Restore notes on portability
diff --git a/src/SUMMARY.md b/src/SUMMARY.md
index 0ee7ea1..bf111bb 100644
--- a/src/SUMMARY.md
+++ b/src/SUMMARY.md
@@ -32,6 +32,7 @@
- [Static Guarantees](./static-guarantees/static-guarantees.md)
<!-- TODO: Define Sections -->
- [Portability](./portability/portability.md)
+ - [The Trait System](./portability/traits.md)
<!-- TODO: Define more sections -->
- [Concurrency](./concurrency/concurrency.md)
<!-- TODO: Define Sections -->
diff --git a/src/portability/portability.md b/src/portability/portability.md
index fec7a58..26fb9c2 100644
--- a/src/portability/portability.md
+++ b/src/portability/portability.md
@@ -1,3 +1,14 @@
# Portability
-> ❌: This section has not yet been written. Please refer to [rust-embedded/book#6](https://github.com/rust-embedded/book/issues/6) for discussion of this section.
+> ❌: This section has not yet been written. Please refer to [embedded-wg#119](https://github.com/rust-lang-nursery/embedded-wg/issues/119) for discussion of this section.
+
+## Building something bigger
+
+> drivers that work for more than one chip
+
+
+
+* Device Crates (one per chip)
+* HAL Implementation Crates (one per chip)
+* `embedded-hal` (only one)
+* Driver Crates (one per external component)
diff --git a/src/portability/traits.md b/src/portability/traits.md
new file mode 100644
index 0000000..2fbf2ff
--- /dev/null
+++ b/src/portability/traits.md
@@ -0,0 +1,50 @@
+# The Trait System
+
+```rust
+/// Single digital push-pull output pin
+pub trait OutputPin {
+ /// Drives the pin low
+ fn set_low(&mut self);
+
+ /// Drives the pin high
+ fn set_high(&mut self);
+}
+```
+
+[rust-embedded/embedded-hal](https://github.com/rust-embedded/embedded-hal/blob/master/src/digital.rs)
+
+```rust
+impl<MODE> OutputPin for OutputGpio<MODE> {
+ fn set_low(&mut self) {
+ self.set_pin_low()
+ }
+
+ fn set_high(&mut self) {
+ self.set_pin_high()
+ }
+}
+```
+
+> this goes in your chip crate
+
+
+```rust
+impl<SPI, CS, E> L3gd20<SPI, CS>
+where
+ SPI: Transfer<u8, Error = E> + Write<u8, Error = E>,
+ CS: OutputPin,
+{
+ /// Creates a new driver from a SPI peripheral
+ /// and a NCS (active low chip select) pin
+ pub fn new(spi: SPI, cs: CS) -> Result<Self, E> {
+ // ...
+ }
+ // ...
+}
+```
+
+[japaric/l3gd20](https://github.com/japaric/l3gd20/blob/master/src/lib.rs)
+
+## N\*M >>> N+M
+
+> to re-use a driver, just implement the embedded-hal interface
\ No newline at end of file
