diff --git a/lib/utils/pyexec.c b/lib/utils/pyexec.c
index 50f73fc17010e..bc47f61829b89 100644
--- a/lib/utils/pyexec.c
+++ b/lib/utils/pyexec.c
@@ -149,7 +149,7 @@ STATIC int parse_compile_execute(const void *source, mp_parse_input_kind_t input
             mp_hal_stdout_tx_strn("\x04", 1);
         }
         // check for SystemExit
-        if (mp_obj_is_subclass_fast(MP_OBJ_FROM_PTR(((mp_obj_base_t *)nlr.ret_val)->type), MP_OBJ_FROM_PTR(&mp_type_SystemExit))) {
+        if (mp_obj_is_subclass_fast(mp_obj_get_type((mp_obj_t)nlr.ret_val), MP_OBJ_FROM_PTR(&mp_type_SystemExit))) {
             // at the moment, the value of SystemExit is unused
             ret = pyexec_system_exit;
         #if CIRCUITPY_ALARM
diff --git a/ports/atmel-samd/audio_dma.c b/ports/atmel-samd/audio_dma.c
index e03800f202eed..883fe0a192519 100644
--- a/ports/atmel-samd/audio_dma.c
+++ b/ports/atmel-samd/audio_dma.c
@@ -273,13 +273,13 @@ audio_dma_result audio_dma_setup_playback(audio_dma_t *dma,
 
     #ifdef SAM_D5X_E5X
     int irq = dma->event_channel < 4 ? EVSYS_0_IRQn + dma->event_channel : EVSYS_4_IRQn;
+    // Only disable and clear on SAMD51 because the SAMD21 shares EVSYS with ticks.
+    NVIC_DisableIRQ(irq);
+    NVIC_ClearPendingIRQ(irq);
     #else
     int irq = EVSYS_IRQn;
     #endif
 
-    NVIC_DisableIRQ(irq);
-    NVIC_ClearPendingIRQ(irq);
-
     DmacDescriptor *first_descriptor = dma_descriptor(dma_channel);
     setup_audio_descriptor(first_descriptor, dma->beat_size, output_spacing, output_register_address);
     if (single_buffer) {
@@ -374,7 +374,7 @@ STATIC void dma_callback_fun(void *arg) {
     audio_dma_load_next_block(dma);
 }
 
-void evsyshandler_common(void) {
+void audio_evsys_handler(void) {
     for (uint8_t i = 0; i < AUDIO_DMA_CHANNEL_COUNT; i++) {
         audio_dma_t *dma = audio_dma_state[i];
         if (dma == NULL) {
@@ -388,26 +388,4 @@ void evsyshandler_common(void) {
     }
 }
 
-#ifdef SAM_D5X_E5X
-void EVSYS_0_Handler(void) {
-    evsyshandler_common();
-}
-void EVSYS_1_Handler(void) {
-    evsyshandler_common();
-}
-void EVSYS_2_Handler(void) {
-    evsyshandler_common();
-}
-void EVSYS_3_Handler(void) {
-    evsyshandler_common();
-}
-void EVSYS_4_Handler(void) {
-    evsyshandler_common();
-}
-#else
-void EVSYS_Handler(void) {
-    evsyshandler_common();
-}
-#endif
-
 #endif
diff --git a/ports/atmel-samd/audio_dma.h b/ports/atmel-samd/audio_dma.h
index bc5beb3bf5c53..d06b5897590ec 100644
--- a/ports/atmel-samd/audio_dma.h
+++ b/ports/atmel-samd/audio_dma.h
@@ -99,4 +99,6 @@ void audio_dma_background(void);
 
 uint8_t find_sync_event_channel_raise(void);
 
+void audio_evsys_handler(void);
+
 #endif  // MICROPY_INCLUDED_ATMEL_SAMD_AUDIO_DMA_H
diff --git a/ports/atmel-samd/boards/adafruit_proxlight_trinkey_m0/mpconfigboard.h b/ports/atmel-samd/boards/adafruit_proxlight_trinkey_m0/mpconfigboard.h
index c792b01272890..878b25f2f8c11 100644
--- a/ports/atmel-samd/boards/adafruit_proxlight_trinkey_m0/mpconfigboard.h
+++ b/ports/atmel-samd/boards/adafruit_proxlight_trinkey_m0/mpconfigboard.h
@@ -2,6 +2,7 @@
 #define MICROPY_HW_MCU_NAME "samd21e18"
 
