Digital &
microcontrollers Where hardware meets code. A microcontroller is a small computer whose pins are wired to the physical world — and almost every frustrating bug at this boundary comes from treating a pin as if it were a software variable rather than a piece of silicon with limits.

Digital is analogue with agreed thresholds. A 3.3V chip typically reads anything above roughly 2.0V as HIGH and below about 0.8V as LOW — and the band between is undefined, where a slowly-changing or noisy signal can be read as either, or oscillate.

• Per-pin current: roughly 20mA source or sink on typical parts (some ESP32 pins less). An LED with a resistor is fine; anything more needs a transistor.
• Total chip current: there is a combined limit across all pins, often around 100–200mA. Eight LEDs at 20mA each will exceed it even though each pin is within spec.
• Input modes: plain input (floating unless something drives it), input with internal pull-up (the usual choice for buttons), and output. Configuring a pin as an output and then connecting it to another output is a short — one of the few ways to destroy a board with code alone.
• Boot-sensitive pins: on the ESP32 in particular, certain pins must be at specific levels at power-up or the chip refuses to boot. Pulling one of them low with a button is a genuinely common and confusing failure. Check the pinout guide for the specific board before assigning pins.

Sinking vs sourcing: an LED can be wired from the pin to ground (pin sources current, HIGH turns it on) or from the supply to the pin (pin sinks current, LOW turns it on). Both are valid, the second is often slightly stronger electrically, and mixing them up produces logic that reads backwards in code.

ADC — voltage in, number out

10-bit ADC, 3.3V reference:
  0    = 0V
  1023 = 3.3V
  resolution = 3.3 / 1024 = 3.2 mV per step

reading a divider:  V = raw × 3.3 / 1023

Practical cautions: the reference voltage is what accuracy depends on, so a sagging supply skews every reading; a high-impedance source (a divider above ~100kΩ) may not charge the ADC's sampling capacitor in time, giving readings that drift or read low; and averaging several samples is nearly free and dramatically improves stability.

PWM — a square wave that pretends to be analogue

Switching a pin on and off rapidly, varying the duty cycle. At 50% duty the average is half the supply. For LEDs and motors the average is all you need — the device's own inertia does the smoothing. For a genuine analogue voltage, add a low-pass filter, as in the Inductors & AC page.

Note that LED brightness perception is logarithmic, so a linear duty ramp looks wrong — most of the visible change happens in the bottom quarter. Squaring the value before writing it produces a fade that looks smooth.

Wiring gotchas, one per bus. UART must be crossed — TX to RX, RX to TX — and connecting TX to TX is the most common serial mistake there is. I²C needs pull-up resistors on both lines (many breakout boards include them, and stacking several boards can over-pull the bus); each device also needs a unique address, and address clashes are why two identical sensors often will not co-exist without an address-select pin. SPI needs a separate chip-select line per device, and getting the clock polarity/phase mode wrong yields data that looks like plausible garbage.