Week 03 · Embedded programming

The RP2040 on the XIAO

The chip is a Raspberry Pi RP2040: two Cortex-M0+ cores at 133 MHz, 264 kB of RAM and 30 GPIO pins. It has no flash of its own, so the XIAO adds a 2 MB flash chip, a USB-C port and a 3.3 V regulator around it.

Seeed Studio XIAO RP2040 under the microscope, next to two 10k resistors — Seeed Studio XIAO RP2040, with R4 and R5 (10k) beside it.

Soldering the QPAD

The QPAD is a board with six copper touch pads. I soldered three things onto it: the XIAO RP2040, two 10 kΩ pull-up resistors for I2C (R4 and R5), and the OLED's four-pin header. Testing each part before writing any game code turned up one bad joint.

Soldering station with a digital microscope, iron and hot air gun — After soldering, I checked every joint under the microscope.

Soldering the XIAO RP2040 onto the QPAD, timelapse.

Row of solder joints seen through the microscope — The OLED's four header pins (GND, VCC, SCL, SDA) under the microscope.

Q1 [D9] didn't respond

I ran the test_touch sketch and pad Q1 [D9] never changed its reading. A multimeter showed no continuity on that pin, so the solder joint was bad. Re-soldering that pad fixed it.

Testing the touch pads: every pad responds except Q1 [D9].

After re-soldering

Re-soldered, and all pads respond.

Programming QPAD Heli

A helicopter flies through a scrolling cave on the QPAD's OLED. Touch pad A is the throttle, and it is analog: a light touch gives a little thrust, a hard press gives a lot. Pad B pauses.

Input. A pad has no sensor. The program pulls it low, lets the pull-up charge it back to 3.3 V and counts how long that takes. A finger adds capacitance, so the count grows with how much of the finger is on the pad: about 0 untouched, 50 to 70 for a light touch, 120 to 150 for a hard press. Thrust is that count, scaled from 50–150 to 0–1.

Output. The OLED is an SSD1306, 128×64 pixels, on I2C address 0x3C at 400 kHz. Sending one full frame takes about 27 ms, and that is what limits the game to about 37 frames per second. Reading the pads takes only about 54 µs. The LED turns green while the throttle is held and red for a moment after a crash.

Timing. Physics always advances in fixed 20 ms steps, however long drawing takes, so the helicopter feels the same whatever the frame rate.

How the cave is generated

Diagram of the scrolling cave: grey walls, a dashed centre line that drifts per column, orange pillars every 70 columns, the helicopter at column 22, and two charts showing the gap shrinking from 44 to 22 px and scroll speed rising with distance — The cave is 128 columns, one per screen pixel. Each new column's gap, centre and pillar follow these rules.

QPAD Heli on the OLED: the cave, a pillar, and a finger on pad A — The same rules on the real screen: the cave, a pillar hanging from the ceiling, and the helicopter at column 22.

AI usage

I used AI as a mentor. It wrote the game code, and I learned by asking questions until I understood every line of it.

The soldering and the debugging were mine: I found the dead pad with the test_touch sketch and a multimeter, and re-soldered it.