fix: set only AF mode for mcu with single AF

Fixes #1798

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/stm32/stm32_def.h

@@ -90,6 +90,20 @@
   #define PinMap_USB PinMap_USB_DRD_FS
 #endif
 
+/**
+ * Some mcu have single AF and thus only AF mode should be configured.
+ * No AFRL/AFRG registers exists so they should not be configured.
+ * In that case the AF does not exists so defining the linked AF
+ * to 0x7F (max value of the AFNUM i.e. STM_PIN_AFNUM_MASK)
+ * See GitHub issue #1798.
+ */
+#if defined(STM32F0xx) && !defined(GPIO_AF0_TIM3)
+  #define GPIO_AF0_TIM3 STM_PIN_AFNUM_MASK
+#endif
+#if defined(STM32L0xx) && !defined(GPIO_AF1_SPI1)
+  #define GPIO_AF1_SPI1 STM_PIN_AFNUM_MASK
+#endif
+
 /**
  * Libc porting layers
  */

libraries/SrcWrapper/src/stm32/pinmap.c

@@ -242,7 +242,9 @@ void pin_function(PinName pin, int function)
     case STM_PIN_ALTERNATE:
       ll_mode = LL_GPIO_MODE_ALTERNATE;
       /* In case of ALT function, also set the afnum */
-      pin_SetAFPin(gpio, pin, afnum);
+      if (afnum != STM_PIN_AFNUM_MASK) {
+        pin_SetAFPin(gpio, pin, afnum);
+      }
       break;
     case STM_PIN_ANALOG:
       ll_mode = LL_GPIO_MODE_ANALOG;