diff --git a/include/libopencm3/stm32/f0/adc.h b/include/libopencm3/stm32/f0/adc.h
index 0510fd2c..9aeddd14 100644
--- a/include/libopencm3/stm32/f0/adc.h
+++ b/include/libopencm3/stm32/f0/adc.h
@@ -78,8 +78,7 @@
 
 /* EXTSEL[2:0]: External trigger selection for regular group */
 #define ADC_CFGR1_EXTSEL_SHIFT		6
-#define ADC_CFGR1_EXTSEL		(0x7 << ADC_CFGR1_EXTSEL_SHIFT)
-#define ADC_CFGR1_EXTSEL_VAL(x)		((x) << ADC_CFGR1_EXTSEL_SHIFT)
+#define ADC_CFGR1_EXTSEL_MASK		0x7
 /** @defgroup adc_cfgr1_extsel ADC external trigger selection values
  *@{*/
 #define ADC_CFGR1_EXTSEL_TIM1_TRGO	0x0
diff --git a/lib/stm32/f0/adc.c b/lib/stm32/f0/adc.c
index c6165e39..d8ae6a4e 100644
--- a/lib/stm32/f0/adc.c
+++ b/lib/stm32/f0/adc.c
@@ -155,18 +155,18 @@ void adc_set_operation_mode(uint32_t adc, enum adc_opmode opmode)
  * sets the polarity of the trigger event: rising or falling edge or both. Note
  * that if the trigger polarity is zero, triggering is disabled.
  *
- * @param[in] adc Unsigned int32. ADC base address (@ref adc_reg_base)
- * @param[in] trigger Unsigned int32. Trigger identifier
- * @ref adc_trigger_regular
- * @param[in] polarity Unsigned int32. Trigger polarity @ref
- * adc_trigger_polarity_regular
+ * @param[in] adc peripheral of choice (@ref adc_reg_base)
+ * @param[in] trigger external trigger @ref adc_cfgr1_extsel
+ * @param[in] polarity Trigger polarity @ref adc_cfgr1_exten
  */
-
 void adc_enable_external_trigger_regular(uint32_t adc, uint32_t trigger,
 				 uint32_t polarity)
 {
-	ADC_CFGR1(adc) = (ADC_CFGR1(adc) & ~ADC_CFGR1_EXTSEL) | trigger;
-	ADC_CFGR1(adc) = (ADC_CFGR1(adc) & ~ADC_CFGR1_EXTEN_MASK) | polarity;
+	uint32_t reg = ADC_CFGR1(adc);
+	reg &= ~(ADC_CFGR1_EXTSEL_MASK << ADC_CFGR1_EXTSEL_SHIFT);
+	reg &= ~(ADC_CFGR1_EXTEN_MASK);
+	reg |= polarity | (trigger << ADC_CFGR1_EXTSEL_SHIFT);
+	ADC_CFGR1(adc) = reg;
 }
 
 /*---------------------------------------------------------------------------*/