diff --git a/CHANGELOG.md b/CHANGELOG.md
index 427404c76..6f8edb0f8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - SPI4 peripheral for supported
   devices. ([#99](https://github.com/stm32-rs/stm32f3xx-hal/pull/99))
 - Support for I2C transfer of more than 255 bytes, and 0 byte write ([#154](https://github.com/stm32-rs/stm32f3xx-hal/pull/154))
+- Support for HSE bypass and CSS ([#156](https://github.com/stm32-rs/stm32f3xx-hal/pull/156))
 
 ### Changed
 
diff --git a/src/rcc.rs b/src/rcc.rs
index d6215fe84..6d543fcfd 100644
--- a/src/rcc.rs
+++ b/src/rcc.rs
@@ -21,13 +21,7 @@ impl RccExt for RCC {
             apb1: APB1 { _0: () },
             apb2: APB2 { _0: () },
             bdcr: BDCR { _0: () },
-            cfgr: CFGR {
-                hse: None,
-                hclk: None,
-                pclk1: None,
-                pclk2: None,
-                sysclk: None,
-            },
+            cfgr: CFGR::default(),
         }
     }
 }
@@ -87,7 +81,7 @@ impl AHB {
 /// ```
 /// let dp = pac::Peripherals::take().unwrap();
 /// let rcc = dp.RCC.constrain();
-/// use_ahb(&mut rcc.apb1)
+/// use_apb1(&mut rcc.apb1)
 /// ```
 pub struct APB1 {
     _0: (),
@@ -112,7 +106,7 @@ impl APB1 {
 /// ```
 /// let dp = pac::Peripherals::take().unwrap();
 /// let rcc = dp.RCC.constrain();
-/// use_ahb(&mut rcc.apb2)
+/// use_apb2(&mut rcc.apb2)
 /// ```
 pub struct APB2 {
     _0: (),
@@ -212,10 +206,13 @@ impl BDCR {
 /// ```
 /// let dp = pac::Peripherals::take().unwrap();
 /// let rcc = dp.RCC.constrain();
-/// use_ahb(&mut rcc.cfgr)
+/// use_cfgr(&mut rcc.cfgr)
 /// ```
+#[derive(Default)]
 pub struct CFGR {
     hse: Option<u32>,
+    hse_bypass: bool,
+    css: bool,
     hclk: Option<u32>,
     pclk1: Option<u32>,
     pclk2: Option<u32>,
@@ -296,6 +293,24 @@ impl CFGR {
         self
     }
 
+    /// Enable HSE bypass.
+    /// Uses user provided clock signal instead of an external oscillator.
+    /// OSC_OUT pin is free and can be used as GPIO.
+    /// No effect if HSE is not enabled.
+    pub fn bypass_hse(mut self) -> Self {
+        self.hse_bypass = true;
+        self
+    }
+
+    /// Enable CSS (Clock Security System).
+    /// System clock is automatically switched to HSI and an interrupt (CSSI) is generated
+    /// when HSE clock failure is detected.
+    /// No effect if HSE is not enabled.
+    pub fn enable_css(mut self) -> Self {
+        self.css = true;
+        self
+    }
+
     /// Sets a frequency for the AHB bus
     pub fn hclk<F>(mut self, freq: F) -> Self
     where
@@ -376,10 +391,11 @@ impl CFGR {
             }
 
             // PLL_MUL maximal value is 16
-            assert!(divisor <= 16);
-            // PRE_DIV maximal value is 16
             assert!(multiplier <= 16);
 
+            // PRE_DIV maximal value is 16
+            assert!(divisor <= 16);
+
             (multiplier, Some(divisor))
         }
         // HSI division is always divided by 2 and has no adjustable division
@@ -583,9 +599,13 @@ impl CFGR {
 
         let rcc = unsafe { &*RCC::ptr() };
 
+        // enable HSE and wait for it to be ready
         if self.hse.is_some() {
-            // enable HSE and wait for it to be ready
-            rcc.cr.modify(|_, w| w.hseon().on());
+            rcc.cr.modify(|_, w| {
+                w.hsebyp().bit(self.hse_bypass);
+                w.csson().bit(self.css);
+                w.hseon().on()
+            });
 
             while rcc.cr.read().hserdy().is_not_ready() {}
         }