 #define MICROPY_HW_NEOPIXEL (&pin_PA15)
+#define MICROPY_HW_NEOPIXEL_COUNT (2)
 
 #define IGNORE_PIN_PA01     1
 #define IGNORE_PIN_PA02     1
diff --git a/ports/atmel-samd/boards/adafruit_proxlight_trinkey_m0/mpconfigboard.mk b/ports/atmel-samd/boards/adafruit_proxlight_trinkey_m0/mpconfigboard.mk
index 3ef6dafac6b86..10a2f4ed379ad 100644
--- a/ports/atmel-samd/boards/adafruit_proxlight_trinkey_m0/mpconfigboard.mk
+++ b/ports/atmel-samd/boards/adafruit_proxlight_trinkey_m0/mpconfigboard.mk
@@ -14,6 +14,7 @@ CIRCUITPY_FULL_BUILD = 0
 CIRCUITPY_ANALOGIO = 0
 CIRCUITPY_AUDIOCORE = 0
 CIRCUITPY_BUSIO_SPI = 0
+CIRCUITPY_BUSIO_UART = 0
 CIRCUITPY_PULSEIO = 0
 CIRCUITPY_PWMIO = 0
 CIRCUITPY_ROTARYIO = 0
diff --git a/ports/atmel-samd/boards/arduino_mkr1300/mpconfigboard.h b/ports/atmel-samd/boards/arduino_mkr1300/mpconfigboard.h
index f0748f35de015..6a3f7a96873cd 100644
--- a/ports/atmel-samd/boards/arduino_mkr1300/mpconfigboard.h
+++ b/ports/atmel-samd/boards/arduino_mkr1300/mpconfigboard.h
@@ -16,3 +16,16 @@
 // USB is always used internally so skip the pin objects for it.
 #define IGNORE_PIN_PA24     1
 #define IGNORE_PIN_PA25     1
+// USD ID
+#define IGNORE_PIN_PA18     1
+
+// Hooked to the external crystal
+#define IGNORE_PIN_PA00     1
+#define IGNORE_PIN_PA01     1
+
+// SWD only
+#define IGNORE_PIN_PA30     1
+#define IGNORE_PIN_PA31     1
+
+// Not connected
+#define IGNORE_PIN_PA28     1
diff --git a/ports/atmel-samd/boards/arduino_nano_33_iot/mpconfigboard.h b/ports/atmel-samd/boards/arduino_nano_33_iot/mpconfigboard.h
index 62883ad4a51d8..6be4f5606aefa 100644
--- a/ports/atmel-samd/boards/arduino_nano_33_iot/mpconfigboard.h
+++ b/ports/atmel-samd/boards/arduino_nano_33_iot/mpconfigboard.h
@@ -16,3 +16,11 @@
 // USB is always used internally so skip the pin objects for it.
 #define IGNORE_PIN_PA24     1
 #define IGNORE_PIN_PA25     1
+
+// Not connected
+#define IGNORE_PIN_PA00     1
+#define IGNORE_PIN_PA01     1
+
+// SWD only
+#define IGNORE_PIN_PA30     1
+#define IGNORE_PIN_PA31     1
diff --git a/ports/atmel-samd/common-hal/pulseio/PulseIn.c b/ports/atmel-samd/common-hal/pulseio/PulseIn.c
index 49cef924783c3..507db7e4372f1 100644
--- a/ports/atmel-samd/common-hal/pulseio/PulseIn.c
+++ b/ports/atmel-samd/common-hal/pulseio/PulseIn.c
@@ -272,6 +272,9 @@ void common_hal_pulseio_pulsein_deinit(pulseio_pulsein_obj_t *self) {
 void common_hal_pulseio_pulsein_pause(pulseio_pulsein_obj_t *self) {
     uint32_t mask = 1 << self->channel;
     EIC->INTENCLR.reg = mask << EIC_INTENSET_EXTINT_Pos;
+    #ifdef SAMD21
+    rtc_end_pulse();
+    #endif
 }
 
 void common_hal_pulseio_pulsein_resume(pulseio_pulsein_obj_t *self,
@@ -299,6 +302,9 @@ void common_hal_pulseio_pulsein_resume(pulseio_pulsein_obj_t *self,
     EIC->INTFLAG.reg = mask << EIC_INTFLAG_EXTINT_Pos;
     EIC->INTENSET.reg = mask << EIC_INTENSET_EXTINT_Pos;
 
