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 SKR PICO <-> RASPBERRY PI — UART SETUP
 FOR:      Raspberry Pi 3 Model B  ·  Pi 3 Model B+  ·  Pi Zero 2 W
 NOT FOR:  Pi 4/400 (own file) · Pi 5/500 (own file — DIFFERENT steps!) · CB1 (own file)
 P5 POWER VERDICT: OK — the Pico's P5 header (5 V, 3.5 A fuse) can power these
                   Pis headless. Keep USB peripherals off the Pi; never connect
                   the Pi's own PSU at the same time as P5 power.
 Source-verified 2026-07-10 against raspberrypi.com official documentation,
 klipper3d.org, and BigTreeTech's SKR-Pico repository. honeybadger.software
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WHY THIS IS NEEDED
On these models Bluetooth owns the good UART (the PL011). One overlay frees it
for the printer link. (BTT's manual says "dtoverlay=pi3-miniuart-bt" — that name
is officially deprecated; the lines below are the current ones and do the same.)

STEP 1 — edit config.txt
File on current Raspberry Pi OS (Bookworm):   /boot/firmware/config.txt
(on older images the same file lives at:      /boot/config.txt)

Add ONE of these two options at the end:

  # Option A (recommended for a headless printer — Bluetooth off):
  dtoverlay=disable-bt

  # Option B (keep Bluetooth working):
  dtoverlay=miniuart-bt
  core_freq=250          # REQUIRED with option B (official docs) — BTT's manual omits it

If you chose Option A, also run once after boot:
  sudo systemctl disable hciuart

STEP 2 — edit cmdline.txt
File (Bookworm):   /boot/firmware/cmdline.txt     (older images: /boot/cmdline.txt)
DELETE this one token from the single long line (leave everything else):
  console=serial0,115200

(TUI alternative for steps 1+2:  sudo raspi-config  ->  3 Interface Options ->
 I6 Serial Port -> login shell over serial? NO -> serial hardware enabled? YES)

STEP 3 — printer.cfg
  [mcu]
  serial: /dev/ttyAMA0
  restart_method: command

STEP 4 — the wiring (BTT's 5-wire lead)
  Pico P5:  5V · 5V · GND · RX0(IO1) · TX0(IO0)
  Pi pins:   2  ·  4 ·  6  ·  8 TXD   ·  10 RXD     (TX crosses to RX both ways)
  Firmware on the Pico: klipper-UART0.uf2 (NOT the USB build).

POWER MATH (why the verdict is OK)
  P5 rail = TPS5450 buck (5 A cont.) behind a 3.5 A fuse, shared with the board's
  own 5 V loads. Headless draw: Pi 3B ~0.4 A typ / 1.34 A stress · 3B+ ~0.5 A typ
  · Zero 2 W ~0.35 A typ. All comfortably under the fuse.
  After first boot, run `vcgencmd get_throttled` — 0x0 means power is clean.

CLASSIC TRAPS
  * Editing /boot/config.txt on Bookworm does nothing — the real file is under
    /boot/firmware/.  * Option B without core_freq=250 breaks Bluetooth quietly.
  * Powering the Pi from P5 AND its own PSU together — never do both.
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