Use HSI48 and clock recovery system to clock USB

Author: kevswims

examples/usb_serial.rs

@@ -14,6 +14,8 @@ use hal::rcc::PllMDiv;
 use hal::rcc::PllNMul;
 use hal::rcc::PllQDiv;
 use hal::rcc::PllRDiv;
+use hal::usb::configure_usb_clock_source;
+use hal::usb::ClockSource;
 use panic_probe as _;
 
 use stm32g4 as _;
@@ -32,6 +34,8 @@ pub fn exit() -> ! {
 
 use hal::rcc::{Config, PLLSrc, Prescaler};
 
+use hal::rcc::clock_recovery_system::{CrsConfig, CrsExt};
+
 use stm32g4xx_hal as hal;
 
 use hal::prelude::*;
@@ -43,8 +47,6 @@ use usbd_serial::{SerialPort, USB_CLASS_CDC};
 
 #[cortex_m_rt::entry]
 fn main() -> ! {
-    // utils::logger::init();
-
     let dp = stm32::Peripherals::take().unwrap();
 
     let rcc = dp.RCC.constrain();
@@ -70,12 +72,11 @@ fn main() -> ! {
             .apb2_psc(Prescaler::Div2),
     );
 
-    {
-        use crate::stm32::RCC;
-        let rcc = unsafe { &*RCC::ptr() };
-        // Set clock source for USB to PLL
-        rcc.ccipr.modify(|_, w| w.clk48sel().pllq());
-    }
+    // Example of setting up the HSI48 with the clock recovery system.
+    let crs = dp.CRS.constrain();
+    let crs_config = CrsConfig {};
+    crs.configure(crs_config, &rcc);
+    configure_usb_clock_source(ClockSource::Hsi48, &rcc);
 
     // Configure an LED
     let gpioa = dp.GPIOA.split(&mut rcc);

src/rcc/clock_recovery_system.rs

@@ -0,0 +1,51 @@
+use crate::stm32::CRS;
+
+use super::Rcc;
+
+use crate::rcc::Enable;
+
+pub struct CrsConfig {}
+
+pub struct Crs {
+    rb: CRS,
+}
+
+impl Crs {
+    /// Sets up the clock recovery system for the HSI48 oscillator.
+    /// TODO: make this configurable for more than just USB applications.
+    pub fn configure(self, crs_config: CrsConfig, rcc: &Rcc) -> Self {
+        // TODO: This needs to ensure that the HSI48 is enabled
+        // and then setup the CRS. For now this just needs to use
+        // the USB sync as the trigger system.
+
+        rcc.enable_hsi48();
+
+        // Enable the clock recovery system
+        CRS::enable(&rcc.rb);
+
+        // Set to b10 for USB SOF as source
+        self.rb
+            .cfgr
+            .modify(|_, w| unsafe { w.syncsrc().bits(0b10) });
+
+        self.rb.cr.modify(|_, w| {
+            // Set autotrim enabled.
+            w.autotrimen().set_bit();
+            // Enable CRS
+            w.cen().set_bit()
+        });
+
+        self
+    }
+}
+
+pub trait CrsExt {
+    /// Constrains the `CRS` peripheral so that it can only be setup once.
+    fn constrain(self) -> Crs;
+}
+
+impl CrsExt for CRS {
+    fn constrain(self) -> Crs {
+        Crs { rb: self }
+    }
+}

src/rcc/mod.rs

@@ -1,6 +1,7 @@
 use crate::stm32::{rcc, FLASH, PWR, RCC};
 use crate::time::{Hertz, U32Ext};
 
+pub mod clock_recovery_system;
 mod clockout;
 mod config;
 mod enable;
@@ -290,6 +291,16 @@ impl Rcc {
         self.rb.csr.modify(|_, w| w.lsion().set_bit());
         while self.rb.csr.read().lsirdy().bit_is_clear() {}
     }
+
+    pub(crate) fn enable_hsi48(&self) {
+        self.rb.crrcr.modify(|_, w| w.hsi48on().set_bit());
+
+        loop {
+            if self.rb.crrcr.read().hsi48rdy().bit() {
+                break;
+            }
+        }
+    }
 }
 
 /// Extension trait that constrains the `RCC` peripheral

src/usb.rs

@@ -12,6 +12,8 @@ use stm32_usbd::UsbPeripheral;
 
 pub use stm32_usbd::UsbBus;
 
+use crate::rcc::Rcc;
+
 pub struct Peripheral {
     pub usb: USB,
 }
@@ -44,4 +46,17 @@ unsafe impl UsbPeripheral for Peripheral {
     }
 }
 
+pub enum ClockSource {
+    Hsi48,
+    PllQ,
+}
+
+#[inline(always)]
+pub fn configure_usb_clock_source(cs: ClockSource, rcc: &Rcc) {
+    rcc.rb.ccipr.modify(|_, w| match cs {
+        ClockSource::Hsi48 => w.clk48sel().hsi48(),
+        ClockSource::PllQ => w.clk48sel().pllq(),
+    });
+}
+
 pub type UsbBusType = UsbBus<Peripheral>;