diff --git a/main.c b/main.c
index ba1e0e4013399..a2e4b9f729761 100644
--- a/main.c
+++ b/main.c
@@ -127,6 +127,7 @@ bool start_mp(safe_mode_t safe_mode) {
 
     pyexec_result_t result;
     bool found_main = false;
+
     if (safe_mode != NO_SAFE_MODE) {
         serial_write(MSG_SAFE_MODE_NO_MAIN);
     } else {
diff --git a/ports/atmel-samd/Makefile b/ports/atmel-samd/Makefile
index f9751415a1abe..ef168dbc90b58 100644
--- a/ports/atmel-samd/Makefile
+++ b/ports/atmel-samd/Makefile
@@ -238,10 +238,10 @@ SRC_C = \
 # Choose which flash filesystem impl to use.
 # (Right now INTERNAL_FLASH_FILESYSTEM and SPI_FLASH_FILESYSTEM are mutually exclusive.
 # But that might not be true in the future.)
-ifdef INTERNAL_FLASH_FILESYSTEM
+ifeq ($(INTERNAL_FLASH_FILESYSTEM),1)
 SRC_C += internal_flash.c
 endif
-ifdef SPI_FLASH_FILESYSTEM
+ifeq ($(SPI_FLASH_FILESYSTEM),1)
 SRC_C += spi_flash.c
 endif
 
@@ -278,7 +278,7 @@ SRC_COMMON_HAL = \
 	usb_hid/__init__.c \
 	usb_hid/Device.c
 
-ifdef INTERNAL_LIBM
+ifeq ($(INTERNAL_LIBM),1)
 SRC_LIBM = $(addprefix lib/,\
 	libm/math.c \
 	libm/fmodf.c \
@@ -338,7 +338,7 @@ OBJ = $(PY_O) $(SUPERVISOR_O) $(addprefix $(BUILD)/, $(SRC_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(SRC_ASF:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(SRC_COMMON_HAL_EXPANDED:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(SRC_SHARED_MODULE_EXPANDED:.c=.o))
-ifdef INTERNAL_LIBM
+ifeq ($(INTERNAL_LIBM),1)
 OBJ += $(addprefix $(BUILD)/, $(SRC_LIBM:.c=.o))
 endif
 
diff --git a/ports/atmel-samd/asf4 b/ports/atmel-samd/asf4
index 8e3e471f481ef..e1d02fde5d23a 160000
--- a/ports/atmel-samd/asf4
+++ b/ports/atmel-samd/asf4
@@ -1 +1 @@
-Subproject commit 8e3e471f481effddd5c23da2342e1e4aab618e1f
+Subproject commit e1d02fde5d23a704d33d74e18fcf624eb56132c5
diff --git a/ports/atmel-samd/background.c b/ports/atmel-samd/background.c
index 6515e448b4991..2fdf059823b18 100644
--- a/ports/atmel-samd/background.c
+++ b/ports/atmel-samd/background.c
@@ -26,9 +26,11 @@
 #include "background.h"
 
 // #include "common-hal/audioio/AudioOut.h"
+#include "usb.h"
 #include "usb_mass_storage.h"
 
 void run_background_tasks(void) {
     // audioout_background();
     usb_msc_background();
+    usb_cdc_background();
 }
diff --git a/ports/atmel-samd/mpconfigport.mk b/ports/atmel-samd/mpconfigport.mk
index acb0916fccb6c..c3b2f22d4f992 100644
--- a/ports/atmel-samd/mpconfigport.mk
+++ b/ports/atmel-samd/mpconfigport.mk
@@ -3,5 +3,5 @@
 # This should correspond to the MICROPY_LONGINT_IMPL definition in mpconfigport.h.
 MPY_TOOL_LONGINT_IMPL = -mlongint-impl=none
 
-INTERNAL_LIBM = (1)
+INTERNAL_LIBM = 1
 
diff --git a/ports/atmel-samd/usb.c b/ports/atmel-samd/usb.c
index e0b9ddc41ca2f..bd712a5d2c071 100644
--- a/ports/atmel-samd/usb.c
+++ b/ports/atmel-samd/usb.c
@@ -56,13 +56,13 @@
 static uint8_t usb_rx_buf[USB_RX_BUF_SIZE];
 
 // Receive buffer head
-static volatile uint8_t usb_rx_buf_head;
+static volatile uint8_t usb_rx_buf_head = 0;
 
 // Receive buffer tail
-static volatile uint8_t usb_rx_buf_tail;
+static volatile uint8_t usb_rx_buf_tail = 0;
 
 // Number of bytes in receive buffer
-volatile uint8_t usb_rx_count;
+volatile uint8_t usb_rx_count = 0;
 
 volatile bool mp_cdc_enabled = false;
 volatile bool usb_transmitting = false;
@@ -133,8 +133,11 @@ static bool read_complete(const uint8_t ep, const enum usb_xfer_code rc, const u
         uint8_t c = cdc_packet_buffer[i];
         if (c == mp_interrupt_char) {
             mp_keyboard_interrupt();
-            // Don't put the interrupt into the buffer, just continue.
-            continue;
+            // If interrupted, flush all the input.
+            usb_rx_count = 0;
+            usb_rx_buf_head = 0;
+            usb_rx_buf_tail = 0;
+            break;
         } else {
             // The count of characters present in receive buffer is
             // incremented.
@@ -144,7 +147,7 @@ static bool read_complete(const uint8_t ep, const enum usb_xfer_code rc, const u
             if (usb_rx_buf_tail == USB_RX_BUF_SIZE) {
                 // Reached the end of buffer, revert back to beginning of
                 // buffer.
-                usb_rx_buf_tail = 0x00;
+                usb_rx_buf_tail = 0;
             }
         }
     }
@@ -219,8 +222,7 @@ void init_usb(void) {
     mscdf_register_callback(MSCDF_CB_TEST_DISK_READY, (FUNC_PTR)usb_msc_disk_is_ready);
     mscdf_register_callback(MSCDF_CB_XFER_BLOCKS_DONE, (FUNC_PTR)usb_msc_xfer_done);
 
-    int32_t result = usbdc_start(&multi_desc);
-    while (result != ERR_NONE) {}
+    usbdc_start(&multi_desc);
     usbdc_attach();
 }
 
@@ -315,3 +317,12 @@ void usb_write(const char* buffer, uint32_t len) {
 bool usb_connected(void) {
     return cdc_enabled();
 }
+
+// Poll for input if keyboard interrupts are enabled,
+// so that we can check for the interrupt char. read_complete() does the checking.
+void usb_cdc_background() {
+    //
+    if (mp_interrupt_char != -1 && cdc_enabled() && !pending_read) {
+        start_read();
+    }
+}
diff --git a/ports/atmel-samd/usb.h b/ports/atmel-samd/usb.h
index 754331211d6a1..45c05df9c0290 100644
--- a/ports/atmel-samd/usb.h
+++ b/ports/atmel-samd/usb.h
@@ -37,5 +37,6 @@ int usb_read(void);
 void usb_write(const char* buffer, uint32_t len);
 bool usb_bytes_available(void);
 bool usb_connected(void);
+void usb_cdc_background(void);
 
 #endif  // __MICROPY_INCLUDED_ATMEL_SAMD_USB_H__