diff --git a/ports/rp2/main.c b/ports/rp2/main.c
index 40374faff9..5fb47cc402 100644
--- a/ports/rp2/main.c
+++ b/ports/rp2/main.c
@@ -76,6 +76,9 @@ int main(int argc, char **argv) {
     // This is a tickless port, interrupts should always trigger SEV.
     SCB->SCR |= SCB_SCR_SEVONPEND_Msk;
 
+    pendsv_init();
+    soft_timer_init();
+
     #if MICROPY_HW_ENABLE_UART_REPL
     bi_decl(bi_program_feature("UART REPL"))
     setup_default_uart();
diff --git a/ports/rp2/mpconfigport.h b/ports/rp2/mpconfigport.h
index a29692d0be..128295f19f 100644
--- a/ports/rp2/mpconfigport.h
+++ b/ports/rp2/mpconfigport.h
@@ -154,6 +154,10 @@
 #define MICROPY_SSL_MBEDTLS                     (1)
 #define MICROPY_PY_LWIP_SOCK_RAW                (MICROPY_PY_LWIP)
 
+// Hardware timer alarm index. Available range 0-3.
+// Number 3 is currently used by pico-sdk (PICO_TIME_DEFAULT_ALARM_POOL_HARDWARE_ALARM_NUM)
+#define MICROPY_HW_SOFT_TIMER_ALARM_NUM         (2)
+
 // fatfs configuration
 #define MICROPY_FATFS_ENABLE_LFN                (1)
 #define MICROPY_FATFS_LFN_CODE_PAGE             437 /* 1=SFN/ANSI 437=LFN/U.S.(OEM) */
@@ -266,6 +270,7 @@ typedef intptr_t mp_off_t;
 #define BINARY_INFO_ID_MP_FROZEN 0x4a99d719
 #define MICROPY_FROZEN_LIST_ITEM(name, file) bi_decl(bi_string(BINARY_INFO_TAG_MICROPYTHON, BINARY_INFO_ID_MP_FROZEN, name))
 
+
 extern uint32_t rosc_random_u32(void);
 extern void lwip_lock_acquire(void);
 extern void lwip_lock_release(void);
diff --git a/ports/rp2/mphalport.c b/ports/rp2/mphalport.c
index 1641eadb7b..a13a0edc30 100644
--- a/ports/rp2/mphalport.c
+++ b/ports/rp2/mphalport.c
@@ -35,6 +35,7 @@
 #include "pendsv.h"
 #include "tusb.h"
 #include "uart.h"
+#include "hardware/irq.h"
 #include "hardware/rtc.h"
 #include "pico/unique_id.h"
 
@@ -46,8 +47,6 @@
 // microseconds since the Epoch.
 static uint64_t time_us_64_offset_from_epoch;
 
-static alarm_id_t soft_timer_alarm_id = 0;
-
 #if MICROPY_HW_ENABLE_UART_REPL || MICROPY_HW_USB_CDC
 
 #ifndef MICROPY_HW_STDIN_BUFFER_LEN
@@ -199,17 +198,31 @@ mp_uint_t mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
     return did_write ? ret : 0;
 }
 
+void mp_hal_delay_us(mp_uint_t us) {
+    // Avoid calling sleep_us() and invoking the alarm pool by splitting long
+    // sleeps into an optional longer sleep and a shorter busy-wait
+    uint64_t end = time_us_64() + us;
+    if (us > 1000) {
+        mp_hal_delay_ms(us / 1000);
+    }
+    while (time_us_64() < end) {
+        // Tight loop busy-wait for accurate timing
+    }
+}
+
 void mp_hal_delay_ms(mp_uint_t ms) {
-    absolute_time_t t = make_timeout_time_ms(ms);
+    mp_uint_t start = mp_hal_ticks_ms();
+    mp_uint_t elapsed = 0;
     do {
-        mp_event_handle_nowait();
-    } while (!best_effort_wfe_or_timeout(t));
+        mp_event_wait_ms(ms - elapsed);
+        elapsed = mp_hal_ticks_ms() - start;
+    } while (elapsed < ms);
 }
 
 void mp_hal_time_ns_set_from_rtc(void) {
-    // Delay at least one RTC clock cycle so it's registers have updated with the most
-    // recent time settings.
-    sleep_us(23);
+    // Outstanding RTC register writes need at least two RTC clock cycles to
+    // update. (See RP2040 datasheet section 4.8.4 "Reference clock").
+    mp_hal_delay_us(44);
 
     // Sample RTC and time_us_64() as close together as possible, so the offset
     // calculated for the latter can be as accurate as possible.
@@ -273,21 +286,40 @@ uint32_t storage_read_blocks(uint8_t *dest, uint32_t block_num, uint32_t num_blo
     panic_unsupported();
 }
 
