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MCU & Digital Circuits

The digital core: a microcontroller (or SoC), its support circuitry, and logic/memory. Most of the work is getting the support circuits right so the chip boots reliably and runs clean.

The blocks around every MCU​

  • Power pins + decoupling β€” clean, well-bypassed supply.
  • Clock β€” internal RC, or an external crystal/oscillator.
  • Reset β€” defined power-on reset.
  • Boot / strap pins β€” select boot mode; must not float.
  • Programming / debug β€” SWD, JTAG or UART header.
  • GPIO / peripherals β€” to the rest of the board.

Decoupling (the #1 rule)​

  • One 100 nF ceramic per power pin, placed right at the pin, with a short via to the ground plane.
  • Add bulk capacitance (1–10 Β΅F) per supply rail near the chip.
  • Short, fat power traces or a power plane β€” never daisy-chain decoupling caps.

Clock / crystal​

  • Keep crystal traces short and symmetric; put load capacitors close.
  • Surround with a ground guard and keep noisy/high-speed signals away.
  • A solid ground under the crystal area; no traces routed through it.

Reset & boot straps​

  • Use the recommended RC reset (or a supervisor IC) β€” don't leave RESET floating.
  • Pull boot/strap pins to their required level with resistors. A floating strap = random boot mode.

Programming / debug header​

  • Always bring out SWD/JTAG (or UART bootloader) on a labelled header β€” you'll need it for bring-up and field updates.
  • Include power and ground on the header; add a few test points for key signals.

Layout checklist​

  • 100 nF at every power pin, bulk cap per rail
  • Crystal close, guarded, over solid ground
  • Reset defined; all strap pins pulled
  • Debug header exposed and labelled
  • 3.3 V vs 5 V logic levels checked across every interface
Common failure

A board that "won't boot" is usually a floating boot/strap pin or missing decoupling β€” not a dead chip. Check these first.

See also​