diff --git a/ports/stm32/boards/stm32f4xx_prefix.c b/ports/stm32/boards/stm32f4xx_prefix.c
index f4ffdab68e..3bcd6e6410 100644
--- a/ports/stm32/boards/stm32f4xx_prefix.c
+++ b/ports/stm32/boards/stm32f4xx_prefix.c
@@ -14,7 +14,7 @@
     .fn = AF_FN_ ## af_fn, \
     .unit = (af_unit), \
     .type = AF_PIN_TYPE_ ## af_fn ## _ ## af_type, \
-    .af_fn = (af_ptr) \
+    .reg = (af_ptr) \
 }
 
 #define PIN(p_port, p_pin, p_af, p_adc_num, p_adc_channel) \
diff --git a/ports/stm32/pin.h b/ports/stm32/pin.h
index b0e05256fe..ea57b0a274 100644
--- a/ports/stm32/pin.h
+++ b/ports/stm32/pin.h
@@ -39,12 +39,7 @@ typedef struct {
   uint8_t fn;
   uint8_t unit;
   uint8_t type;
-
-  union {
-    void          *reg;
-
-    PIN_DEFS_PORT_AF_UNION
-  };
+  void *reg; // The peripheral associated with this AF
 } pin_af_obj_t;
 
 typedef struct {
diff --git a/ports/stm32/pin_defs_stm32.h b/ports/stm32/pin_defs_stm32.h
index c5b2862835..feaf56ae97 100644
--- a/ports/stm32/pin_defs_stm32.h
+++ b/ports/stm32/pin_defs_stm32.h
@@ -115,17 +115,5 @@ enum {
   PIN_ADC3  = (1 << 2),
 };
 
-// Note that SPI and I2S are really the same peripheral as far as the HAL
-// is concerned, so there is no I2S_TypeDef.
-// We use void* for SDMMC because not all MCUs have the SDMMC_TypeDef type.
-#define PIN_DEFS_PORT_AF_UNION \
-    TIM_TypeDef   *TIM; \
-    I2C_TypeDef   *I2C; \
-    USART_TypeDef *USART; \
-    USART_TypeDef *UART; \
-    SPI_TypeDef   *SPI;\
-    SPI_TypeDef   *I2S; \
-    void          *SDMMC; \
-
 typedef GPIO_TypeDef pin_gpio_t;
 
diff --git a/ports/teensy/mk20dx256_prefix.c b/ports/teensy/mk20dx256_prefix.c
index d8e7480b58..58ab07d6ec 100644
--- a/ports/teensy/mk20dx256_prefix.c
+++ b/ports/teensy/mk20dx256_prefix.c
@@ -15,7 +15,7 @@
     .fn = AF_FN_ ## af_fn, \
     .unit = (af_unit), \
     .type = AF_PIN_TYPE_ ## af_fn ## _ ## af_type, \
-    .af_fn = (af_ptr) \
+    .reg = (af_ptr) \
 }
 
 #define PIN(p_port, p_pin, p_num_af, p_af, p_adc_num, p_adc_channel) \
diff --git a/ports/teensy/pin_defs_teensy.h b/ports/teensy/pin_defs_teensy.h
index 54a6055f1a..d3a700be21 100644
--- a/ports/teensy/pin_defs_teensy.h
+++ b/ports/teensy/pin_defs_teensy.h
@@ -40,10 +40,4 @@ enum {
     AF_PIN_TYPE_UART_RTS,
 };
 
-#define PIN_DEFS_PORT_AF_UNION \
-    FTM_TypeDef   *FTM; \
-    I2C_TypeDef   *I2C; \
-    UART_TypeDef  *UART; \
-    SPI_TypeDef   *SPI;
-
 typedef GPIO_TypeDef pin_gpio_t;