diff --git a/hal/targets/cmsis/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF51822/system_nrf51.c b/hal/targets/cmsis/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF51822/system_nrf51.c
index 3414e5b38d..165b67f431 100644
--- a/hal/targets/cmsis/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF51822/system_nrf51.c
+++ b/hal/targets/cmsis/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF51822/system_nrf51.c
@@ -94,6 +94,16 @@ void SystemInit(void)
         }
         *(uint32_t volatile *)0x4006EC14 = 0xC0;
     }
+
+    // Start the external 32khz crystal oscillator.
+    NRF_CLOCK->LFCLKSRC             = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);
+    NRF_CLOCK->EVENTS_LFCLKSTARTED  = 0;
+    NRF_CLOCK->TASKS_LFCLKSTART     = 1;
+
+    // Wait for the external oscillator to start up.
+    while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0) {
+        // Do nothing.
+    }
 }
 
 
diff --git a/hal/targets/cmsis/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF52832/system_nrf52.c b/hal/targets/cmsis/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF52832/system_nrf52.c
index 72e067cad7..4b101c519a 100644
--- a/hal/targets/cmsis/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF52832/system_nrf52.c
+++ b/hal/targets/cmsis/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF52832/system_nrf52.c
@@ -187,6 +187,16 @@ void SystemInit(void)
     #endif
 
     SystemCoreClockUpdate();
+
+    // Start the external 32khz crystal oscillator.
+    NRF_CLOCK->LFCLKSRC             = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);
+    NRF_CLOCK->EVENTS_LFCLKSTARTED  = 0;
+    NRF_CLOCK->TASKS_LFCLKSTART     = 1;
+
+    // Wait for the external oscillator to start up.
+    while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0) {
+        // Do nothing.
+    }
 }