diff --git a/rtos/TARGET_CORTEX/SysTimer.cpp b/rtos/TARGET_CORTEX/SysTimer.cpp
index 41a8a2157e..3f56ac36f7 100644
--- a/rtos/TARGET_CORTEX/SysTimer.cpp
+++ b/rtos/TARGET_CORTEX/SysTimer.cpp
@@ -45,6 +45,10 @@ SysTimer::SysTimer() :
         TimerEvent(get_lp_ticker_data()), _start_time(0), _tick(0)
 {
     _start_time = ticker_read_us(_ticker_data);
+}
+
+void SysTimer::setup_irq()
+{
 #if (defined(NO_SYSTICK))
     NVIC_SetVector(mbed_get_m0_tick_irqn(), (uint32_t)SysTick_Handler);
     NVIC_SetPriority(mbed_get_m0_tick_irqn(), 0xFF); /* RTOS requires lowest priority */
@@ -96,16 +100,26 @@ SysTimer::~SysTimer()
 {
 }
 
-void SysTimer::handler()
+void SysTimer::set_irq_pending()
 {
 #if (defined(NO_SYSTICK))
     NVIC_SetPendingIRQ(mbed_get_m0_tick_irqn());
 #else
     SCB->ICSR = SCB_ICSR_PENDSTSET_Msk;
 #endif
+}
+
+void SysTimer::increment_tick()
+{
     _tick++;
 }
 
+void SysTimer::handler()
+{
+    set_irq_pending();
+    increment_tick();
+}
+
 }
 }
 
diff --git a/rtos/TARGET_CORTEX/SysTimer.h b/rtos/TARGET_CORTEX/SysTimer.h
index c902629481..2af7c8ec33 100644
--- a/rtos/TARGET_CORTEX/SysTimer.h
+++ b/rtos/TARGET_CORTEX/SysTimer.h
@@ -51,6 +51,11 @@ public:
     SysTimer();
     virtual ~SysTimer();
 
+    /**
+     * Enable an IRQ/SysTick with the correct priority.
+     */
+    static void setup_irq();
+
     /**
      * Schedule an os tick to fire
      *
@@ -92,6 +97,8 @@ public:
 
 protected:
     virtual void handler();
+    void increment_tick();
+    static void set_irq_pending();
     us_timestamp_t _start_time;
     uint64_t _tick;
 };
diff --git a/rtos/TARGET_CORTEX/mbed_rtx_idle.cpp b/rtos/TARGET_CORTEX/mbed_rtx_idle.cpp
index c027552864..ac0ad4c53b 100644
--- a/rtos/TARGET_CORTEX/mbed_rtx_idle.cpp
+++ b/rtos/TARGET_CORTEX/mbed_rtx_idle.cpp
@@ -47,6 +47,7 @@ int32_t OS_Tick_Enable (void)
     // Do not use SingletonPtr since this relies on the RTOS
     if (NULL == os_timer) {
         os_timer = new (os_timer_data) rtos::internal::SysTimer();
+        os_timer->setup_irq();
     }
 
     // set to fire interrupt on next tick