diff --git a/ports/atmel-samd/common-hal/digitalio/DigitalInOut.c b/ports/atmel-samd/common-hal/digitalio/DigitalInOut.c
index cd0e3f91e57d3..03bd225b14738 100644
--- a/ports/atmel-samd/common-hal/digitalio/DigitalInOut.c
+++ b/ports/atmel-samd/common-hal/digitalio/DigitalInOut.c
@@ -60,7 +60,7 @@ void common_hal_digitalio_digitalinout_deinit(digitalio_digitalinout_obj_t* self
 void common_hal_digitalio_digitalinout_switch_to_input(
         digitalio_digitalinout_obj_t* self, digitalio_pull_t pull) {
     self->output = false;
-
+    // This also sets direction to input.
     common_hal_digitalio_digitalinout_set_pull(self, pull);
 }
 
@@ -69,13 +69,13 @@ void common_hal_digitalio_digitalinout_switch_to_output(
         digitalio_drive_mode_t drive_mode) {
     const uint8_t pin = self->pin->pin;
     gpio_set_pin_pull_mode(pin, GPIO_PULL_OFF);
-    gpio_set_pin_direction(pin, GPIO_DIRECTION_OUT);
-
     // Turn on "strong" pin driving (more current available). See DRVSTR doc in datasheet.
     hri_port_set_PINCFG_DRVSTR_bit(PORT, (enum gpio_port)GPIO_PORT(pin), GPIO_PIN(pin));
 
     self->output = true;
     self->open_drain = drive_mode == DRIVE_MODE_OPEN_DRAIN;
+
+    // Direction is set in set_value. We don't need to do it here.
     common_hal_digitalio_digitalinout_set_value(self, value);
 }
 
@@ -86,16 +86,22 @@ digitalio_direction_t common_hal_digitalio_digitalinout_get_direction(
 
 void common_hal_digitalio_digitalinout_set_value(
         digitalio_digitalinout_obj_t* self, bool value) {
+    const uint8_t pin = self->pin->pin;
+    const uint8_t port = GPIO_PORT(pin);
+    const uint32_t pin_mask = 1U << GPIO_PIN(pin);
     if (value) {
         if (self->open_drain) {
-            gpio_set_pin_direction(self->pin->pin, GPIO_DIRECTION_IN);
+            // Assertion: pull is off, so the pin is floating in this case.
+            // We do open-drain high output (no sinking of current)
+            // by changing the direction to input with no pulls.
+            hri_port_clear_DIR_DIR_bf(PORT, port, pin_mask);
         } else {
-            gpio_set_pin_level(self->pin->pin, true);
-            gpio_set_pin_direction(self->pin->pin, GPIO_DIRECTION_OUT);
+            hri_port_set_DIR_DIR_bf(PORT, port, pin_mask);
+            hri_port_set_OUT_OUT_bf(PORT, port, pin_mask);
         }
     } else {
-        gpio_set_pin_level(self->pin->pin, false);
-        gpio_set_pin_direction(self->pin->pin, GPIO_DIRECTION_OUT);
+        hri_port_set_DIR_DIR_bf(PORT, port, pin_mask);
+        hri_port_clear_OUT_OUT_bf(PORT,port, pin_mask);
     }
 }
 
