diff --git a/ports/atmel-samd/Makefile b/ports/atmel-samd/Makefile
index 62f058c3d7..865210e2f5 100644
--- a/ports/atmel-samd/Makefile
+++ b/ports/atmel-samd/Makefile
@@ -115,7 +115,7 @@ else
   # -finline-limit=80 or so is similar to not having it on.
   # There is no simple default value, though.
   ifdef INTERNAL_FLASH_FILESYSTEM
-    CFLAGS += -finline-limit=55
+    CFLAGS += -finline-limit=50
   endif
   ifdef CFLAGS_INLINE_LIMIT
     CFLAGS += -finline-limit=$(CFLAGS_INLINE_LIMIT)
diff --git a/ports/atmel-samd/board_busses.c b/ports/atmel-samd/board_busses.c
index 4bf1a40faf..6a4c4a84d6 100644
--- a/ports/atmel-samd/board_busses.c
+++ b/ports/atmel-samd/board_busses.c
@@ -33,81 +33,96 @@
 #include "samd/pins.h"
 #include "py/runtime.h"
 
-#if !defined(DEFAULT_I2C_BUS_SDA) || !defined(DEFAULT_I2C_BUS_SCL)
-    STATIC mp_obj_t board_i2c(void) {
-        mp_raise_NotImplementedError("No default I2C bus");
-        return NULL;
+#define BOARD_I2C (defined(DEFAULT_I2C_BUS_SDA) && defined(DEFAULT_I2C_BUS_SCL))
+#define BOARD_SPI (defined(DEFAULT_SPI_BUS_SCK) && defined(DEFAULT_SPI_BUS_MISO) && defined(DEFAULT_SPI_BUS_MOSI))
+#define BOARD_UART (defined(DEFAULT_UART_BUS_RX) && defined(DEFAULT_UART_BUS_TX))
+
+#if BOARD_I2C
+STATIC mp_obj_t i2c_singleton = NULL;
+
+STATIC mp_obj_t board_i2c(void) {
+
+    if (i2c_singleton == NULL) {
+        busio_i2c_obj_t *self = m_new_obj(busio_i2c_obj_t);
+        self->base.type = &busio_i2c_type;
+
+        assert_pin_free(DEFAULT_I2C_BUS_SDA);
+        assert_pin_free(DEFAULT_I2C_BUS_SCL);
+        common_hal_busio_i2c_construct(self, DEFAULT_I2C_BUS_SCL, DEFAULT_I2C_BUS_SDA, 400000, 0);
+        i2c_singleton = (mp_obj_t)self;
     }
+    return i2c_singleton;
+}
 #else
-    STATIC mp_obj_t i2c_singleton = NULL;
-
-    STATIC mp_obj_t board_i2c(void) {
-
-        if (i2c_singleton == NULL) {
-            busio_i2c_obj_t *self = m_new_obj(busio_i2c_obj_t);
-            self->base.type = &busio_i2c_type;
-
-            assert_pin_free(DEFAULT_I2C_BUS_SDA);
-            assert_pin_free(DEFAULT_I2C_BUS_SCL);
-            common_hal_busio_i2c_construct(self, DEFAULT_I2C_BUS_SCL, DEFAULT_I2C_BUS_SDA, 400000, 0);
-            i2c_singleton = (mp_obj_t)self;
-        }
-        return i2c_singleton;
-
-    }
+STATIC mp_obj_t board_i2c(void) {
+    mp_raise_NotImplementedError("No default I2C bus");
+    return NULL;
+}
 #endif
 MP_DEFINE_CONST_FUN_OBJ_0(board_i2c_obj, board_i2c);
 
