add most of exercise text

embedded-workshop-book/src/uarte-implementation.md

@@ -1,10 +1,53 @@
 # Write the Uarte implementation
 ## Step-by-Step Solution
-* add field to the board struct
-* add struct for the instance, how to figure out what the type of the inner field is
-* create instance in init, add baudrate, parity etc. 
-* add to instantiation of board struct
-* impl fmt::Write for the Uarte struct, simple write does not work because of dma 
-* example code with button is not a good idea for the simple button implementation.
+1. Check Documentation.
+   The UART protocol requires four pins, they are usually labelled:
+* RXD
+* TXD
+* CTS
+* RTS
+  Check the documentation to find out which pins are reserved for these and what their configuration needs to be.  
+2. Explore the `nrf-hal` to find out what needs to be done. 
+
+
+The `nrf52840-hal` is a crate that exports all the `52840` flagged features from the `nrf-hal-common`. Let's take a look at the [Uarte module](https://github.com/nrf-rs/nrf-hal/blob/v0.14.1/nrf-hal-common/src/uarte.rs). 
+
+In line 16 we see, that the nRF52840 uses the `hal::pac::UARTE1` peripheral.
+In line 44 you find the `struct Uarte<T>(T)`, the interface to a UARTE instance `T`. Besides the instance `T`, the instantiating method takes variables of the following types as arguments: `Pins`, `Parity` and `Baudrate`.
+
+A quick search of the document reveals where to find all of them:
+* `Pins`: Line 463
+* `Parity` and `Baudrate`: Re-export on line 34
+
+Add the following lines as import:
+```rust
+use hal::pac::uarte0::{
+    baudrate::BAUDRATE_A as Baudrate, config::PARITY_A as Parity};
+use hal::uarte;
+```
+
+3.  Add `struct Uarte` that serves as a wrapper for the `UARTE1` instance.
+
+The struct has one field labelled `inner`, it contains the `UARTE1` instance.
+
+4. Bring up the peripheral in the `fn init()`
+
+Take a closer look at the definition of the `Pins` struct. Import the types of the pin configuration that you don't have yet. Note that the third and fourth pin are each wrapped in an `Option`. 
+Level?
+Create an instance of this struct in `fn init()` with the appropriate pins and configurations.
+
+Create an interface to the UARTE1 instance with `uarte::Uarte::new(...)`. The UARTE0 instance can be found in the `periph` variable. Set parity to `INCLUDED` and the baud rate to `BAUD115200`.
+
+
+5. Board struct
+
+Add a field for the Uarte struct in the Board struct. 
+add the field to the instance of the Board struct in `fn init()`.
+
+6. Implementing the `fmt::Write` trait
+
+We can't just write to the Uarte instance. A simple write would write from flash memory. This does not work because of EasyDMA. We have to write a function that implements the `fmt::Write` trait. This trait guarantees that the buffer is fully and successfully written on a stack allocated buffer, before it returns. 
+
+
 
 I think this is plenty for an hour.