@@ -105,10 +111,10 @@ bool common_hal_digitalio_digitalinout_get_value(
     if (!self->output) {
         return gpio_get_pin_level(pin);
     } else {
-        if (self->open_drain && hri_port_get_DIR_reg(PORT, (enum gpio_port)GPIO_PORT(pin), 1U << GPIO_PIN(pin)) == 0) {
+        if (self->open_drain && hri_port_get_DIR_reg(PORT, GPIO_PORT(pin), 1U << GPIO_PIN(pin)) == 0) {
             return true;
         } else {
-            return hri_port_get_OUT_reg(PORT, (enum gpio_port)GPIO_PORT(pin), 1U << GPIO_PIN(pin));
+            return hri_port_get_OUT_reg(PORT, GPIO_PORT(pin), 1U << GPIO_PIN(pin));
         }
     }
 }
@@ -119,7 +125,7 @@ void common_hal_digitalio_digitalinout_set_drive_mode(
     bool value = common_hal_digitalio_digitalinout_get_value(self);
     self->open_drain = drive_mode == DRIVE_MODE_OPEN_DRAIN;
     // True is implemented differently between modes so reset the value to make
-    // sure its correct for the new mode.
+    // sure it's correct for the new mode.
     if (value) {
         common_hal_digitalio_digitalinout_set_value(self, value);
     }
@@ -148,7 +154,9 @@ void common_hal_digitalio_digitalinout_set_pull(
         default:
             break;
     }
+    // Set pull first to avoid glitches.
     gpio_set_pin_pull_mode(self->pin->pin, asf_pull);
+    gpio_set_pin_direction(self->pin->pin, GPIO_DIRECTION_IN);
 }
 
 digitalio_pull_t common_hal_digitalio_digitalinout_get_pull(
@@ -158,9 +166,9 @@ digitalio_pull_t common_hal_digitalio_digitalinout_get_pull(
         mp_raise_AttributeError("Cannot get pull while in output mode");
         return PULL_NONE;
     } else {
-        if (hri_port_get_PINCFG_PULLEN_bit(PORT, (enum gpio_port)GPIO_PORT(pin), GPIO_PIN(pin)) == 0) {
+        if (hri_port_get_PINCFG_PULLEN_bit(PORT, GPIO_PORT(pin), GPIO_PIN(pin)) == 0) {
             return PULL_NONE;
-        } if (hri_port_get_OUT_reg(PORT, (enum gpio_port)GPIO_PORT(pin), 1U << GPIO_PIN(pin)) > 0) {
+        } if (hri_port_get_OUT_reg(PORT, GPIO_PORT(pin), 1U << GPIO_PIN(pin)) > 0) {
             return PULL_UP;
         } else {
             return PULL_DOWN;
diff --git a/ports/atmel-samd/mphalport.c b/ports/atmel-samd/mphalport.c
index c1b8ce5504462..a4eca1c748c9c 100644
--- a/ports/atmel-samd/mphalport.c
+++ b/ports/atmel-samd/mphalport.c
@@ -1,3 +1,29 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2016 Scott Shawcroft
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
 #include <string.h>
 
 #include "lib/mp-readline/readline.h"
@@ -14,6 +40,7 @@
 #include "hal/include/hal_delay.h"
 #include "hal/include/hal_gpio.h"
 #include "hal/include/hal_sleep.h"
+#include "sam.h"
 
 #include "mpconfigboard.h"
 #include "mphalport.h"
@@ -22,6 +49,7 @@
 #include "usb.h"
 
 extern struct usart_module usart_instance;
+extern uint32_t common_hal_mcu_processor_get_frequency(void);
 
 int mp_hal_stdin_rx_chr(void) {
     for (;;) {
@@ -71,8 +99,23 @@ void mp_hal_delay_ms(mp_uint_t delay) {
     }
 }
 
+// Use mp_hal_delay_us() for timing of less than 1ms.
+// Do a simple timing loop to wait for a certain number of microseconds.
+// Can be used when interrupts are disabled, which makes tick_delay() unreliable.
+//
+// Testing done at 48 MHz on SAMD21 and 120 MHz on SAMD51, multiplication and division cancel out.
+// But get the frequency just in case.
+#ifdef SAMD21
+#define DELAY_LOOP_ITERATIONS_PER_US ( (10U*48000000U) / common_hal_mcu_processor_get_frequency())
+#endif
+#ifdef SAMD51
+#define DELAY_LOOP_ITERATIONS_PER_US ( (30U*120000000U) / common_hal_mcu_processor_get_frequency())
+#endif
+
 void mp_hal_delay_us(mp_uint_t delay) {
-    tick_delay(delay);
+    for (uint32_t i = delay*DELAY_LOOP_ITERATIONS_PER_US; i > 0; i--) {
+        asm volatile("nop");
+    }
 }
 
