diff --git a/ports/atmel-samd/Makefile b/ports/atmel-samd/Makefile
index bb5c39e999..2546c416d6 100644
--- a/ports/atmel-samd/Makefile
+++ b/ports/atmel-samd/Makefile
@@ -10,8 +10,8 @@ endif
 BUILD ?= build-$(BOARD)
 
 include ../../py/mkenv.mk
--include mpconfigport.mk
 include boards/$(BOARD)/mpconfigboard.mk
+-include mpconfigport.mk
 
 # qstr definitions (must come before including py.mk)
 QSTR_DEFS = qstrdefsport.h
diff --git a/ports/atmel-samd/mpconfigport.h b/ports/atmel-samd/mpconfigport.h
index 88a467c202..e6728d83d2 100644
--- a/ports/atmel-samd/mpconfigport.h
+++ b/ports/atmel-samd/mpconfigport.h
@@ -59,8 +59,6 @@
 #define MICROPY_PY_URANDOM_EXTRA_FUNCS (0)
 #define MICROPY_PY_STRUCT           (0)
 #define MICROPY_PY_SYS              (1)
-// If you change MICROPY_LONGINT_IMPL, also change MPY_TOOL_LONGINT_IMPL in mpconfigport.mk.
-#define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_NONE)
 #define MICROPY_FLOAT_IMPL          (MICROPY_FLOAT_IMPL_FLOAT)
 #define MICROPY_STREAMS_NON_BLOCK   (1)
 
@@ -141,12 +139,17 @@ typedef long mp_off_t;
 #define CIRCUITPY_MCU_FAMILY samd21
 #define MICROPY_PY_SYS_PLATFORM                     "Atmel SAMD21"
 #define PORT_HEAP_SIZE (16384 + 4096)
+// If you change MICROPY_LONGINT_IMPL, also change MPY_TOOL_LONGINT_IMPL in mpconfigport.mk.
+#define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_NONE)
 #endif
 
 #ifdef SAMD51
 #define CIRCUITPY_MCU_FAMILY samd51
 #define MICROPY_PY_SYS_PLATFORM                     "MicroChip SAMD51"
 #define PORT_HEAP_SIZE (0x20000) // 128KiB
+// If you change MICROPY_LONGINT_IMPL, also change MPY_TOOL_LONGINT_IMPL in mpconfigport.mk.
+#define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_MPZ)
+#define MP_SSIZE_MAX (0x7fffffff)
 #endif
 
 // extra built in modules to add to the list of known ones
diff --git a/ports/atmel-samd/mpconfigport.mk b/ports/atmel-samd/mpconfigport.mk
index c3b2f22d4f..0f3f399e22 100644
--- a/ports/atmel-samd/mpconfigport.mk
+++ b/ports/atmel-samd/mpconfigport.mk
@@ -1,7 +1,12 @@
 # Define an equivalent for MICROPY_LONGINT_IMPL, to pass to $(MPY-TOOL) in py/mkrules.mk
 # $(MPY-TOOL) needs to know what kind of longint to use (if any) to freeze long integers.
 # This should correspond to the MICROPY_LONGINT_IMPL definition in mpconfigport.h.
+ifeq ($(CHIP_FAMILY), samd21)
 MPY_TOOL_LONGINT_IMPL = -mlongint-impl=none
+endif
+
+ifeq ($(CHIP_FAMILY), samd51)
+MPY_TOOL_LONGINT_IMPL = -mlongint-impl=mpz
+endif
 
 INTERNAL_LIBM = 1
-
diff --git a/ports/atmel-samd/usb.c b/ports/atmel-samd/usb.c
index 993ac7eaad..740486aa1e 100644
--- a/ports/atmel-samd/usb.c
+++ b/ports/atmel-samd/usb.c
@@ -107,7 +107,11 @@ static volatile bool pending_read;
 
 static int32_t start_read(void) {
     pending_read = true;
-    return cdcdf_acm_read(cdc_packet_buffer, 64);
+    int32_t result = cdcdf_acm_read(cdc_packet_buffer, 64);
+    if (result != ERR_NONE) {
+        pending_read = false;
+    }
+    return result;
 }
 
 static bool read_complete(const uint8_t ep, const enum usb_xfer_code rc, const uint32_t count) {
@@ -222,7 +226,7 @@ void init_usb(void) {
     usbdc_attach();
 }
 
-static inline bool cdc_enabled(void) {
+static bool cdc_enabled(void) {
     if (mp_cdc_enabled) {
         return true;
     }
@@ -239,11 +243,10 @@ static inline bool cdc_enabled(void) {
 }
 
 bool usb_bytes_available(void) {
-    if (!pending_read) {
+    if (cdc_enabled() && !pending_read) {
         start_read();
     }
     if (usb_rx_count == 0) {
-        cdc_enabled();
         return false;
     }
     return usb_rx_count > 0;