-#if !defined(DEFAULT_SPI_BUS_SCK) || !defined(DEFAULT_SPI_BUS_MISO) || !defined(DEFAULT_SPI_BUS_MOSI)
-    STATIC mp_obj_t board_spi(void) {
-        mp_raise_NotImplementedError("No default SPI bus");
-        return NULL;
+#if BOARD_SPI
+STATIC mp_obj_t spi_singleton = NULL;
+
+STATIC mp_obj_t board_spi(void) {
+    if (spi_singleton == NULL) {
+        busio_spi_obj_t *self = m_new_obj(busio_spi_obj_t);
+        self->base.type = &busio_spi_type;
+        assert_pin_free(DEFAULT_SPI_BUS_SCK);
+        assert_pin_free(DEFAULT_SPI_BUS_MOSI);
+        assert_pin_free(DEFAULT_SPI_BUS_MISO);
+        const mcu_pin_obj_t* clock = MP_OBJ_TO_PTR(DEFAULT_SPI_BUS_SCK);
+        const mcu_pin_obj_t* mosi = MP_OBJ_TO_PTR(DEFAULT_SPI_BUS_MOSI);
+        const mcu_pin_obj_t* miso = MP_OBJ_TO_PTR(DEFAULT_SPI_BUS_MISO);
+        common_hal_busio_spi_construct(self, clock, mosi, miso);
+        spi_singleton = (mp_obj_t)self;
     }
+    return spi_singleton;
+}
 #else
-    STATIC mp_obj_t spi_singleton = NULL;
-
-    STATIC mp_obj_t board_spi(void) {
-
-        if (spi_singleton == NULL) {
-            busio_spi_obj_t *self = m_new_obj(busio_spi_obj_t);
-            self->base.type = &busio_spi_type;
-            assert_pin_free(DEFAULT_SPI_BUS_SCK);
-            assert_pin_free(DEFAULT_SPI_BUS_MOSI);
-            assert_pin_free(DEFAULT_SPI_BUS_MISO);
-            const mcu_pin_obj_t* clock = MP_OBJ_TO_PTR(DEFAULT_SPI_BUS_SCK);
-            const mcu_pin_obj_t* mosi = MP_OBJ_TO_PTR(DEFAULT_SPI_BUS_MOSI);
-            const mcu_pin_obj_t* miso = MP_OBJ_TO_PTR(DEFAULT_SPI_BUS_MISO);
-            common_hal_busio_spi_construct(self, clock, mosi, miso);
-            spi_singleton = (mp_obj_t)self;
-        }
-        return spi_singleton;
-    }
+STATIC mp_obj_t board_spi(void) {
+    mp_raise_NotImplementedError("No default SPI bus");
+    return NULL;
+}
 #endif
 MP_DEFINE_CONST_FUN_OBJ_0(board_spi_obj, board_spi);
 
-#if !defined(DEFAULT_UART_BUS_RX) || !defined(DEFAULT_UART_BUS_TX)
-    STATIC mp_obj_t board_uart(void) {
-        mp_raise_NotImplementedError("No default UART bus");
-        return NULL;
+#if BOARD_UART
+STATIC mp_obj_t uart_singleton = NULL;
+
+STATIC mp_obj_t board_uart(void) {
+    if (uart_singleton == NULL) {
+        busio_uart_obj_t *self = m_new_obj(busio_uart_obj_t);
+        self->base.type = &busio_uart_type;
+
+        assert_pin_free(DEFAULT_UART_BUS_RX);
+        assert_pin_free(DEFAULT_UART_BUS_TX);
+
+        const mcu_pin_obj_t* rx = MP_OBJ_TO_PTR(DEFAULT_UART_BUS_RX);
+        const mcu_pin_obj_t* tx = MP_OBJ_TO_PTR(DEFAULT_UART_BUS_TX);
+
+        common_hal_busio_uart_construct(self, tx, rx, 9600, 8, PARITY_NONE, 1, 1000, 64);
+        uart_singleton = (mp_obj_t)self;
     }
+    return uart_singleton;
+}
 #else
-    STATIC mp_obj_t uart_singleton = NULL;
-
-    STATIC mp_obj_t board_uart(void) {
-        if (uart_singleton == NULL) {
-            busio_uart_obj_t *self = m_new_obj(busio_uart_obj_t);
-            self->base.type = &busio_uart_type;
-
-            assert_pin_free(DEFAULT_UART_BUS_RX);
-            assert_pin_free(DEFAULT_UART_BUS_TX);
-
-            const mcu_pin_obj_t* rx = MP_OBJ_TO_PTR(DEFAULT_UART_BUS_RX);
-            const mcu_pin_obj_t* tx = MP_OBJ_TO_PTR(DEFAULT_UART_BUS_TX);
-
-            common_hal_busio_uart_construct(self, tx, rx, 9600, 8, PARITY_NONE, 1, 1000, 64);
-            uart_singleton = (mp_obj_t)self;
-        }
-        return uart_singleton;
-    }
+STATIC mp_obj_t board_uart(void) {
+    mp_raise_NotImplementedError("No default UART bus");
+    return NULL;
+}
 #endif
 MP_DEFINE_CONST_FUN_OBJ_0(board_uart_obj, board_uart);
+
+
+void reset_board_busses(void) {
+#if BOARD_I2C
+    i2c_singleton = NULL;
+#endif
+#if BOARD_SPI
+    spi_singleton = NULL;
+#endif
+#if BOARD_UART
+    uart_singleton = NULL;
+#endif
+}
diff --git a/ports/atmel-samd/board_busses.h b/ports/atmel-samd/board_busses.h
index a368885a58..08dd1aae12 100644
--- a/ports/atmel-samd/board_busses.h
+++ b/ports/atmel-samd/board_busses.h
@@ -36,4 +36,6 @@ extern mp_obj_fun_builtin_fixed_t board_spi_obj;
 void board_uart(void);
 extern mp_obj_fun_builtin_fixed_t board_uart_obj;
 
