cpu/stm32l1: fix ADC initialization & resolution setting

Co-authored-by: benpicco <benpicco@googlemail.com>

cpu/stm32/periph/adc_l1.c

@@ -131,8 +131,6 @@ int adc_init(adc_t line)
         ADC->CCR |= ADC_CCR_TSVREFE;
     }
 
-    /* enable the ADC module */
-    ADC1->CR2 = ADC_CR2_ADON;
     /* turn off during idle phase*/
     ADC1->CR1 = ADC_CR1_PDI;
 
@@ -157,12 +155,18 @@ int32_t adc_sample(adc_t line, adc_res_t res)
     /* lock and power on the ADC device  */
     prep();
 
-    /* set resolution, conversion channel and single read */
+    /* mask and set resolution, conversion channel and single read */
+    ADC1->CR1 &= ~ADC_CR1_RES_Msk;
     ADC1->CR1 |= res & ADC_CR1_RES;
     ADC1->SQR1 &= ~ADC_SQR1_L;
     ADC1->SQR5 = adc_config[line].chan;
 
-    /* wait for regulat channel to be ready*/
+    /* only set ADON when ADONS bit is cleared (ADC not ready) */
+    if (!(ADC1->SR & ADC_SR_ADONS_Msk)) {
+        ADC1->CR2 |= ADC_CR2_ADON;
+    }
+
+    /* wait for regular channel to be ready*/
     while (!(ADC1->SR & ADC_SR_RCNR)) {}
     /* start conversion and wait for results */
     ADC1->CR2 |= ADC_CR2_SWSTART;
@@ -171,6 +175,10 @@ int32_t adc_sample(adc_t line, adc_res_t res)
     sample = (int)ADC1->DR;
     ADC1 -> SR &= ~ADC_SR_STRT;
 
+    /* wait for ADC to become ready before disabling it */
+    while (!(ADC1->SR & ADC_SR_ADONS)) {}
+    ADC1->CR2 &= ~ADC_CR2_ADON_Msk;
+
     /* power off and unlock device again */
     done();