SAI example with wolfson 8960 codec

Provides a sample application for playing back audio without DMA
directly writing to the FIFO of the SAI peripheral, which should result
in an I2S signaling to the WM8960 codec on the EVK boards.

examples/rtic_sai_wm8960.rs

@@ -0,0 +1,87 @@
+//! Audio playback using sai peripheral and imxrt-hal.
+//!
+//! Plays back a simple 440Hz (A note) simple square wave tone with the SAI peripheral
+//! to a Wolfson WM8960 codec on a number of the EVK boards.
+//!
+//! The audio stream itself is expected to be a 48000Hz 16bit stereo signal.
+
+#![no_main]
+#![no_std]
+#[rtic::app(device = board, peripherals = false, dispatchers = [BOARD_SWTASK0])]
+mod app {
+
+    //
+    // Configure the demo below.
+    //
+
+    /// How frequently (milliseconds) should we poll audio
+    const AUDIO_POLL_MS: u32 = board::PIT_FREQUENCY / 1_000 * 250;
+
+    use imxrt_hal as hal;
+    //
+    // End configurations.
+    //
+
+    #[local]
+    struct Local {
+        /// Toggle when we poll.
+        led: board::Led,
+        /// This timer tells us how frequently work on audio.
+        audio_pit: hal::pit::Pit<2>,
+    }
+
+    #[shared]
+    struct Shared {}
+
+    #[init]
+    fn init(cx: init::Context) -> (Shared, Local, init::Monotonics) {
+        let mut cortex_m = cx.core;
+        let (
+            board::Common {
+                pit: (_, _, mut audio_pit, _),
+                ..
+            },
+            board::Specifics { led, .. },
+        ) = board::new();
+        cortex_m.DCB.enable_trace();
+        cortex_m::peripheral::DWT::unlock();
+        cortex_m.DWT.enable_cycle_counter();
+
+        audio_pit.set_load_timer_value(AUDIO_POLL_MS);
+        audio_pit.set_interrupt_enable(true);
+        audio_pit.enable();
+
+        (Shared {}, Local { led, audio_pit }, init::Monotonics())
+    }
+
+    #[task(binds = BOARD_PIT, local = [led, audio_pit, counter: u32 = 0], priority = 1)]
+    fn pit_interrupt(cx: pit_interrupt::Context) {
+        let pit_interrupt::LocalResources {
+            audio_pit,
+            led,
+            counter,
+        } = cx.local;
+
+        // Is it time for us to send a new log message?
+        if audio_pit.is_elapsed() {
+            led.toggle();
+            while audio_pit.is_elapsed() {
+                audio_pit.clear_elapsed();
+            }
+
+            let count = cycles(|| {});
+
+            defmt::println!("Audio synthesis took {=u32} cycles", count);
+
+            *counter += 1;
+        }
+    }
+
+    /// Count the clock cycles required to execute `f`
+    fn cycles<F: FnOnce()>(f: F) -> u32 {
+        let start = cortex_m::peripheral::DWT::cycle_count();
+        f();
+        let end = cortex_m::peripheral::DWT::cycle_count();
+        end - start
+    }
+}