+    #ifdef SAMD21
+    rtc_start_pulse();
+    #endif
     pulsein_set_config(self, true);
 }
 
diff --git a/ports/atmel-samd/supervisor/port.c b/ports/atmel-samd/supervisor/port.c
index c03b79ce4f2e2..9ad54e5604cf4 100644
--- a/ports/atmel-samd/supervisor/port.c
+++ b/ports/atmel-samd/supervisor/port.c
@@ -91,26 +91,19 @@
 #if CIRCUITPY_PEW
 #include "common-hal/_pew/PewPew.h"
 #endif
-volatile bool hold_interrupt = false;
+static volatile bool sleep_ok = true;
 #ifdef SAMD21
-static void rtc_set_continuous(bool continuous) {
-    while (RTC->MODE0.STATUS.bit.SYNCBUSY) {
-        ;
-    }
-    RTC->MODE0.READREQ.reg = (continuous ? RTC_READREQ_RCONT : 0) | 0x0010;
-    while (RTC->MODE0.STATUS.bit.SYNCBUSY) {
-        ;
-    }
-}
+static uint8_t _tick_event_channel = 0;
 
+// Sleeping requires a register write that can stall interrupt handling. Turning
+// off sleeps allows for more accurate interrupt timing. (Python still thinks
+// it is sleeping though.)
 void rtc_start_pulse(void) {
-    rtc_set_continuous(true);
-    hold_interrupt = true;
+    sleep_ok = false;
 }
 
 void rtc_end_pulse(void) {
-    hold_interrupt = false;
-    rtc_set_continuous(false);
+    sleep_ok = true;
 }
 #endif
 
@@ -161,6 +154,20 @@ static void save_usb_clock_calibration(void) {
 }
 #endif
 
+static void rtc_continuous_mode(void) {
+    #ifdef SAMD21
+    while (RTC->MODE0.STATUS.bit.SYNCBUSY) {
+    }
+    RTC->MODE0.READREQ.reg = RTC_READREQ_RCONT | 0x0010;
+    while (RTC->MODE0.STATUS.bit.SYNCBUSY) {
+    }
+    // Do the first request and wait for it.
+    RTC->MODE0.READREQ.reg = RTC_READREQ_RREQ | RTC_READREQ_RCONT | 0x0010;
+    while (RTC->MODE0.STATUS.bit.SYNCBUSY) {
+    }
+    #endif
+}
+
 static void rtc_init(void) {
     #ifdef SAMD21
     _gclk_enable_channel(RTC_GCLK_ID, GCLK_CLKCTRL_GEN_GCLK2_Val);
@@ -168,9 +175,17 @@ static void rtc_init(void) {
     while (RTC->MODE0.CTRL.bit.SWRST != 0) {
     }
 
+    // Turn on periodic events to use as tick. We control whether it interrupts
+    // us with the EVSYS INTEN register.
+    RTC->MODE0.EVCTRL.reg = RTC_MODE0_EVCTRL_PEREO2;
+
     RTC->MODE0.CTRL.reg = RTC_MODE0_CTRL_ENABLE |
         RTC_MODE0_CTRL_MODE_COUNT32 |
         RTC_MODE0_CTRL_PRESCALER_DIV2;
+
+    // Turn on continuous sync of the count register. This will speed up all
+    // tick reads.
+    rtc_continuous_mode();
     #endif
     #ifdef SAM_D5X_E5X
     hri_mclk_set_APBAMASK_RTC_bit(MCLK);
@@ -363,6 +378,9 @@ void reset_port(void) {
     #endif
 
     reset_event_system();
+    #ifdef SAMD21
+    _tick_event_channel = EVSYS_SYNCH_NUM;
+    #endif
 
     reset_all_pins();
 
