tests: drivers: uart: baudrate_test: Improve accuracy measurement

Test was reporting high deviation 17-25% which seemed high.
Such measurement inaccuracy was due to 3 reasons:
- using gpio Zephyr API for reading pin state which is twice longer
than using GPIO HAL
- assumption that stop bits are also 0 and should be included in the
measurement. UART stop bits are 1 so they should not be included
- starting UART transfer and then starting the measurement. For
high baudrates beginning of the byte was not included in the
measurement as first gpio read was already 0.

All issues have been fixed:
- Reduced number of bits in the frame (biggest gain).
- Using GPIO HAL for reading the input - sampling frequency is
doubled.
- Transferring 2 bytes and measuring the second byte which improved
accuracy for higher baudrates.

Additionally, reduced number of repeats from 10 to 3 to shorten
the test.

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

Author: nordic-krch

tests/drivers/uart/uart_baudrate_test/Kconfig

@@ -8,7 +8,7 @@ source "Kconfig.zephyr"
 
 config TEST_ALLOWED_DEVIATION
 	int "Allowed deviation (%) for UART timing checks"
-	default 25
+	default 5
 	range 0 100
 	help
 	  Maximum allowed deviation (%) from the programmed values for the test to be

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

@@ -10,19 +10,24 @@
 #include <zephyr/kernel.h>
 #include <zephyr/ztest.h>
 #include <zephyr/drivers/gpio.h>
+#include <hal/nrf_gpio.h>
 #include <stdlib.h>
 #include <math.h>
 
+
+static NRF_GPIO_Type *gpio_port =
+	((NRF_GPIO_Type *)DT_REG_ADDR(DT_GPIO_CTLR(DT_PATH(zephyr_user), gpios)));
+static uint32_t pin_mask = BIT(DT_GPIO_PIN(DT_PATH(zephyr_user), gpios));
 static const struct gpio_dt_spec gpio_spec =
 	GPIO_DT_SPEC_GET_BY_IDX(DT_PATH(zephyr_user), gpios, 0);
 static const struct device *const uart_dev = DEVICE_DT_GET(DT_NODELABEL(dut));
 
-static const uint8_t tx_buf[1] = {0x00};
+static const uint8_t tx_buf[] = {0x00, 0x00};
 
-#define PIN_STATE_SIZE 16384
-static int pin_state[PIN_STATE_SIZE] = {};
+#define PIN_STATE_SIZE 32768
+static uint8_t pin_state[PIN_STATE_SIZE] = {};
 
-#define REPEAT_NUMBER 10
+#define REPEAT_NUMBER 3
 
 #ifdef CONFIG_UART_INTERRUPT_DRIVEN
 static void uart_fifo_callback(const struct device *dev, void *user_data)
@@ -53,16 +58,18 @@ static void check_timing(uint32_t baudrate)
 	double gpio_read_time_us_mean;
 	int32_t start_index;
 	int32_t stop_index;
+	int32_t idle_found;
 	int32_t start_index_count_zero;
 	double bit_diviation_mean;
 	double symbol_diviation_mean;
+	bool once = true;
 	int key;
 
 	ret = uart_config_get(uart_dev, &test_uart_config);
 	zassert_equal(ret, 0, "uart_config_get: %d\n", ret);
 
 	test_uart_config.parity = UART_CFG_PARITY_EVEN;
-	test_uart_config.stop_bits = UART_CFG_STOP_BITS_2;
+	test_uart_config.stop_bits = UART_CFG_STOP_BITS_1;
 	test_uart_config.flow_ctrl = UART_CFG_FLOW_CTRL_NONE;
 	test_uart_config.baudrate = baudrate;
 	ret = uart_configure(uart_dev, &test_uart_config);
@@ -75,37 +82,11 @@ static void check_timing(uint32_t baudrate)
 	cycles_s_sys = (uint64_t)sys_clock_hw_cycles_per_sec();
 	TC_PRINT("Cycles: %llu cycles\n", cycles_s_sys);
 
