Add minimal microcontroller example.

src/bare-metal/microcontrollers.md

@@ -1,5 +1,17 @@
 # Microcontrollers
 
- * Peripheral Access Crates
- * HAL crates
- * Other projects
+The `cortex_m_rt` crate provides (among other things) a reset handler for Cortex M microcontrollers.
+
+```rust,editable,compile_fail
+{{#include microcontrollers/examples/src/bin/minimal.rs:Example}}
+```
+
+Next we'll look at how to access peripherals, with increasing levels of abstraction.
+
+<details>
+
+* The `cortex_m_rt::entry` macro requires that the function have type `fn() -> !`, because returning
+  to the reset handler doesn't make sense.
+* Run the example with `cargo embed --bin minimal`
+
+</details>

src/bare-metal/microcontrollers/examples/Cargo.toml

@@ -19,6 +19,9 @@ name = "board_support"
 [[bin]]
 name = "hal"
 
+[[bin]]
+name = "minimal"
+
 [[bin]]
 name = "mmio"
 

src/bare-metal/microcontrollers/examples/src/bin/minimal.rs

@@ -0,0 +1,28 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// ANCHOR: Example
+#![no_main]
+#![no_std]
+
+extern crate panic_halt as _;
+
+mod interrupts;
+
+use cortex_m_rt::entry;
+
+#[entry]
+fn main() -> ! {
+    loop {}
+}