tpiu: bitfield ACPR, verify set_swo_baud_rate args and ops

src/peripheral/tpiu.rs

@@ -16,7 +16,7 @@ pub struct RegisterBlock {
  pub cspsr: RW<u32>,
  reserved0: [u32; 2],
  /// Asynchronous Clock Prescaler
- pub acpr: RW<u32>,
+ pub acpr: RW<Acpr>,
  reserved1: [u32; 55],
  /// Selected Pin Control
  pub sppr: RW<Sppr>,
@@ -41,6 +41,14 @@ bitfield! {
  enfcont, set_enfcont: 1;
 }
 
+bitfield! {
+ /// TPIU ACPR Register.
+ #[repr(C)]
+ #[derive(Clone, Copy)]
+ pub struct Acpr(u32);
+ u16, swoscaler, set_swoscaler: 15, 0;
+}
+
 bitfield! {
  /// TPIU Type Register.
  #[repr(C)]
@@ -101,15 +109,54 @@ pub struct SWOSupports {
  pub min_queue_size: u8,
 }
 
+/// Possible errors on [`TPIU::set_swo_baud_rate`].
+#[derive(Debug, Eq, PartialEq, Copy, Clone)]
+pub enum ACPRError {
+ /// The reference clock frequency divided by the requested baud rate
+ /// did not yield an integer.
+ NonInteger,
+ /// Required prescaler value is too large. Largest supported
+ /// prescaler value is [`u16::MAX`].
+ TooLarge,
+}
+
 impl TPIU {
  /// Sets the prescaler value for a wanted baud rate of the Serial
  /// Wire Output (SWO) in relation to a given asynchronous refernce
- /// clock rate.
+ /// clock rate. Returns `true` if a prescaler was correctly
+ /// calculated and applied, `false` otherwise.
+ ///
+ /// See C1.10.4 "Asynchronous Clock Prescaler Register, TPIU_ACPR".
  #[inline]
- pub fn set_swo_baud_rate(&mut self, ref_clk_rate: u32, baud_rate: u32) {
+ pub fn set_swo_baud_rate(
+ &mut self,
+ ref_clk_rate: u32,
+ baud_rate: u32,
+ ) -> Result<(), ACPRError> {
+ use ACPRError as Error;
+
+ if ref_clk_rate % baud_rate != 0 {
+ return Err(Error::NonInteger);
+ }
+
+ use core::convert::TryInto;
+ let prescaler: u16 = match { ((ref_clk_rate / baud_rate) - 1).try_into() } {
+ Ok(ps) => ps,
+ Err(_) => return Err(Error::TooLarge),
+ };
+
  unsafe {
- self.acpr.write((ref_clk_rate / baud_rate) - 1);
+ self.acpr.modify(|mut r| {
+ r.set_swoscaler(prescaler);
+ r
+ });
+ }
+
+ if self.acpr.read().swoscaler() != prescaler {
+ return Err(Error::TooLarge);
  }
+
+ Ok(())
  }
 
  /// The used protocol for the trace output. Return `None` if an