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 SKR PICO <-> RASPBERRY PI — UART SETUP
 FOR:      Raspberry Pi 4 Model B  ·  Pi 400
 NOT FOR:  Pi 3/Zero 2 W (own file) · Pi 5/500 (DIFFERENT steps!) · CB1 (own file)
 P5 POWER VERDICT: CAUTION — a lean HEADLESS Pi 4 runs fine from the Pico's P5
                   header (measured stress ~1.25 A vs the 3.5 A fuse), but P5
                   cannot honor the Pi 4's full official 3 A envelope. No USB
                   peripherals. Pi 400: use its own USB-C supply, period.
 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 the Pi 4, Bluetooth owns the good UART (the PL011/UART0) and the mini UART
sits on the header by default. One overlay swaps that. (BTT's manual line
"dtoverlay=pi3-miniuart-bt" is a deprecated alias — current names below.)

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

Add ONE of:

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

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

If Option A, also run once:  sudo systemctl disable hciuart

STEP 2 — edit cmdline.txt
File (Bookworm):  /boot/firmware/cmdline.txt    (older: /boot/cmdline.txt)
DELETE the token:  console=serial0,115200

(TUI alternative:  sudo raspi-config -> 3 Interface Options -> I6 Serial Port ->
 login shell NO -> serial hardware 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 CAUTION)
  Official Pi 4 recommendation: 3.0 A supply (includes a 1.2 A USB-peripheral
  budget). P5's rail = 3.5 A fuse SHARED with the Pico's own 5 V loads
  (endstops, probe, servo, RGB). Headless Pi 4 stress ~1.25 A = fine;
  add a camera/touchscreen/USB drives and you are gambling with the fuse.
  Check with `vcgencmd get_throttled` after install — 0x0 = clean.

CLASSIC TRAPS
  * Bookworm's real files live under /boot/firmware/ — the old paths edit dead
    copies.  * Option B without core_freq=250 quietly breaks Bluetooth.
  * Never feed the Pi from P5 and its USB-C supply at the same time.
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