-	/*
-	 * Measure time needed to read gpios
-	 */
-	gpio_read_time_us_mean = 0;
-	key = irq_lock();
-	for (uint32_t t = 0; t < REPEAT_NUMBER; ++t) {
-		cycle_start_time = k_cycle_get_32();
-		for (uint32_t i = 0; i < PIN_STATE_SIZE; ++i) {
-			pin_state[i] = gpio_pin_get_dt(&gpio_spec);
-		}
-		cycle_stop_time = k_cycle_get_32();
-		double gpio_read_time_us =
-			((cycle_stop_time - cycle_start_time) / (double)PIN_STATE_SIZE) *
-			(1e6 / cycles_s_sys);
-		gpio_read_time_us_mean += gpio_read_time_us;
-	}
-	irq_unlock(key);
-	gpio_read_time_us_mean /= (double)REPEAT_NUMBER;
-	TC_PRINT("GPIO get takes: %.2f us\n", gpio_read_time_us_mean);
-
 	double expected_bit_period_us = 1e6 / (double)baudrate;
 	double number_of_bits = 8;
 
 	number_of_bits += 1;
-	if (test_uart_config.stop_bits == UART_CFG_STOP_BITS_1) {
-		number_of_bits += 1;
-	} else if (test_uart_config.stop_bits == UART_CFG_STOP_BITS_2) {
-		number_of_bits += 2;
-	} else {
-		zassert_true(false, "Unsupported stop_bits: %d", test_uart_config.stop_bits);
-	}
+	/* Stop bit is 1 so it is not counted. */
 	if (test_uart_config.parity != UART_CFG_PARITY_NONE) {
 		number_of_bits += 1;
 	}
@@ -114,11 +95,6 @@ static void check_timing(uint32_t baudrate)
 	TC_PRINT("[%d] Expected symbol time: %.2f us, expected bit time: %.2f us\n", baudrate,
 		 expected_symbol_period_us, expected_bit_period_us);
 
-	if (expected_bit_period_us < gpio_read_time_us_mean) {
-		TC_PRINT("[%d] Not supported - gpio measurement is too slow.\n", baudrate);
-		ztest_test_skip();
-	}
-
 	start_index_count_zero = 0;
 	bit_diviation_mean = 0;
 	symbol_diviation_mean = 0;
@@ -136,26 +112,49 @@ static void check_timing(uint32_t baudrate)
 		/*
 		 * Check gpio
 		 */
+
 		key = irq_lock();
+		cycle_start_time = k_cycle_get_32();
 		for (uint32_t i = 0; i < PIN_STATE_SIZE; ++i) {
-			pin_state[i] = gpio_pin_get_dt(&gpio_spec);
+			pin_state[i] = nrf_gpio_port_in_read(gpio_port) & pin_mask ? 1 : 0;
 		}
+		cycle_stop_time = k_cycle_get_32();
 		irq_unlock(key);
 
+		if (once) {
+			/* Calculate only for the first iteration. */
+			uint32_t t_us = k_cyc_to_us_ceil32(cycle_stop_time - cycle_start_time);
+
+			gpio_read_time_us_mean = (double)t_us / PIN_STATE_SIZE;
+			once = false;
+			TC_PRINT("GPIO get takes: %.2f us\n", gpio_read_time_us_mean);
+		}
+
+		if (expected_bit_period_us < gpio_read_time_us_mean) {
+			TC_PRINT("[%d] Not supported - gpio measurement is too slow.\n", baudrate);
+			ztest_test_skip();
+		}
+
 		/*
-		 * Find start of start bit and end of stop bit
+		 * Find start of start bit and end of stop bit. For higher baudrates it is
+		 * possible that first byte is already being transferred. In search for
+		 * byte start and end, start from searching for idle state between byte 0 and 1.
 		 */
 		start_index = -1;
 		stop_index = -1;
+		idle_found = -1;
 		for (uint32_t i = 0; i < PIN_STATE_SIZE; ++i) {
 			if (-1 == start_index) {
-				if (0 == pin_state[i]) {
+				if (1 == pin_state[i]) {
+					idle_found = 0;
+				} else if ((idle_found == 0) && (0 == pin_state[i])) {
 					start_index = i;
 				}
 			} else {
 				if (-1 == stop_index) {
 					if (1 == pin_state[i]) {
 						stop_index = i;
+						break;
 					}
 				} else {
 					zassert_true(1 == pin_state[i], "Unexpected low at %d\n",