 void mp_hal_disable_all_interrupts(void) {
diff --git a/ports/atmel-samd/mphalport.h b/ports/atmel-samd/mphalport.h
index 3b5955a4ffc65..3bc824d4e9baa 100644
--- a/ports/atmel-samd/mphalport.h
+++ b/ports/atmel-samd/mphalport.h
@@ -48,7 +48,6 @@ int receive_usb(void);
 void mp_hal_set_interrupt_char(int c);
 
 void mp_hal_disable_all_interrupts(void);
-
 void mp_hal_enable_all_interrupts(void);
 
 #endif // MICROPY_INCLUDED_ATMEL_SAMD_MPHALPORT_H
diff --git a/ports/atmel-samd/tick.c b/ports/atmel-samd/tick.c
index 9753a1a0c7494..149347793395a 100644
--- a/ports/atmel-samd/tick.c
+++ b/ports/atmel-samd/tick.c
@@ -64,8 +64,8 @@ void tick_init() {
     for (uint16_t i = 0; i < PERIPH_COUNT_IRQn; i++) {
         NVIC_SetPriority(i, (1UL << __NVIC_PRIO_BITS) - 1UL);
     }
-    // Bump up the systick interrupt.
-    NVIC_SetPriority(SysTick_IRQn, 1);
+    // Bump up the systick interrupt so nothing else interferes with timekeeping.
+    NVIC_SetPriority(SysTick_IRQn, 0);
     #ifdef SAMD21
     NVIC_SetPriority(USB_IRQn, 1);
     #endif
diff --git a/ports/nrf/common-hal/microcontroller/__init__.c b/ports/nrf/common-hal/microcontroller/__init__.c
index e26acff1b4cbc..4399e7708c480 100644
--- a/ports/nrf/common-hal/microcontroller/__init__.c
+++ b/ports/nrf/common-hal/microcontroller/__init__.c
@@ -33,6 +33,8 @@
 #include "shared-bindings/microcontroller/Processor.h"
 
 // TODO porting common_hal_mcu
+// This routine should work even when interrupts are disabled. Used by OneWire
+// for precise timing.
 void common_hal_mcu_delay_us(uint32_t delay) {
 //    os_delay_us(delay);
 }
@@ -58,4 +60,3 @@ const mcu_processor_obj_t common_hal_mcu_processor_obj = {
         .type = &mcu_processor_type,
     },
 };
-
diff --git a/ports/nrf/mphalport.h b/ports/nrf/mphalport.h
index 3334fd7fe96a7..a872477370221 100644
--- a/ports/nrf/mphalport.h
+++ b/ports/nrf/mphalport.h
@@ -76,4 +76,3 @@ bool mp_hal_stdin_any(void);
 #define mp_hal_ticks_cpu() (0)
 
 #endif
-
diff --git a/shared-module/bitbangio/OneWire.c b/shared-module/bitbangio/OneWire.c
index e77bdb3b2961f..f5f4790876fbc 100644
--- a/shared-module/bitbangio/OneWire.c
+++ b/shared-module/bitbangio/OneWire.c
@@ -49,6 +49,9 @@ void shared_module_bitbangio_onewire_deinit(bitbangio_onewire_obj_t* self) {
     common_hal_digitalio_digitalinout_deinit(&self->pin);
 }
 
+// We use common_hal_mcu_delay_us(). It should not be dependent on interrupts
+// to do accurate timekeeping, since we disable interrupts during the delays below.
+
 bool shared_module_bitbangio_onewire_reset(bitbangio_onewire_obj_t* self) {
     common_hal_mcu_disable_interrupts();
     common_hal_digitalio_digitalinout_switch_to_output(&self->pin, false, DRIVE_MODE_OPEN_DRAIN);