@@ -430,21 +448,14 @@ uint32_t port_get_saved_word(void) {
 // TODO: Move this to an RTC backup register so we can preserve it when only the BACKUP power domain
 // is enabled.
 static volatile uint64_t overflowed_ticks = 0;
-#ifdef SAMD21
-static volatile bool _ticks_enabled = false;
-#endif
 
 static uint32_t _get_count(uint64_t *overflow_count) {
     #ifdef SAM_D5X_E5X
     while ((RTC->MODE0.SYNCBUSY.reg & (RTC_MODE0_SYNCBUSY_COUNTSYNC | RTC_MODE0_SYNCBUSY_COUNT)) != 0) {
     }
     #endif
-    #ifdef SAMD21
-    // Request a read so we don't stall the bus later. See section 14.3.1.5 Read Request
-    RTC->MODE0.READREQ.reg = RTC_READREQ_RREQ | 0x0010;
-    while (RTC->MODE0.STATUS.bit.SYNCBUSY != 0) {
-    }
-    #endif
+    // SAMD21 does continuous sync so we don't need to wait here.
+
     // Disable interrupts so we can grab the count and the overflow.
     common_hal_mcu_disable_interrupts();
     uint32_t count = RTC->MODE0.COUNT.reg;
@@ -458,29 +469,6 @@ static uint32_t _get_count(uint64_t *overflow_count) {
 
 volatile bool _woken_up;
 
-static void _port_interrupt_after_ticks(uint32_t ticks) {
-    uint32_t current_ticks = _get_count(NULL);
-    if (ticks > 1 << 28) {
-        // We'll interrupt sooner with an overflow.
-        return;
-    }
-    #ifdef SAMD21
-    if (hold_interrupt) {
-        return;
-    }
-    #endif
-    uint32_t target = current_ticks + (ticks << 4);
-    RTC->MODE0.COMP[0].reg = target;
-    #ifdef SAM_D5X_E5X
-    while ((RTC->MODE0.SYNCBUSY.reg & (RTC_MODE0_SYNCBUSY_COMP0)) != 0) {
-    }
-    #endif
-    RTC->MODE0.INTFLAG.reg = RTC_MODE0_INTFLAG_CMP0;
-    RTC->MODE0.INTENSET.reg = RTC_MODE0_INTENSET_CMP0;
-    current_ticks = _get_count(NULL);
-    _woken_up = current_ticks >= target;
-}
-
 void RTC_Handler(void) {
     uint32_t intflag = RTC->MODE0.INTFLAG.reg;
     if (intflag & RTC_MODE0_INTFLAG_OVF) {
@@ -497,19 +485,10 @@ void RTC_Handler(void) {
     }
     #endif
     if (intflag & RTC_MODE0_INTFLAG_CMP0) {
-        // Clear the interrupt because we may have hit a sleep and _ticks_enabled
+        // Clear the interrupt because we may have hit a sleep
         RTC->MODE0.INTFLAG.reg = RTC_MODE0_INTFLAG_CMP0;
         _woken_up = true;
-        #ifdef SAMD21
-        if (_ticks_enabled) {
-            // Do things common to all ports when the tick occurs.
-            supervisor_tick();
-            // Check _ticks_enabled again because a tick handler may have turned it off.
-            if (_ticks_enabled) {
-                _port_interrupt_after_ticks(1);
-            }
-        }
-        #endif
+        // SAMD21 ticks are handled by EVSYS
         #ifdef SAM_D5X_E5X
         RTC->MODE0.INTENCLR.reg = RTC_MODE0_INTENCLR_CMP0;
         #endif
@@ -526,6 +505,39 @@ uint64_t port_get_raw_ticks(uint8_t *subticks) {
     return overflow_count + current_ticks / 16;
 }
 
