Write about embedded-hal

src/SUMMARY.md

@@ -242,6 +242,7 @@
 - [Microcontrollers](bare-metal/microcontrollers.md)
   - [PACs](bare-metal/microcontrollers/pacs.md)
   - [HAL crates](bare-metal/microcontrollers/hals.md)
+  - [embedded-hal](bare-metal/microcontrollers/embedded-hal.md)
   - [The type state pattern]()
 
 # Day 5A: Afternoon

src/bare-metal/microcontrollers/embedded-hal.md

@@ -0,0 +1,23 @@
+# `embedded-hal`
+
+The [`embedded-hal`](https://crates.io/crates/embedded-hal) crate provides a number of traits
+covering common microcontroller peripherals.
+
+ * GPIO
+ * ADC
+ * I2C, SPI, UART, CAN
+ * RNG
+ * Timers
+ * Watchdogs
+
+Other crates then implement
+[drivers](https://github.com/rust-embedded/awesome-embedded-rust#driver-crates) in terms of these
+traits, e.g. an accelerometer driver might need an I2C or SPI bus implementation.
+
+<details>
+
+ * There are implementations for many microcontrollers, as well as other platforms such as Linux on
+Raspberry Pi.
+ * There is work in progress on an `async` version of `embedded-hal`, but it isn't stable yet.
+
+</details>

src/bare-metal/microcontrollers/hals.md

@@ -1,5 +1,9 @@
 # HAL crates
 
+[HAL crates](https://github.com/rust-embedded/awesome-embedded-rust#hal-implementation-crates) for
+many microcontrollers provide wrappers around various peripherals. These generally implement traits
+from [`embedded-hal`](https://crates.io/crates/embedded-hal).
+
 ```rust,editable,compile_fail
 {{#include examples/src/bin/hal.rs:Example}}
 ```