STM32: analog_out: Enable Buffer and switch

ADC1 channel2 and ADC2 of few targets only have an output switch and no
buffer. This switch needs to be enabled, and also the buffer can be enabled
in order to reduce the output impedance on output, and to drive external
loads directly without having to add an external operational amplifier.

Author: LMESTM

targets/TARGET_STM/TARGET_STM32F3/analogout_api.c

@@ -87,8 +87,15 @@ void analogout_init(dac_t *obj, PinName pin) {
     obj->handle.Instance = (DAC_TypeDef *)(obj->dac);
 
     sConfig.DAC_Trigger      = DAC_TRIGGER_NONE;
-    sConfig.DAC_OutputBuffer = DAC_OUTPUTBUFFER_DISABLE;
-
+    /* Enable both Buffer and Switch in the configuration,
+     * letting HAL layer in charge of selecting either one
+     * or the other depending on the actual DAC instance and
+     * channel being configured.
+     */
+    sConfig.DAC_OutputBuffer = DAC_OUTPUTBUFFER_ENABLE;
+#if defined(DAC_OUTPUTSWITCH_ENABLE)
+    sConfig.DAC_OutputSwitch = DAC_OUTPUTSWITCH_ENABLE;
+#endif
 
     if (pin == PA_4) {
         pa4_used = 1;