+void evsyshandler_common(void) {
+    #ifdef SAMD21
+    if (_tick_event_channel < EVSYS_SYNCH_NUM && event_interrupt_active(_tick_event_channel)) {
+        supervisor_tick();
+    }
+    #endif
+    #if CIRCUITPY_AUDIOIO || CIRCUITPY_AUDIOBUSIO
+    audio_evsys_handler();
+    #endif
+}
+
+#ifdef SAM_D5X_E5X
+void EVSYS_0_Handler(void) {
+    evsyshandler_common();
+}
+void EVSYS_1_Handler(void) {
+    evsyshandler_common();
+}
+void EVSYS_2_Handler(void) {
+    evsyshandler_common();
+}
+void EVSYS_3_Handler(void) {
+    evsyshandler_common();
+}
+void EVSYS_4_Handler(void) {
+    evsyshandler_common();
+}
+#else
+void EVSYS_Handler(void) {
+    evsyshandler_common();
+}
+#endif
+
 // Enable 1/1024 second tick.
 void port_enable_tick(void) {
     #ifdef SAM_D5X_E5X
@@ -533,9 +545,23 @@ void port_enable_tick(void) {
     RTC->MODE0.INTENSET.reg = RTC_MODE0_INTENSET_PER2;
     #endif
     #ifdef SAMD21
-    // TODO: Switch to using the PER *event* from the RTC to generate an interrupt via EVSYS.
-    _ticks_enabled = true;
-    _port_interrupt_after_ticks(1);
+    // SAMD21 ticks won't survive port_reset(). This *should* be ok since it'll
+    // be triggered by ticks and no Python will be running.
+    if (_tick_event_channel >= EVSYS_SYNCH_NUM) {
+        turn_on_event_system();
+        _tick_event_channel = find_sync_event_channel();
+    }
+    // This turns on both the event detected interrupt (EVD) and overflow (OVR).
+    init_event_channel_interrupt(_tick_event_channel, CORE_GCLK, EVSYS_ID_GEN_RTC_PER_2);
+    // Disable overflow interrupt because we ignore it.
+    if (_tick_event_channel >= 8) {
+        uint8_t value = 1 << (_tick_event_channel - 8);
+        EVSYS->INTENCLR.reg = EVSYS_INTENSET_OVRp8(value);
+    } else {
+        uint8_t value = 1 << _tick_event_channel;
+        EVSYS->INTENCLR.reg = EVSYS_INTENSET_OVR(value);
+    }
+    NVIC_EnableIRQ(EVSYS_IRQn);
     #endif
 }
 
@@ -545,21 +571,48 @@ void port_disable_tick(void) {
     RTC->MODE0.INTENCLR.reg = RTC_MODE0_INTENCLR_PER2;
     #endif
     #ifdef SAMD21
-    _ticks_enabled = false;
-    RTC->MODE0.INTENCLR.reg = RTC_MODE0_INTENCLR_CMP0;
+    if (_tick_event_channel >= 8) {
+        uint8_t value = 1 << (_tick_event_channel - 8);
+        EVSYS->INTENCLR.reg = EVSYS_INTENSET_EVDp8(value);
+    } else {
+        uint8_t value = 1 << _tick_event_channel;
+        EVSYS->INTENCLR.reg = EVSYS_INTENSET_EVD(value);
+    }
     #endif
 }
 
-// This is called by sleep, we ignore it when our ticks are enabled because
-// they'll wake us up earlier. If we don't, we'll mess up ticks by overwriting
-// the next RTC wake up time.
 void port_interrupt_after_ticks(uint32_t ticks) {
+    uint32_t current_ticks = _get_count(NULL);
+    if (ticks > 1 << 28) {
+        // We'll interrupt sooner with an overflow.
+        return;
+    }
     #ifdef SAMD21
-    if (_ticks_enabled) {
+    if (!sleep_ok) {
         return;
     }
     #endif
-    _port_interrupt_after_ticks(ticks);
+
+    uint32_t target = current_ticks + (ticks << 4);
+    #ifdef SAMD21
+    // Try and avoid a bus stall when writing COMP by checking for an obvious
+    // existing sync.
+    while (RTC->MODE0.STATUS.bit.SYNCBUSY == 1) {
+    }
+    #endif
+    // Writing the COMP register can take up to 180us to synchronize. During
+    // this time, the bus will stall and no interrupts will be serviced.
+    RTC->MODE0.COMP[0].reg = target;
+    #ifdef SAM_D5X_E5X
+    while ((RTC->MODE0.SYNCBUSY.reg & (RTC_MODE0_SYNCBUSY_COMP0)) != 0) {
+    }
+    #endif
+    RTC->MODE0.INTFLAG.reg = RTC_MODE0_INTFLAG_CMP0;
+    RTC->MODE0.INTENSET.reg = RTC_MODE0_INTENSET_CMP0;
+    // Set continuous mode again because setting COMP may disable it.
+    rtc_continuous_mode();
+    current_ticks = _get_count(NULL);
+    _woken_up = current_ticks >= target;
 }
 
