Implements continuous sampling for the nRF SAADC

Implements continuous sampling for the nRF SAADC and also renames `OneShot` to `Saadc`. The one-shot behaviour is retained with the `sample` method and a new `run_sampler` method is provided for efficiently (i.e. zero copying) sampler processing. A double buffer is used for continuously sampling, which wlll be swapped once sampling has taken place.

A sample frequency is provided and will set the internal timer of the SAADC when there is just the one channel being sampled. Otherwise, PPI will be used to hook up the TIMER peripheral to drive the sampling task.

examples/nrf/src/bin/saadc_continuous.rs

@@ -0,0 +1,49 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+#[path = "../example_common.rs"]
+mod example_common;
+use defmt::panic;
+use embassy::executor::Spawner;
+use embassy_nrf::ppi::Ppi;
+use embassy_nrf::saadc::{ChannelConfig, Config, Mode, Saadc, SamplerState};
+use embassy_nrf::timer::{Frequency, Timer};
+use embassy_nrf::{interrupt, Peripherals};
+use example_common::*;
+
+// Demonstrates both continuous sampling and scanning multiple channels driven by a PPI linked timer
+
+#[embassy::main]
+async fn main(_spawner: Spawner, mut p: Peripherals) {
+    let config = Config::default();
+    let channel_1_config = ChannelConfig::single_ended(&mut p.P0_02);
+    let channel_2_config = ChannelConfig::single_ended(&mut p.P0_03);
+    let channel_3_config = ChannelConfig::single_ended(&mut p.P0_04);
+    let mut saadc = Saadc::new(
+        p.SAADC,
+        interrupt::take!(SAADC),
+        config,
+        [channel_1_config, channel_2_config, channel_3_config],
+    );
+
+    let mut timer = Timer::new(p.TIMER0);
+    timer.set_frequency(Frequency::F1MHz);
+    timer.cc(0).write(100); // We want to sample at 10KHz
+    timer.cc(0).short_compare_clear();
+
+    let mut ppi = Ppi::new(p.PPI_CH0);
+    ppi.set_event(timer.cc(0).event_compare());
+    ppi.set_task(saadc.task_sample());
+    ppi.enable();
+
+    timer.start();
+
+    let mut bufs = [[0; 3 * 500]; 2]; // Each buffer of the double buffer has to be large enough for all channels.
+    saadc
+        .run_sampler(&mut bufs, Mode::Task, |buf| {
+            info!("sample len={}", buf.len());
+            SamplerState::Sampled
+        })
+        .await;
+}