Test: Sleep: Increase the UART flush wait

TESTS/mbed_hal/sleep/sleep_test_utils.h

@@ -29,21 +29,20 @@
 #include "hal/us_ticker_api.h"
 #include "hal/lp_ticker_api.h"
 
-/* Flush serial buffer before deep sleep
+/* To prevent a loss of Greentea data, the serial buffers have to be flushed
+ * before the UART peripheral shutdown. The UART shutdown happens when the
+ * device is entering the deepsleep mode or performing a reset.
  *
- * Since deepsleep() may shut down the UART peripheral, we wait for some time
- * to allow for hardware serial buffers to completely flush.
+ * With the current API, it is not possible to check if the hardware buffers
+ * are empty. However, it is possible to determine the time required for the
+ * buffers to flush.
  *
- * Take NUMAKER_PFM_NUC472 as an example:
- * Its UART peripheral has 16-byte Tx FIFO. With baud rate set to 9600, flush
- * Tx FIFO would take: 16 * 8 * 1000 / 9600 = 13.3 (ms). So set wait time to
- * 20ms here for safe.
- *
- * 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 busy_wait_ms() function for now.
+ * Assuming the biggest Tx FIFO of 128 bytes (as for CY8CPROTO_062_4343W)
+ * and a default UART config (9600, 8N1), flushing the Tx FIFO wold take:
+ * (1 start_bit + 8 data_bits + 1 stop_bit) * 128 * 1000 / 9600 = 133.3 ms.
+ * To be on the safe side, set the wait time to 150 ms.
  */
-#define SERIAL_FLUSH_TIME_MS 20
+#define SERIAL_FLUSH_TIME_MS 150
 
 #define US_PER_S 1000000