diff --git a/src/machine/machine_atsamd21.go b/src/machine/machine_atsamd21.go
index 92f21b0a..9b18a313 100644
--- a/src/machine/machine_atsamd21.go
+++ b/src/machine/machine_atsamd21.go
@@ -1742,6 +1742,7 @@ type USBCDC struct {
 	waitTxc           bool
 	waitTxcRetryCount uint8
 	sent              bool
+	configured        bool
 }
 
 var (
@@ -1923,6 +1924,13 @@ func (usbcdc *USBCDC) Configure(config UARTConfig) {
 	// enable IRQ
 	intr := interrupt.New(sam.IRQ_USB, handleUSB)
 	intr.Enable()
+
+	usbcdc.configured = true
+}
+
+// Configured returns whether usbcdc is configured or not.
+func (usbcdc *USBCDC) Configured() bool {
+	return usbcdc.configured
 }
 
 func handlePadCalibration() {
diff --git a/src/machine/machine_atsamd51.go b/src/machine/machine_atsamd51.go
index ba556c6a..3ba5ea5b 100644
--- a/src/machine/machine_atsamd51.go
+++ b/src/machine/machine_atsamd51.go
@@ -1965,6 +1965,7 @@ type USBCDC struct {
 	waitTxc           bool
 	waitTxcRetryCount uint8
 	sent              bool
+	configured        bool
 }
 
 var (
@@ -2149,6 +2150,13 @@ func (usbcdc *USBCDC) Configure(config UARTConfig) {
 	interrupt.New(sam.IRQ_USB_SOF_HSOF, handleUSBIRQ).Enable()
 	interrupt.New(sam.IRQ_USB_TRCPT0, handleUSBIRQ).Enable()
 	interrupt.New(sam.IRQ_USB_TRCPT1, handleUSBIRQ).Enable()
+
+	usbcdc.configured = true
+}
+
+// Configured returns whether usbcdc is configured or not.
+func (usbcdc *USBCDC) Configured() bool {
+	return usbcdc.configured
 }
 
 func handlePadCalibration() {
diff --git a/src/runtime/runtime_atsamd21.go b/src/runtime/runtime_atsamd21.go
index 3f66ad2f..8888a711 100644
--- a/src/runtime/runtime_atsamd21.go
+++ b/src/runtime/runtime_atsamd21.go
@@ -30,6 +30,9 @@ func init() {
 
 	// connect to USB CDC interface
 	machine.Serial.Configure(machine.UARTConfig{})
+	if !machine.USB.Configured() {
+		machine.USB.Configure(machine.UARTConfig{})
+	}
 }
 
 func putchar(c byte) {
diff --git a/src/runtime/runtime_atsamd51.go b/src/runtime/runtime_atsamd51.go
index ed2aa9c3..a2438bac 100644
--- a/src/runtime/runtime_atsamd51.go
+++ b/src/runtime/runtime_atsamd51.go
@@ -30,6 +30,9 @@ func init() {
 
 	// connect to USB CDC interface
 	machine.Serial.Configure(machine.UARTConfig{})
+	if !machine.USB.Configured() {
+		machine.USB.Configure(machine.UARTConfig{})
+	}
 }
 
 func putchar(c byte) {