3 files changed, 10 insertions, 4 deletions
diff --git a/os/hal/platforms/STM32F2xx/adc_lld.c b/os/hal/platforms/STM32F2xx/adc_lld.c
index fd91ba76a..6d889d4c5 100644
--- a/os/hal/platforms/STM32F2xx/adc_lld.c
+++ b/os/hal/platforms/STM32F2xx/adc_lld.c
@@ -174,8 +174,6 @@ CH_IRQ_HANDLER(ADC1_2_3_IRQHandler) {
  */
 void adc_lld_init(void) {
 
-  ADC->CCR = STM32_ADC_ADCPRE;
-
 #if STM32_ADC_USE_ADC1
   /* Driver initialization.*/
   adcObjectInit(&ADCD1);
@@ -270,6 +268,10 @@ void adc_lld_start(ADCDriver *adcp) {
     }
 #endif /* STM32_ADC_USE_ADC3 */
 
+    /* This is a common register but apparently it requires that at least one
+       of the ADCs is clocked in order to allow writing, see bug 3575297.*/
+    ADC->CCR = STM32_ADC_ADCPRE << 16;
+
     /* ADC initial setup, starting the analog part here in order to reduce
        the latency when starting a conversion.*/
     adcp->adc->CR1 = 0;
diff --git a/os/hal/platforms/STM32F4xx/adc_lld.c b/os/hal/platforms/STM32F4xx/adc_lld.c
index bf0788dea..9f447f9a6 100644
--- a/os/hal/platforms/STM32F4xx/adc_lld.c
+++ b/os/hal/platforms/STM32F4xx/adc_lld.c
@@ -174,8 +174,6 @@ CH_IRQ_HANDLER(ADC1_2_3_IRQHandler) {
  */
 void adc_lld_init(void) {
 
-  ADC->CCR = STM32_ADC_ADCPRE;
-
 #if STM32_ADC_USE_ADC1
   /* Driver initialization.*/
   adcObjectInit(&ADCD1);
@@ -270,6 +268,10 @@ void adc_lld_start(ADCDriver *adcp) {
     }
 #endif /* STM32_ADC_USE_ADC3 */
 
+    /* This is a common register but apparently it requires that at least one
+       of the ADCs is clocked in order to allow writing, see bug 3575297.*/
+    ADC->CCR = STM32_ADC_ADCPRE << 16;
+
     /* ADC initial setup, starting the analog part here in order to reduce
        the latency when starting a conversion.*/
     adcp->adc->CR1 = 0;
diff --git a/readme.txt b/readme.txt
index 37aaa7d02..f1a7289da 100644
--- a/readme.txt
+++ b/readme.txt
@@ -83,6 +83,8 @@
 *****************************************************************************
 
 *** 2.5.1 ***
+- FIX: Fixed STM32F4 ADC prescaler incorrectly initialized (bug 3575297)
+  (backported to 2.4.3).
 - FIX: Fixed RCC_APB2ENR_IOPEEN undeclared on STM32F10X_LD_VL devices (bug
   3575098)(backported to 2.4.3).
 - FIX: Fixed misplaced declarations in lwip_bindings sys_arch.c (bug 3571053)