+void reset_board_busses(void);
+
 #endif  // MICROPY_INCLUDED_ATMEL_SAMD_BOARD_BUSSES_H
diff --git a/ports/atmel-samd/common-hal/busio/UART.c b/ports/atmel-samd/common-hal/busio/UART.c
index fbed1d506b..b8b0ce74ca 100644
--- a/ports/atmel-samd/common-hal/busio/UART.c
+++ b/ports/atmel-samd/common-hal/busio/UART.c
@@ -273,7 +273,7 @@ size_t common_hal_busio_uart_read(busio_uart_obj_t *self, uint8_t *data, size_t
 #ifdef MICROPY_VM_HOOK_LOOP
         MICROPY_VM_HOOK_LOOP
 #endif
-       // If we are zero timeout, make sure we don't loop again (in the event 
+       // If we are zero timeout, make sure we don't loop again (in the event
        // we read in under 1ms)
        if (self->timeout_ms == 0)
             break;
@@ -349,7 +349,18 @@ void common_hal_busio_uart_set_baudrate(busio_uart_obj_t *self, uint32_t baudrat
 }
 
 uint32_t common_hal_busio_uart_rx_characters_available(busio_uart_obj_t *self) {
-    return self->buffer_size;
+    // This assignment is only here because the usart_async routines take a *const argument.
+    struct usart_async_descriptor * const usart_desc_p = (struct usart_async_descriptor * const) &self->usart_desc;
+    struct usart_async_status async_status;
+    usart_async_get_status(usart_desc_p, &async_status);
+    return async_status.rxcnt;
+}
+
+void common_hal_busio_uart_clear_rx_buffer(busio_uart_obj_t *self) {
+    // This assignment is only here because the usart_async routines take a *const argument.
+    struct usart_async_descriptor * const usart_desc_p = (struct usart_async_descriptor * const) &self->usart_desc;
+    usart_async_flush_rx_buffer(usart_desc_p);
+
 }
 
 bool common_hal_busio_uart_ready_to_tx(busio_uart_obj_t *self) {
diff --git a/ports/atmel-samd/common-hal/busio/UART.h b/ports/atmel-samd/common-hal/busio/UART.h
index 685755a5d1..f94df040f8 100644
--- a/ports/atmel-samd/common-hal/busio/UART.h
+++ b/ports/atmel-samd/common-hal/busio/UART.h
@@ -42,10 +42,6 @@ typedef struct {
     bool rx_error;
     uint32_t baudrate;
     uint32_t timeout_ms;
-    // Index of the oldest received character.
-    uint32_t buffer_start;
-    // Index of the next available spot to store a character.
-    uint32_t buffer_size;
     uint32_t buffer_length;
     uint8_t* buffer;
 } busio_uart_obj_t;
diff --git a/ports/atmel-samd/supervisor/port.c b/ports/atmel-samd/supervisor/port.c
index 422bf1f64c..a63b85a9f7 100644
--- a/ports/atmel-samd/supervisor/port.c
+++ b/ports/atmel-samd/supervisor/port.c
@@ -61,6 +61,7 @@
 #include "samd/external_interrupts.h"
 #include "samd/dma.h"
 #include "shared-bindings/rtc/__init__.h"
+#include "board_busses.h"
 #include "tick.h"
 #include "usb.h"
 
@@ -270,6 +271,8 @@ void reset_port(void) {
 
     reset_all_pins();
 
+    reset_board_busses();
+
     // Output clocks for debugging.
     // not supported by SAMD51G; uncomment for SAMD51J or update for 51G
     // #ifdef SAMD51
diff --git a/ports/esp8266/common-hal/busio/UART.c b/ports/esp8266/common-hal/busio/UART.c
index 9b5d86ef5c..8b5bb49d42 100644
--- a/ports/esp8266/common-hal/busio/UART.c
+++ b/ports/esp8266/common-hal/busio/UART.c
@@ -137,6 +137,9 @@ uint32_t common_hal_busio_uart_rx_characters_available(busio_uart_obj_t *self) {
     return 0;
 }
 