-static int64_t soft_timer_callback(alarm_id_t id, void *user_data) {
-    soft_timer_alarm_id = 0;
-    pendsv_schedule_dispatch(PENDSV_DISPATCH_SOFT_TIMER, soft_timer_handler);
-    return 0; // don't reschedule this alarm
-}
-
 uint32_t soft_timer_get_ms(void) {
     return mp_hal_ticks_ms();
 }
 
 void soft_timer_schedule_at_ms(uint32_t ticks_ms) {
-    if (soft_timer_alarm_id != 0) {
-        cancel_alarm(soft_timer_alarm_id);
-    }
     int32_t ms = soft_timer_ticks_diff(ticks_ms, mp_hal_ticks_ms());
     ms = MAX(0, ms);
-    soft_timer_alarm_id = add_alarm_in_ms(ms, soft_timer_callback, NULL, true);
+    if (hardware_alarm_set_target(MICROPY_HW_SOFT_TIMER_ALARM_NUM, delayed_by_ms(get_absolute_time(), ms))) {
+        // "missed" hardware alarm target
+        hardware_alarm_force_irq(MICROPY_HW_SOFT_TIMER_ALARM_NUM);
+    }
+}
+
+static void soft_timer_hardware_callback(unsigned int alarm_num) {
+    // The timer alarm ISR needs to call here and trigger PendSV dispatch via
+    // a second ISR, as PendSV may be currently suspended by the other CPU.
+    pendsv_schedule_dispatch(PENDSV_DISPATCH_SOFT_TIMER, soft_timer_handler);
+}
+
+void soft_timer_init(void) {
+    hardware_alarm_claim(MICROPY_HW_SOFT_TIMER_ALARM_NUM);
+    hardware_alarm_set_callback(MICROPY_HW_SOFT_TIMER_ALARM_NUM, soft_timer_hardware_callback);
+}
+
+void mp_wfe_or_timeout(uint32_t timeout_ms) {
+    soft_timer_entry_t timer;
+
+    // Note the timer doesn't have an associated callback, it just exists to create a
+    // hardware interrupt to wake the CPU
+    soft_timer_static_init(&timer, SOFT_TIMER_MODE_ONE_SHOT, 0, NULL);
+    soft_timer_insert(&timer, timeout_ms);
+
+    __wfe();
+
+    // Clean up the timer node if it's not already
+    soft_timer_remove(&timer);
 }
diff --git a/ports/rp2/mphalport.h b/ports/rp2/mphalport.h
index d2d74d783a..bc09f8c4a5 100644
--- a/ports/rp2/mphalport.h
+++ b/ports/rp2/mphalport.h
@@ -62,23 +62,22 @@
         if ((TIMEOUT_MS) < 0) { \
             __wfe(); \
         } else { \
-            best_effort_wfe_or_timeout(make_timeout_time_ms(TIMEOUT_MS)); \
+            mp_wfe_or_timeout(TIMEOUT_MS); \
         } \
     } while (0)
 
 extern int mp_interrupt_char;
 extern ringbuf_t stdin_ringbuf;
 
+// Port-specific function to create a wakeup interrupt after timeout_ms and enter WFE
+void mp_wfe_or_timeout(uint32_t timeout_ms);
+
 uint32_t mp_thread_begin_atomic_section(void);
 void mp_thread_end_atomic_section(uint32_t);
 
 void mp_hal_set_interrupt_char(int c);
 void mp_hal_time_ns_set_from_rtc(void);
 
-static inline void mp_hal_delay_us(mp_uint_t us) {
-    sleep_us(us);
-}
-
 static inline void mp_hal_delay_us_fast(mp_uint_t us) {
     busy_wait_us(us);
 }
diff --git a/ports/rp2/mpnetworkport.c b/ports/rp2/mpnetworkport.c
index 09a543c2db..4690a4e511 100644
--- a/ports/rp2/mpnetworkport.c
+++ b/ports/rp2/mpnetworkport.c
@@ -66,7 +66,7 @@ static void gpio_irq_handler(void) {
 void cyw43_irq_init(void) {
     gpio_add_raw_irq_handler_with_order_priority(CYW43_PIN_WL_HOST_WAKE, gpio_irq_handler, CYW43_SHARED_IRQ_HANDLER_PRIORITY);
     irq_set_enabled(IO_IRQ_BANK0, true);
-    NVIC_SetPriority(PendSV_IRQn, PICO_LOWEST_IRQ_PRIORITY);
+    NVIC_SetPriority(PendSV_IRQn, IRQ_PRI_PENDSV);
 }
 
 void cyw43_post_poll_hook(void) {
diff --git a/ports/rp2/pendsv.c b/ports/rp2/pendsv.c
index e4f662715f..d24b4cb012 100644
--- a/ports/rp2/pendsv.c
+++ b/ports/rp2/pendsv.c
@@ -25,6 +25,7 @@
  */
 
