add new hardware peripheral

README.md

@@ -11,6 +11,29 @@
 
 See [blue-pill.md](docs/hardware/blue-pill.md) for wiring notes.
 
+## Attached Hardware
+
+- internal LED: `PC13`
+- RGB LED:
+  - `PA5` = red
+  - `PA6` = green
+  - `PA7` = blue
+  - `PA4` = drive low for the LED return path
+- 5-way button board:
+  - `PB12` = GND, drive low
+  - `PA10` = VCC, drive high
+  - `PB13` = right
+  - `PB14` = down
+  - `PB15` = left
+  - `PA8` = center
+  - `PA9` = up
+- HMC5883L module:
+  - `PB4` = VCC, drive high
+  - `PB5` = GND, drive low
+  - `PB6` = SCL
+  - `PB7` = SDA
+  - `PB8` = DRDY
+
 ## Important Target Note
 
 - Blue Pill `STM32F103` is Cortex-M3 and uses `thumbv7m-none-eabi`.
@@ -71,14 +94,15 @@ probe-rs chip list | rg STM32F103
 
 - `blinky-basic` - plain `no_std` blink with a simple delay
 - `blinky-timer` - periodic blink driven by a timer abstraction
-- `button-input` - poll a button on `PA0`, mirror state on the LED, log transitions
+- `button-input` - poll the center button on `PA8`, mirror state on the LED, log transitions
 - `embassy-blinky` - minimal async blink with Embassy
-- `embassy-button` - minimal Embassy polling loop for a button on `PA0`
+- `embassy-button` - minimal Embassy polling loop for the center button on `PA8`
 
-The button examples assume a simple external button:
+The button examples assume the 5-way board is wired like this:
 
-- one side to `PA0`
-- one side to `3V3`
+- `PB12` forced low for ground
+- `PA10` forced high for power
+- `PA8` used as the center button input
 - internal pull-down enabled in firmware
 - common ground still required for the probe