 void port_idle_until_interrupt(void) {
@@ -571,7 +624,7 @@ void port_idle_until_interrupt(void) {
     }
     #endif
     common_hal_mcu_disable_interrupts();
-    if (!tud_task_event_ready() && !hold_interrupt && !_woken_up) {
+    if (!tud_task_event_ready() && sleep_ok && !_woken_up) {
         __DSB();
         __WFI();
     }
diff --git a/py/circuitpy_mpconfig.mk b/py/circuitpy_mpconfig.mk
index 4df9432efa308..be68904d18593 100644
--- a/py/circuitpy_mpconfig.mk
+++ b/py/circuitpy_mpconfig.mk
@@ -121,9 +121,15 @@ CFLAGS += -DCIRCUITPY_BUILTINS_POW3=$(CIRCUITPY_BUILTINS_POW3)
 CIRCUITPY_BUSIO ?= 1
 CFLAGS += -DCIRCUITPY_BUSIO=$(CIRCUITPY_BUSIO)
 
+# These two flags pretend to implement their class but raise a ValueError due to
+# unsupported pins. This should be used sparingly on boards that don't break out
+# generic IO but need parts of busio.
 CIRCUITPY_BUSIO_SPI ?= 1
 CFLAGS += -DCIRCUITPY_BUSIO_SPI=$(CIRCUITPY_BUSIO_SPI)
 
+CIRCUITPY_BUSIO_UART ?= 1
+CFLAGS += -DCIRCUITPY_BUSIO_UART=$(CIRCUITPY_BUSIO_UART)
+
 CIRCUITPY_CAMERA ?= 0
 CFLAGS += -DCIRCUITPY_CAMERA=$(CIRCUITPY_CAMERA)
 
@@ -184,7 +190,7 @@ CFLAGS += -DCIRCUITPY_FREQUENCYIO=$(CIRCUITPY_FREQUENCYIO)
 CIRCUITPY_GAMEPADSHIFT ?= 0
 CFLAGS += -DCIRCUITPY_GAMEPADSHIFT=$(CIRCUITPY_GAMEPADSHIFT)
 
-CIRCUITPY_GETPASS ?= 1
+CIRCUITPY_GETPASS ?= $(CIRCUITPY_FULL_BUILD)
 CFLAGS += -DCIRCUITPY_GETPASS=$(CIRCUITPY_GETPASS)
 
 CIRCUITPY_GNSS ?= 0
@@ -330,7 +336,7 @@ CFLAGS += -DCIRCUITPY_TOUCHIO_USE_NATIVE=$(CIRCUITPY_TOUCHIO_USE_NATIVE)
 CIRCUITPY_TOUCHIO ?= 1
 CFLAGS += -DCIRCUITPY_TOUCHIO=$(CIRCUITPY_TOUCHIO)
 
-CIRCUITPY_TRACEBACK ?= 1
+CIRCUITPY_TRACEBACK ?= $(CIRCUITPY_FULL_BUILD)
 CFLAGS += -DCIRCUITPY_TRACEBACK=$(CIRCUITPY_TRACEBACK)
 
 # For debugging.
diff --git a/shared-bindings/busio/SPI.c b/shared-bindings/busio/SPI.c
index 31f3a9021eac3..a611d8e1d581a 100644
--- a/shared-bindings/busio/SPI.c
+++ b/shared-bindings/busio/SPI.c
@@ -104,7 +104,7 @@ STATIC mp_obj_t busio_spi_make_new(const mp_obj_type_t *type, size_t n_args, con
     common_hal_busio_spi_construct(self, clock, mosi, miso);
     return MP_OBJ_FROM_PTR(self);
     #else
-    mp_raise_NotImplementedError(NULL);
+    mp_raise_ValueError(translate("Invalid pins"));
     #endif // CIRCUITPY_BUSIO_SPI
 }
 
diff --git a/shared-bindings/busio/UART.c b/shared-bindings/busio/UART.c
index bd97910192343..efb0643d0b8e6 100644
--- a/shared-bindings/busio/UART.c
+++ b/shared-bindings/busio/UART.c
@@ -72,13 +72,16 @@ typedef struct {
 extern const busio_uart_parity_obj_t busio_uart_parity_even_obj;
 extern const busio_uart_parity_obj_t busio_uart_parity_odd_obj;
 
