diff --git a/TESTS/mbed_drivers/lp_timeout/main.cpp b/TESTS/mbed_drivers/lp_timeout/main.cpp
index 6effbe39090..8d4412b5010 100644
--- a/TESTS/mbed_drivers/lp_timeout/main.cpp
+++ b/TESTS/mbed_drivers/lp_timeout/main.cpp
@@ -43,8 +43,18 @@ void lp_timeout_1s_deepsleep(void)
 {
     complete = false;
 
-    /* 
-     * We use here lp_ticker_read() instead of us_ticker_read() for start and 
+    /*
+     * Since deepsleep() may shut down the UART peripheral, we wait for 10ms
+     * to allow for hardware serial buffers to completely flush.
+
+     * This should be replaced with a better function that checks if the
+     * hardware buffers are empty. However, such an API does not exist now,
+     * so we'll use the wait_ms() function for now.
+     */
+    wait_ms(10);
+
+    /*
+     * We use here lp_ticker_read() instead of us_ticker_read() for start and
      * end because the microseconds timer might be disable during deepsleep.
      */
     timestamp_t start = lp_ticker_read();
diff --git a/TESTS/mbed_hal/lp_ticker/main.cpp b/TESTS/mbed_hal/lp_ticker/main.cpp
index a634ff6a73a..b706087d1d6 100644
--- a/TESTS/mbed_hal/lp_ticker/main.cpp
+++ b/TESTS/mbed_hal/lp_ticker/main.cpp
@@ -65,12 +65,22 @@ void lp_ticker_1s_deepsleep()
     complete = false;
     uint32_t delay_ts;
 
+    /*
+     * Since deepsleep() may shut down the UART peripheral, we wait for 10ms
+     * to allow for hardware serial buffers to completely flush.
+
+     * This should be replaced with a better function that checks if the
+     * hardware buffers are empty. However, such an API does not exist now,
+     * so we'll use the wait_ms() function for now.
+     */
+    wait_ms(10);
+
     ticker_set_handler(lp_ticker_data, cb_done);
     ticker_remove_event(lp_ticker_data, &delay_event);
     delay_ts = lp_ticker_read() + 1000000;
 
-    /* 
-     * We use here lp_ticker_read() instead of us_ticker_read() for start and 
+    /*
+     * We use here lp_ticker_read() instead of us_ticker_read() for start and
      * end because the microseconds timer might be disable during deepsleep.
      */
     timestamp_t start = lp_ticker_read();