+void common_hal_busio_uart_clear_rx_buffer(busio_uart_obj_t *self) {
+}
+
 bool common_hal_busio_uart_ready_to_tx(busio_uart_obj_t *self) {
     return true;
 }
diff --git a/shared-bindings/busio/UART.c b/shared-bindings/busio/UART.c
index 9bad2468e5..864fb974a2 100644
--- a/shared-bindings/busio/UART.c
+++ b/shared-bindings/busio/UART.c
@@ -252,6 +252,36 @@ const mp_obj_property_t busio_uart_baudrate_obj = {
               (mp_obj_t)&mp_const_none_obj},
 };
 
+//|   .. attribute:: in_waiting
+//|
+//|     The number of bytes in the input buffer, available to be read
+//|
+STATIC mp_obj_t busio_uart_obj_get_in_waiting(mp_obj_t self_in) {
+    busio_uart_obj_t *self = MP_OBJ_TO_PTR(self_in);
+    raise_error_if_deinited(common_hal_busio_uart_deinited(self));
+    return MP_OBJ_NEW_SMALL_INT(common_hal_busio_uart_rx_characters_available(self));
+}
+MP_DEFINE_CONST_FUN_OBJ_1(busio_uart_get_in_waiting_obj, busio_uart_obj_get_in_waiting);
+
+const mp_obj_property_t busio_uart_in_waiting_obj = {
+    .base.type = &mp_type_property,
+    .proxy = {(mp_obj_t)&busio_uart_get_in_waiting_obj,
+              (mp_obj_t)&mp_const_none_obj,
+              (mp_obj_t)&mp_const_none_obj},
+};
+
+//|   .. method:: reset_input_buffer()
+//|
+//|     Discard any unread characters in the input buffer.
+//|
+STATIC mp_obj_t busio_uart_obj_reset_input_buffer(mp_obj_t self_in) {
+    busio_uart_obj_t *self = MP_OBJ_TO_PTR(self_in);
+    raise_error_if_deinited(common_hal_busio_uart_deinited(self));
+    common_hal_busio_uart_clear_rx_buffer(self);
+    return mp_const_none;
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(busio_uart_reset_input_buffer_obj, busio_uart_obj_reset_input_buffer);
+
 //| .. class:: busio.UART.Parity
 //|
 //|     Enum-like class to define the parity used to verify correct data transfer.
@@ -306,9 +336,12 @@ STATIC const mp_rom_map_elem_t busio_uart_locals_dict_table[] = {
     { MP_OBJ_NEW_QSTR(MP_QSTR_readinto), MP_ROM_PTR(&mp_stream_readinto_obj) },
     { MP_OBJ_NEW_QSTR(MP_QSTR_write),    MP_ROM_PTR(&mp_stream_write_obj) },
 
+    { MP_OBJ_NEW_QSTR(MP_QSTR_reset_input_buffer), MP_ROM_PTR(&busio_uart_reset_input_buffer_obj) },
+
     // Properties
     { MP_ROM_QSTR(MP_QSTR_baudrate), MP_ROM_PTR(&busio_uart_baudrate_obj) },
-    
+    { MP_ROM_QSTR(MP_QSTR_in_waiting), MP_ROM_PTR(&busio_uart_in_waiting_obj) },
+
     // Nested Enum-like Classes.
     { MP_ROM_QSTR(MP_QSTR_Parity),       MP_ROM_PTR(&busio_uart_parity_type) },
 };
diff --git a/shared-bindings/busio/UART.h b/shared-bindings/busio/UART.h
index fa39d8ad5f..513ff50c83 100644
--- a/shared-bindings/busio/UART.h
+++ b/shared-bindings/busio/UART.h
@@ -60,6 +60,7 @@ extern void common_hal_busio_uart_set_baudrate(busio_uart_obj_t *self, uint32_t
 
 
 extern uint32_t common_hal_busio_uart_rx_characters_available(busio_uart_obj_t *self);
+extern void common_hal_busio_uart_clear_rx_buffer(busio_uart_obj_t *self);
 extern bool common_hal_busio_uart_ready_to_tx(busio_uart_obj_t *self);
 
 #endif  // MICROPY_INCLUDED_SHARED_BINDINGS_BUSIO_UART_H