Added improvements with regards to SOF timing jitter

Author: skuep

src/portable/st/stm32_fsdev/dcd_stm32_fsdev.c

@@ -627,6 +627,13 @@ void dcd_int_handler(uint8_t rhport) {
   // dcd_ep_ctr_handler(), so less need to loop here. The other interrupts shouldn't
   // be triggered repeatedly.
 
+  /* Put SOF flag at the beginning of ISR in case to get least amount of jitter if it is used for timing purposes */
+  if(int_status & USB_ISTR_SOF) {
+    clear_istr_bits(USB_ISTR_SOF);
+    if (tu_stm32_sof_cb) tu_stm32_sof_cb();
+    dcd_event_sof(0, USB->FNR & USB_FNR_FN, true);
+  }
+
   if(int_status & USB_ISTR_RESET) {
     // USBRST is start of reset.
     clear_istr_bits(USB_ISTR_RESET);
@@ -664,11 +671,6 @@ void dcd_int_handler(uint8_t rhport) {
     dcd_event_bus_signal(0, DCD_EVENT_SUSPEND, true);
   }
 
-  if(int_status & USB_ISTR_SOF) {
-    clear_istr_bits(USB_ISTR_SOF);
-    dcd_event_sof(0, USB->FNR & USB_FNR_FN, true);
-  }
-
   if(int_status & USB_ISTR_ESOF) {
     if(remoteWakeCountdown == 1u)
     {

src/portable/st/stm32_fsdev/dcd_stm32_fsdev_pvt_st.h

@@ -130,6 +130,9 @@ static __IO uint16_t * const pma = (__IO uint16_t*)USB_PMAADDR;
 // this callback and defining a custom mapping in your application
 TU_ATTR_WEAK uint8_t tu_stm32_edpt_number_cb(uint8_t addr);
 
+// This callback is called on SOF and can be used to e.g. capture a timer value for timing purposes
+TU_ATTR_WEAK void tu_stm32_sof_cb(void);
+
 // prototypes
 TU_ATTR_ALWAYS_INLINE static inline __IO uint16_t* pcd_ep_rx_cnt_ptr(USB_TypeDef * USBx, uint32_t bEpNum);
 TU_ATTR_ALWAYS_INLINE static inline __IO uint16_t* pcd_ep_tx_cnt_ptr(USB_TypeDef * USBx, uint32_t bEpNum);