tests: drivers: uart: uart_baudrate_test: Handle low baudrates

Fast UARTE instances cannot generate precisely low baudrates
as BAUDRATE register which holds the divider has only top
16 bits implemented so if peripheral clock frequency is high
and baudrate is low then those missing bits results in
deviation of the baudrate. Mainly visible on 4800 baudrate
with UARTE clocked by 320 MHz clock.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

Author: nordic-krch

tests/drivers/uart/uart_baudrate_test/src/main.c

@@ -64,6 +64,16 @@ static void check_timing(uint32_t baudrate)
 	double symbol_diviation_mean;
 	bool once = true;
 	int key;
+	uint32_t uart_freq = NRF_PERIPH_GET_FREQUENCY(DT_NODELABEL(dut));
+	uint32_t accepted_deviation = CONFIG_TEST_ALLOWED_DEVIATION;
+
+	/* Baudrate register has only top 16 bits implemented so if baudrate is
+	 * significantly lower than the peripheral frequency, accuracy of the baudrate
+	 * decreases.
+	 */
+	if (uart_freq / 10000) {
+		accepted_deviation += 5;
+	}
 
 	ret = uart_config_get(uart_dev, &test_uart_config);
 	zassert_equal(ret, 0, "uart_config_get: %d\n", ret);
@@ -195,12 +205,12 @@ static void check_timing(uint32_t baudrate)
 		 uart_dev->name, baudrate, symbol_diviation_mean, bit_diviation_mean);
 
 	if (start_index_count_zero == 0) {
-		zassert_true(symbol_diviation_mean <= (double)CONFIG_TEST_ALLOWED_DEVIATION,
-			     "Symbol diviation %0.f%%  higher than %d%%\n", symbol_diviation_mean,
-			     CONFIG_TEST_ALLOWED_DEVIATION);
-		zassert_true(bit_diviation_mean <= (double)CONFIG_TEST_ALLOWED_DEVIATION,
-			     "Bit diviation %0.f%% higher than %d%%\n", bit_diviation_mean,
-			     CONFIG_TEST_ALLOWED_DEVIATION);
+		zassert_true(symbol_diviation_mean <= (double)accepted_deviation,
+			     "Symbol diviation %0.f%%  higher than %d%%\n",
+			     symbol_diviation_mean, accepted_deviation);
+		zassert_true(bit_diviation_mean <= (double)accepted_deviation,
+			     "Bit diviation %0.f%% higher than %d%%\n",
+			     bit_diviation_mean, accepted_deviation);
 	} else {
 		TC_PRINT("Not checking diviation due to lost start of start bit\n");
 	}