diff --git a/main.c b/main.c
index a2e4b9f729..ba1e0e4013 100644
--- a/main.c
+++ b/main.c
@@ -127,7 +127,6 @@ 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 ef168dbc90..f9751415a1 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.)
-ifeq ($(INTERNAL_FLASH_FILESYSTEM),1)
+ifdef INTERNAL_FLASH_FILESYSTEM
 SRC_C += internal_flash.c
 endif
-ifeq ($(SPI_FLASH_FILESYSTEM),1)
+ifdef SPI_FLASH_FILESYSTEM
 SRC_C += spi_flash.c
 endif
 
@@ -278,7 +278,7 @@ SRC_COMMON_HAL = \
 	usb_hid/__init__.c \
 	usb_hid/Device.c
 
-ifeq ($(INTERNAL_LIBM),1)
+ifdef INTERNAL_LIBM
 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)/, $(SRC_SHARED_MODULE_EXPANDED:.c=.o))
-ifeq ($(INTERNAL_LIBM),1)
+ifdef INTERNAL_LIBM
 OBJ += $(addprefix $(BUILD)/, $(SRC_LIBM:.c=.o))
 endif
 
diff --git a/ports/atmel-samd/background.c b/ports/atmel-samd/background.c
index 2fdf059823..6515e448b4 100644
--- a/ports/atmel-samd/background.c
+++ b/ports/atmel-samd/background.c
@@ -26,11 +26,9 @@
 #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 c3b2f22d4f..acb0916fcc 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 bd712a5d2c..e0b9ddc41c 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 = 0;
+static volatile uint8_t usb_rx_buf_head;
 
 // Receive buffer tail
-static volatile uint8_t usb_rx_buf_tail = 0;
+static volatile uint8_t usb_rx_buf_tail;
 
 // Number of bytes in receive buffer
-volatile uint8_t usb_rx_count = 0;
+volatile uint8_t usb_rx_count;
 
 volatile bool mp_cdc_enabled = false;
 volatile bool usb_transmitting = false;
@@ -133,11 +133,8 @@ 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();
-            // If interrupted, flush all the input.
-            usb_rx_count = 0;
-            usb_rx_buf_head = 0;
-            usb_rx_buf_tail = 0;
-            break;
+            // Don't put the interrupt into the buffer, just continue.
+            continue;
         } else {
             // The count of characters present in receive buffer is
             // incremented.
@@ -147,7 +144,7 @@ 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 = 0;
+                usb_rx_buf_tail = 0x00;
             }
         }
     }
@@ -222,7 +219,8 @@ 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);
 
-    usbdc_start(&multi_desc);
+    int32_t result = usbdc_start(&multi_desc);
+    while (result != ERR_NONE) {}
     usbdc_attach();
 }
 
@@ -317,12 +315,3 @@ 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 45c05df9c0..754331211d 100644
--- a/ports/atmel-samd/usb.h
+++ b/ports/atmel-samd/usb.h
@@ -37,6 +37,5 @@ 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__