Accelerometer

Our last sensor is a 3-axis Accelerometer. It has many more features than we'll use during the exercise, but it can read acceleration in three axis: X, Y, and Z.

It reads accleration, e.g. due to gravity, as a positive number. It can measure up to 8g of acceleration, returning i16::MAX for 8.0g, or 4096 as 1.0g, or -4096 for -1.0g.

If the board is sitting level, it should read approximately:

• x: 0
• y: 0
• z: 4096

If you tilt the board so the potentometer is facing RIGHT, it should read approximately:

• x: 4096
• y: 0
• z: 0

If you tilt the board so the potentiometer is facing AWAY from you, it should read approximately:

• x: 0
• y: 4096
• z: 0

Running the code

We can start the code by running the hello-03 project. It will begin immediately printing out acceleration values at 4Hz.

cargo run --release --bin hello-03
    Finished release [optimized + debuginfo] target(s) in 0.10s
     Running `probe-rs run --chip RP2040 --speed 12000 --protocol swd target/thumbv6m-none-eabi/release/hello-03`
      Erasing ✔ [00:00:00] [######################################################] 40.00 KiB/40.00 KiB @ 75.76 KiB/s (eta 0s )
  Programming ✔ [00:00:00] [#####################################################] 40.00 KiB/40.00 KiB @ 132.75 KiB/s (eta 0s )    Finished in 0.841s
 WARN defmt_decoder::log::format: logger format contains timestamp but no timestamp implementation was provided; consider removing the timestamp (`{t}` or `{T}`) from the logger format or provide a `defmt::timestamp!` implementation
0.000976 INFO  Start
└─ hello_03::____embassy_main_task::{async_fn#0} @ src/bin/hello-03.rs:38
0.002457 INFO  id: E4629076D3222C21
└─ hello_03::____embassy_main_task::{async_fn#0} @ src/bin/hello-03.rs:42
0.253936 INFO  accelerometer: AccelReading { x: -80, y: 0, z: 4064 }
└─ hello_03::__accel_task_task::{async_fn#0} @ src/bin/hello-03.rs:82
0.503916 INFO  accelerometer: AccelReading { x: -112, y: -16, z: 4048 }
└─ hello_03::__accel_task_task::{async_fn#0} @ src/bin/hello-03.rs:82
0.753916 INFO  accelerometer: AccelReading { x: -112, y: 16, z: 4048 }
└─ hello_03::__accel_task_task::{async_fn#0} @ src/bin/hello-03.rs:82

Reading the code

Similar to the other exercises, we've added some new tasks:

#![allow(unused)]
fn main() {
// in main
let accel = Accelerometer::new(
    p.SPI0, p.PIN_6, p.PIN_7, p.PIN_4, p.PIN_5, p.DMA_CH1, p.DMA_CH2,
)
.await;

// as a task
#[embassy_executor::task]
async fn accel_task(mut accel: Accelerometer) {
    let mut ticker = Ticker::every(Duration::from_millis(250));
    loop {
        ticker.next().await;
        let reading = accel.read().await;
        info!("accelerometer: {:?}", reading);
    }
}
}

One thing to note is that the constructor, Accelerometer::new() is an async function. This is because the driver establishes the connection, and ensures we are talking to the accelerometer using async SPI methods.

You can access the raw driver through the lis3dh-async crate.

We have also wired up the accelerometer's interrupt pins, which can serve as a "notification" when some event has happened, however we will not use that as part of the exercise today.