diff --git a/src/machine/machine_atsamd21.go b/src/machine/machine_atsamd21.go
index 69abee25..c01b69b9 100644
--- a/src/machine/machine_atsamd21.go
+++ b/src/machine/machine_atsamd21.go
@@ -1646,6 +1646,7 @@ func (usbcdc *USBCDC) Flush() error {
 
 			if usbcdc.waitTxc {
 				// waiting for the next flush(), because the transmission is not complete
+				usbcdc.waitTxcRetryCount++
 				return nil
 			}
 			usbcdc.waitTxc = true
@@ -1700,7 +1701,7 @@ func (usbcdc *USBCDC) WriteByte(c byte) error {
 				mask := interrupt.Disable()
 				UART0.waitTxc = false
 				UART0.waitTxcRetryCount = 0
-				usbcdc.TxIdx.Set(0)
+				UART0.TxIdx.Set(0)
 				usbLineInfo.lineState = 0
 				interrupt.Restore(mask)
 				break
@@ -1868,6 +1869,7 @@ func handleUSB(intr interrupt.Interrupt) {
 
 	// Start of frame
 	if (flags & sam.USB_DEVICE_INTFLAG_SOF) > 0 {
+		UART0.Flush()
 		// if you want to blink LED showing traffic, this would be the place...
 	}
 
@@ -1931,13 +1933,7 @@ func handleUSB(intr interrupt.Interrupt) {
 				}
 			}
 		}
-
-		if i == usb_CDC_ENDPOINT_IN && UART0.waitTxc {
-			UART0.waitTxcRetryCount++
-		}
 	}
-
-	UART0.Flush()
 }
 
 func initEndpoint(ep, config uint32) {
diff --git a/src/machine/machine_atsamd51.go b/src/machine/machine_atsamd51.go
index 5cb7df40..97165874 100644
--- a/src/machine/machine_atsamd51.go
+++ b/src/machine/machine_atsamd51.go
@@ -1850,6 +1850,7 @@ func (usbcdc *USBCDC) Flush() error {
 
 			if usbcdc.waitTxc {
 				// waiting for the next flush(), because the transmission is not complete
+				usbcdc.waitTxcRetryCount++
 				return nil
 			}
 			usbcdc.waitTxc = true
@@ -1904,7 +1905,7 @@ func (usbcdc *USBCDC) WriteByte(c byte) error {
 				mask := interrupt.Disable()
 				UART0.waitTxc = false
 				UART0.waitTxcRetryCount = 0
-				usbcdc.TxIdx.Set(0)
+				UART0.TxIdx.Set(0)
 				usbLineInfo.lineState = 0
 				interrupt.Restore(mask)
 				break
@@ -2074,6 +2075,7 @@ func handleUSBIRQ(interrupt.Interrupt) {
 
 	// Start of frame
 	if (flags & sam.USB_DEVICE_INTFLAG_SOF) > 0 {
+		UART0.Flush()
 		// if you want to blink LED showing traffic, this would be the place...
 	}
 
@@ -2137,13 +2139,7 @@ func handleUSBIRQ(interrupt.Interrupt) {
 				}
 			}
 		}
-
-		if i == usb_CDC_ENDPOINT_IN && UART0.waitTxc {
-			UART0.waitTxcRetryCount++
-		}
 	}
-
-	UART0.Flush()
 }
 
 func initEndpoint(ep, config uint32) {
diff --git a/src/machine/machine_nrf52840_usb.go b/src/machine/machine_nrf52840_usb.go
index 3a04599f..26f240c3 100644
--- a/src/machine/machine_nrf52840_usb.go
+++ b/src/machine/machine_nrf52840_usb.go
@@ -65,7 +65,7 @@ func (usbcdc *USBCDC) Flush() error {
 }
 
 // WriteByte writes a byte of data to the USB CDC interface.
-func (usbcdc USBCDC) WriteByte(c byte) error {
+func (usbcdc *USBCDC) WriteByte(c byte) error {
 	// Supposedly to handle problem with Windows USB serial ports?
 	if usbLineInfo.lineState > 0 {
 		ok := false
@@ -199,6 +199,7 @@ func (usbcdc *USBCDC) handleInterrupt(interrupt.Interrupt) {
 	if nrf.USBD.EVENTS_SOF.Get() == 1 {
 		nrf.USBD.EVENTS_SOF.Set(0)
 		USB.Flush()
+		// if you want to blink LED showing traffic, this would be the place...
 	}
 
 	// USBD ready event