their own microcontroller (MCU). An MCU is a whole computer shrunk into one chip, CPU, memory and interface circuits together, designed to do specific jobs with guaranteed timing, low power and low cost, unlike general-purpose computers that can do many things but guarantee no timing.

The whole course uses one hypothetical case: building an add-on sensor module for a drone on a Raspberry Pi Pico 2 that reads sensors, measures voltage, drives a servo and sends data to the flight controller. The Python code of every module can be downloaded from /downloads/uat-204/.

Inside a microcontroller

• The CPU core executes instructions; most MCUs in drone work use Arm Cortex-M cores
• Flash stores the program and constants, and keeps them when powered off
• SRAM holds variables while running, and loses them when powered off
• The bus carries data between all parts
• Peripherals are dedicated circuits on the chip, such as GPIO (digital inputs and outputs), timers, the ADC (analog-to-digital converter) and communication circuits, which work alongside the CPU

MCUs in drone work

All figures come from the PX4, Raspberry Pi and Espressif documentation. Flight controllers use faster MCUs with more memory because they must estimate state and control attitude hundreds of times a second.

The ADC: from voltage to numbers

An ADC (analog-to-digital converter) turns a voltage into an integer. An $N$-bit ADC divides 0 to $V_{\text{ref}}$ into $2^N$ steps, each called an LSB. The RP2350 has a 12-bit ADC, so 4,096 steps.

An ADC pin can take no more than the chip’s reference voltage (about 3.3 V), but a 6S battery reaches 25.2 V, so it must first be reduced with a voltage divider.

vref, bits = 3.3, 12
lsb = vref / 2 ** bits
r1, r2 = 100_000, 10_000
ratio = r2 / (r1 + r2)
print(f"LSB = {lsb * 1000:.3f} mV at the pin = {lsb / ratio * 1000:.2f} mV of battery voltage")
for v_batt in (25.2, 22.2, 19.8):
    v_pin = v_batt * ratio
    code = round(v_pin / lsb)
    print(f"battery {v_batt:5.1f} V -> pin {v_pin:.3f} V -> code {code:4d} -> read back {code * lsb / ratio:.3f} V")

LSB = 0.806 mV at the pin = 8.86 mV of battery voltage
battery  25.2 V -> pin 2.291 V -> code 2844 -> read back 25.204 V
battery  22.2 V -> pin 2.018 V -> code 2505 -> read back 22.200 V
battery  19.8 V -> pin 1.800 V -> code 2234 -> read back 19.798 V

Module lab

Common mistakes

Summary

• An MCU combines CPU, memory, bus and peripherals on one chip, doing specific jobs with guaranteed timing
• The Pixhawk 6X uses an STM32H753 as its main processor and an STM32F100 as its IO processor
• An $N$-bit ADC divides the voltage range into $2^N$ steps
• Measure high voltages through a voltage divider, and calibrate

Check your understanding

1. What is the resolution of a 10-bit ADC with a 3.3 V reference, in mV?
2. A divider with $R_1=47$ kΩ and $R_2=10$ kΩ has 16.8 V in. What is the pin voltage?
3. A 12-bit ADC with a 3.3 V reference reads 2048. What is the pin voltage?
4. Which loses its data at power-off, Flash or SRAM?
5. Which MCU is the Pixhawk 6X main processor?