embedded-trainings-2020/embedded-workshop-book/src/uarte-implementation.md

Write the Uarte implementation

Step-by-Step Solution

Step 1: Check Documentation.

The UART protocol requires four pins, they are usually labelled:

• RXD
• TXD
• CTS
• RTS

✅ Check the User Guide in section 7.2 to find to find out which pins are reserved for these and what their configuration needs to be.

Step 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 nrf52840-hal's Uarte module.

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:

use hal::pac::uarte0::{
    baudrate::BAUDRATE_A as Baudrate, config::PARITY_A as Parity};
use hal::uarte;

Step 3: Add struct Uarte

✅ Add struct Uarte that serves as a wrapper for underlying HAL UART driver.

The struct has one field labelled inner which is of type hal::Uarte but with the type parameter T being set to hal::pac::UARTE1.

The main difference between using our Uarte object and the underlying HAL Uarte object is that ours will be pre-configured for the correct pins and baud rate, according to the layout of our nRF52840-DK board and the Virtual COM Port interface chip on the other end of the UART link.

Solution

pub struct Uarte {
    inner: hal::Uarte<hal::pac::UARTE1>,
}

Step 4: Bring up the peripheral in the fn init()

✅ Take a closer look at the definition of the uarte::Pins struct in the nrf-hal. Compare the pin type configurations with the ones you have already imported in lib.rs. Add the ones you're missing.

✅ Create an instance of this struct in fn init() with the appropriate pins and configurations. Set the output pin's level to Level::High. Note, that the third and fourth pin are each wrapped in an Option.

✅ Create a Uarte driver with hal::uarte::Uarte::new(...) and bind it to a variable called uarte - we will stash this in our own Uarte struct later.

Creating the Uarte driver requires four arguments:

• The UARTE1 instance can be found in the periph variable.
• Your instance of the uarte::Pins struct.
• Set parity to Parity::INCLUDED
• set the baud rate to Baudrate::BAUD115200.

Solution

  let pins =  hal::uarte::Pins {
            rxd: pins.p0_08.degrade().into_floating_input(),
            txd: pins.p0_06.degrade().into_push_pull_output(Level::High),
            cts: Some(pins.p0_07.degrade().into_floating_input()),
            rts: Some(pins.p0_05.degrade().into_push_pull_output(Level::High)),
        };
       

        let uarte = hal::uarte::Uarte::new(periph.UARTE1, pins, Parity::INCLUDED, Baudrate::BAUD115200);

Step 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().

Solution

pub struct Board {
    /// LEDs
    pub leds: Leds,
    /// Buttons
    pub buttons: Buttons,
    /// Timer
    pub timer: Timer,
    /// uarte interface
    pub uarte: Uarte,
}

// ...

pub fn init() -> Result<Board, ()> {

    // ... 

        Ok(Board {
            leds: Leds {
                // ...
            },

            buttons: Buttons {
                // ...
            },
            // 🔼  --- Button Exercise --- 🔼 

            timer: Timer { inner: timer },

            uarte: Uarte { inner: uarte },
        })
    } else {
        Err(())
    }

Step 6: Implementing the fmt::Write trait

We want to implement the fmt::Write trait so that users can call write! on our Uarte object

When implementing this, we can't just write to the Uarte instance because a simple write of a string literal would try and read the string literal from flash memory. This does not work because the EasyDMA peripheral in the nRF52 series can only access RAM, not flash.

Instead our implementation must ensure all the strings are copied to a stack allocated buffer and that buffer is passed to the Uarte's write method.

✅ Add use::core::fmt; to your imports.

✅ Create a public method write_str. It takes a mutable reference to self and a &str as argument. It returns an fmt::Result

✅ Create a buffer. The type is an array of 16 u8, set to all 0.

✅ To copy all data into an on-stack buffer, iterate over chunks of the string and copy them into the buffer (noting that the chunk length may be less than the requested size if you are at the end of the input string).

Solution

impl fmt::Write for Uarte {

    fn write_str(&mut self, s: &str) -> fmt::Result {
        // Copy all data into an on-stack buffer so we never try to EasyDMA from
        // flash.
        let mut buf: [u8; 16] = [0; 16];
        for block in s.as_bytes().chunks(16) {
            buf[..block.len()].copy_from_slice(block);
            self.inner.write(&buf[..block.len()]).map_err(|_| fmt::Error)?;
        }

        Ok(())
    }
}

Step 7: Connect your computer to the virtual UART

[todo!] [directions for mac present, linux and windows are missing.]

✅ Use the following command to find the address of the nRF52840-DK on your computer.

ls /dev/tty.usbmodem*

✅ Run the following command to run screen with the nRF52840-DK with 115200 baud.

screen <address of mc> 115200

Step 8: Run the example.

✅ In another terminal window go into the folder down-the-stack/apps and use the following command.

cargo run --bin uarte_print

On your terminal window where screen runs, "Hello, World" should appear.

You need to terminate screen manually.