 #include <assert.h>
+#include "pico/mutex.h"
 #include "py/mpconfig.h"
 #include "pendsv.h"
 #include "RP2040.h"
@@ -34,15 +35,21 @@
 #endif
 
 static pendsv_dispatch_t pendsv_dispatch_table[PENDSV_DISPATCH_NUM_SLOTS];
-static int pendsv_lock;
+static recursive_mutex_t pendsv_mutex;
+
+void pendsv_init(void) {
+    recursive_mutex_init(&pendsv_mutex);
+}
 
 void pendsv_suspend(void) {
-    pendsv_lock++;
+    // Recursive Mutex here as either core may call pendsv_suspend() and expect
+    // both mutual exclusion (other core can't enter pendsv_suspend() at the
+    // same time), and that no PendSV handler will run.
+    recursive_mutex_enter_blocking(&pendsv_mutex);
 }
 
 void pendsv_resume(void) {
-    pendsv_lock--;
-    assert(pendsv_lock >= 0);
+    recursive_mutex_exit(&pendsv_mutex);
     // Run pendsv if needed.  Find an entry with a dispatch and call pendsv dispatch
     // with it.  If pendsv runs it will service all slots.
     int count = PENDSV_DISPATCH_NUM_SLOTS;
@@ -55,9 +62,11 @@ void pendsv_resume(void) {
 }
 
 void pendsv_schedule_dispatch(size_t slot, pendsv_dispatch_t f) {
-    assert(pendsv_lock >= 0);
     pendsv_dispatch_table[slot] = f;
-    if (pendsv_lock == 0) {
+    if (pendsv_mutex.enter_count == 0) {
+        // There is a race here where other core calls pendsv_suspend() before
+        // ISR can execute, but dispatch will happen later when other core
+        // calls pendsv_resume().
         SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
     } else {
         #if MICROPY_PY_NETWORK_CYW43
@@ -68,7 +77,14 @@ void pendsv_schedule_dispatch(size_t slot, pendsv_dispatch_t f) {
 
 // PendSV interrupt handler to perform background processing.
 void PendSV_Handler(void) {
-    assert(pendsv_lock == 0);
+
+    if (!recursive_mutex_try_enter(&pendsv_mutex, NULL)) {
+        // Failure here means core 1 holds pendsv_mutex. ISR will
+        // run again after core 1 calls pendsv_resume().
+        return;
+    }
+    // Core 0 should not already have locked pendsv_mutex
+    assert(pensv_mutex.enter_count == 1);
 
     #if MICROPY_PY_NETWORK_CYW43
     CYW43_STAT_INC(PENDSV_RUN_COUNT);
@@ -81,4 +97,6 @@ void PendSV_Handler(void) {
             f();
         }
     }
+
+    recursive_mutex_exit(&pendsv_mutex);
 }
diff --git a/ports/rp2/pendsv.h b/ports/rp2/pendsv.h
index bc8e8d61c8..a5d9440211 100644
--- a/ports/rp2/pendsv.h
+++ b/ports/rp2/pendsv.h
@@ -42,8 +42,12 @@ enum {
 
 #define PENDSV_DISPATCH_NUM_SLOTS PENDSV_DISPATCH_MAX
 
+// PendSV IRQ priority, to run system-level tasks that preempt the main thread.
+#define IRQ_PRI_PENDSV PICO_LOWEST_IRQ_PRIORITY
+
 typedef void (*pendsv_dispatch_t)(void);
 
+void pendsv_init(void);
 void pendsv_suspend(void);
 void pendsv_resume(void);
 void pendsv_schedule_dispatch(size_t slot, pendsv_dispatch_t f);
diff --git a/shared/runtime/softtimer.c b/shared/runtime/softtimer.c
index 13a6e78777..92a0d6d511 100644
--- a/shared/runtime/softtimer.c
+++ b/shared/runtime/softtimer.c
@@ -89,7 +89,7 @@ void soft_timer_handler(void) {
         heap = (soft_timer_entry_t *)mp_pairheap_pop(soft_timer_lt, &heap->pairheap);
         if (entry->flags & SOFT_TIMER_FLAG_PY_CALLBACK) {
             mp_sched_schedule(entry->py_callback, MP_OBJ_FROM_PTR(entry));
-        } else {
+        } else if (entry->c_callback) {
             entry->c_callback(entry);
         }
         if (entry->mode == SOFT_TIMER_MODE_PERIODIC) {
diff --git a/shared/runtime/softtimer.h b/shared/runtime/softtimer.h
index fe5d02b907..c7b374d755 100644
--- a/shared/runtime/softtimer.h
+++ b/shared/runtime/softtimer.h
@@ -81,6 +81,10 @@ static inline void soft_timer_reinsert(soft_timer_entry_t *entry, uint32_t initi
 //   pend-SV IRQ level, or equivalent.
 uint32_t soft_timer_get_ms(void);
 void soft_timer_schedule_at_ms(uint32_t ticks_ms);
+
+// Optional port-specific initialisation function (provided by the port and called
+// by the port if needed.)
+void soft_timer_init(void);
 #endif
 
 #endif // MICROPY_INCLUDED_SHARED_RUNTIME_SOFTTIMER_H