Power Rails

A split-supply scheme: USB powers the Pico, a separate external 3.3 V supply powers the 65C02, RAM, and pull-ups.

Recommended setup (USB + external supply)

USB ──────────────→ Pico (5 V via USB, onboard reg → 3.3 V logic)

External +3.3 V ──┬─→ 65C02 pin 8 (VDD)
                  ├─→ RAM pin 28 (VCC)
                  ├─→ RAM pin 22 (OE#)
                  └─→ Top of all 6 pull-up resistors (R1–R6)

Common GND ───────┬─→ Pico pins 3, 8, 13, 18, 23, 28, 33, 38
                  ├─→ 65C02 pin 21 (VSS)
                  ├─→ RAM pin 14 (VSS)
                  └─→ External supply GND

Warning

Do not connect external +3.3 V to Pico pin 39 (VSYS) or pin 36 (3V3 OUT) while USB is plugged in. USB back-feeds ~4.5 V on VSYS and fights the breadboard rail, which freezes the CPU with a stuck address bus.

Notes

• Use the breadboard’s power rails for the external supply: run +3.3 V along one rail and GND along the other. Every chip’s VCC/GND wire is then a short hop to the nearest rail.

• A common ground between the Pico and breadboard is required even with split supplies.

• Alternative (USB disconnected): a single external 3.3 V supply can feed the breadboard rail and Pico pin 36 (3V3 OUT) directly. Never use VSYS for a regulated 3.3 V input when USB is also connected.