+#if CIRCUITPY_BUSIO_UART
 STATIC void validate_timeout(mp_float_t timeout) {
     if (timeout < (mp_float_t)0.0f || timeout > (mp_float_t)100.0f) {
         mp_raise_ValueError(translate("timeout must be 0.0-100.0 seconds"));
     }
 }
+#endif  // CIRCUITPY_BUSIO_UART
 
 STATIC mp_obj_t busio_uart_make_new(const mp_obj_type_t *type, size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {
+    #if CIRCUITPY_BUSIO_UART
     // Always initially allocate the UART object within the long-lived heap.
     // This is needed to avoid crashes with certain UART implementations which
     // cannot accomodate being moved after creation. (See
@@ -141,8 +144,12 @@ STATIC mp_obj_t busio_uart_make_new(const mp_obj_type_t *type, size_t n_args, co
         args[ARG_baudrate].u_int, bits, parity, stop, timeout,
         args[ARG_receiver_buffer_size].u_int, NULL, false);
     return (mp_obj_t)self;
+    #else
+    mp_raise_ValueError(translate("Invalid pins"));
+    #endif  // CIRCUITPY_BUSIO_UART
 }
 
+#if CIRCUITPY_BUSIO_UART
 
 // Helper to ensure we have the native super class instead of a subclass.
 busio_uart_obj_t *native_uart(mp_obj_t uart_obj) {
@@ -358,6 +365,7 @@ STATIC mp_obj_t busio_uart_obj_reset_input_buffer(mp_obj_t self_in) {
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(busio_uart_reset_input_buffer_obj, busio_uart_obj_reset_input_buffer);
+#endif  // CIRCUITPY_BUSIO_UART
 
 //| class Parity:
 //|     """Enum-like class to define the parity used to verify correct data transfer."""
@@ -400,6 +408,7 @@ const mp_obj_type_t busio_uart_parity_type = {
 };
 
 STATIC const mp_rom_map_elem_t busio_uart_locals_dict_table[] = {
+    #if CIRCUITPY_BUSIO_UART
     { MP_ROM_QSTR(MP_QSTR___del__),      MP_ROM_PTR(&busio_uart_deinit_obj) },
     { MP_ROM_QSTR(MP_QSTR_deinit),       MP_ROM_PTR(&busio_uart_deinit_obj) },
     { MP_ROM_QSTR(MP_QSTR___enter__),    MP_ROM_PTR(&default___enter___obj) },
@@ -417,12 +426,14 @@ STATIC const mp_rom_map_elem_t busio_uart_locals_dict_table[] = {
     { 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) },
     { MP_ROM_QSTR(MP_QSTR_timeout),      MP_ROM_PTR(&busio_uart_timeout_obj) },
+    #endif  // CIRCUITPY_BUSIO_UART
 
     // Nested Enum-like Classes.
     { MP_ROM_QSTR(MP_QSTR_Parity),       MP_ROM_PTR(&busio_uart_parity_type) },
 };
 STATIC MP_DEFINE_CONST_DICT(busio_uart_locals_dict, busio_uart_locals_dict_table);
 
+#if CIRCUITPY_BUSIO_UART
 STATIC const mp_stream_p_t uart_stream_p = {
     MP_PROTO_IMPLEMENT(MP_QSTR_protocol_stream)
     .read = busio_uart_read,
@@ -445,3 +456,11 @@ const mp_obj_type_t busio_uart_type = {
         .protocol = &uart_stream_p,
         ),
 };
+#else
+const mp_obj_type_t busio_uart_type = {
+    { &mp_type_type },
+    .name = MP_QSTR_UART,
+    .make_new = busio_uart_make_new,
+    .locals_dict = (mp_obj_dict_t *)&busio_uart_locals_dict,
+};
+#endif  // CIRCUITPY_BUSIO_UART