rtc: add calibration functionality

Author: newAM

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Added
 - Added `rcc::Lsco` to use the low-speed oscillator output.
+- Added `Rtc::calibrate_lp` to calibrate the RTC.
+- Added `Rtc::recalibration_pending`.
 
 ## [0.5.1] - 2022-05-14
 ### Added

hal/src/rtc/mod.rs

@@ -6,6 +6,7 @@ pub use alarm::{Alarm, AlarmDay};
 
 use crate::{pac, rcc::lsi_hz};
 use chrono::{Datelike, NaiveDate, NaiveDateTime, NaiveTime, Timelike};
+use core::cmp::min;
 use pac::{
     rcc::{
         bdcr::RTCSEL_A,
@@ -356,6 +357,34 @@ impl Rtc {
         }
     }
 
+    /// Calibrate the RTC using the low-power mode.
+    ///
+    /// This does not poll for completion, use [`recalibration_pending`] if you
+    /// need to wait for completion.
+    ///
+    /// The calibration argument is in units of 0.9537 ppm.
+    /// The calibration range is -487.1 ppm to +488.5 ppm (-511 to 512), values
+    /// outside of this range will saturate.
+    ///
+    /// [`recalibration_pending`]: Self::recalibration_pending
+    pub fn calibrate_lp(&mut self, calibration: i16) {
+        while self.recalibration_pending() {}
+        let (calp, calm): (bool, u16) = if let Ok(calibration_pos) = u16::try_from(calibration) {
+            (true, 512_u16.saturating_sub(calibration_pos))
+        } else {
+            (false, min(calibration.abs_diff(0), 511))
+        };
+        self.rtc
+            .calr
+            .write(|w| w.calp().bit(calp).lpcal().set_bit().calm().bits(calm))
+    }
+
+    /// Returns `true` if recalibration is pending.
+    #[inline]
+    pub fn recalibration_pending(&self) -> bool {
+        self.rtc.icsr.read().recalpf().is_pending()
+    }
+
     /// Calendar Date
     ///
     /// Returns `None` if the calendar has not been initialized.