samples: drivers: uart: add RS485 echo bot sample

rlevano77 · rlevano77 · commit 8a07907ed36b · 2025-12-05T05:06:35.000-05:00
Add a sample application demonstrating RS485 transceiver control
using the SAM0 UART driver. The sample shows how to:

- Configure UART in RS485 mode at runtime
- Use de-gpios and re-gpios for transceiver control
- Implement echo bot with interrupt-driven RX and polling TX

The sample includes complete documentation and overlay for
SAME54 Xplained Pro board using SERCOM0 with DE/RE control
on PA22.

Hardware tested:
- SAME54 Xplained Pro board
- SparkFun RS-485 Transceiver Breakout
- DTECH USB-to-RS485 converter

Signed-off-by: Ricardo Levano &lt;rlevano77@gmail.com&gt;
diff --git a/samples/drivers/uart/echo_bot_rs485/CMakeLists.txt b/samples/drivers/uart/echo_bot_rs485/CMakeLists.txt
@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(uart_echo_bot)
+
+target_sources(app PRIVATE src/main.c)
diff --git a/samples/drivers/uart/echo_bot_rs485/README.rst b/samples/drivers/uart/echo_bot_rs485/README.rst
@@ -0,0 +1,96 @@
+.. zephyr:code-sample:: uart-rs485
+   :name: UART RS485 echo bot
+   :relevant-api: uart_interface
+
+   Read data from RS485 bus and echo it back.
+
+Overview
+********
+
+This sample demonstrates how to use the UART serial driver with RS485 transceiver
+control. It reads data from the RS485 bus and echoes the characters back after
+an end of line (return key) is received.
+
+The sample automatically configures the UART in RS485 mode and controls the
+Driver Enable (DE) and Receiver Enable (RE) signals to manage bus direction.
+
+The polling API is used for sending data and the interrupt-driven API
+for receiving, so that in theory the thread could do something else
+while waiting for incoming data.
+
+Hardware Requirements
+*********************
+
+This sample requires a UART peripheral with RS485 transceiver control support.
+The hardware must have:
+
+* A UART peripheral (e.g., SERCOM on SAM devices)
+* GPIO pins for DE (Driver Enable) and RE (Receiver Enable) control
+* An RS485 transceiver chip connected to the UART
+
+Test Hardware
+=============
+
+This sample was tested with the following hardware:
+
+* **Development Board**: SAME54 Xplained Pro
+* **RS485 Transceiver**: `SparkFun Transceiver Breakout - RS-485 <https://www.sparkfun.com/sparkfun-transceiver-breakout-rs-485.html>`_
+* **USB-to-RS485 Adapter**: `DTECH USB to RS422/RS485 Converter Cable <https://www.dtechelectronics.com/high-quality-usb-2-0-to-rs422-rs485-converter-cable-1-2m-gold-plated-db9-male-usb-to-rs422-rs485-adapter-cable_p368.html>`_
+
+SAME54 Xplained Pro Configuration
+**********************************
+
+For the SAME54 Xplained Pro board, the sample is configured to use:
+
+* **SERCOM0** as UART
+* **PA04** - TX (SERCOM0 PAD0)
+* **PA05** - RX (SERCOM0 PAD1)
+* **PA22** - DE/RE (Driver/Receiver Enable, shared pin)
+* **Baud rate**: 115200
+
+Connect your RS485 transceiver as follows:
+
+1. Connect SERCOM0 TX (PA04) to transceiver DI (Driver Input)
+2. Connect SERCOM0 RX (PA05) to transceiver RO (Receiver Output)
+3. Connect PA22 to both DE and ~RE pins on the transceiver
+4. Connect A and B lines to your RS485 bus
+5. Add 120Ω termination resistor if at bus end
+
+Building and Running
+********************
+
+Build and flash the sample for SAME54 Xplained Pro:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/drivers/uart/echo_bot_rs485
+   :board: same54_xpro
+   :goals: build flash
+   :compact:
+
+Sample Output
+=============
+
+.. code-block:: console
+
+    *** Booting Zephyr OS build v4.3.0-rc3 ***
+    RS485 mode enabled
+    Hello! I'm your echo bot.
+    Tell me something and press enter:
+    # Type e.g. "Hi there!" and hit enter!
+    Echo: Hi there!
+
+Testing
+*******
+
+To test the RS485 functionality:
+
+1. Connect another RS485 device to the bus
+2. Configure it with 115200 baud, 8N1
+3. Send text data followed by newline
+4. The echo bot will respond with "Echo: <your message>"
+
+The DE/RE GPIO will automatically:
+
+* Switch to transmit mode (DE=HIGH, RE=LOW) when sending
+* Switch to receive mode (DE=LOW, RE=HIGH) when receiving
+* Transition happens automatically in the driver
diff --git a/samples/drivers/uart/echo_bot_rs485/boards/same54_xpro.overlay b/samples/drivers/uart/echo_bot_rs485/boards/same54_xpro.overlay
@@ -0,0 +1,69 @@
+/*
+ * Copyright (c) 2025 Altronix
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/*
+ * RS485 Echo Bot configuration for SAME54 Xplained Pro
+ * Using SERCOM0 with:
+ *   - TX on PA04 (SERCOM0 PAD0)
+ *   - RX on PA05 (SERCOM0 PAD1)
+ *   - DE/RE on PA22 (Driver Enable and Receiver Enable tied together)
+ */
+
+#include <dt-bindings/pinctrl/same54p-pinctrl.h>
+
+/* Add SERCOM0 pinctrl configuration */
+&pinctrl {
+	sercom0_uart_rs485: sercom0_uart_rs485 {
+		group1 {
+			pinmux = <PA4D_SERCOM0_PAD0>,  /* TX */
+				 <PA5D_SERCOM0_PAD1>;  /* RX */
+		};
+	};
+};
+
+/* Configure SERCOM0 as UART with RS485 support */
+&sercom0 {
+	status = "okay";
+	compatible = "atmel,sam0-uart";
+	current-speed = <115200>;
+	rxpo = <1>;  /* RX on PAD1 */
+	txpo = <0>;  /* TX on PAD0 */
+
+	/*
+	 * RS485 transceiver control
+	 *
+	 * Option 1: DE and RE tied together (RECOMMENDED - most common)
+	 * ---------------------------------------------------------------
+	 * If your RS485 transceiver has DE and RE pins connected together,
+	 * only specify de-gpios:
+	 *
+	 *   de-gpios = <&porta 22 GPIO_ACTIVE_HIGH>;
+	 *
+	 * The driver will:
+	 *   - Set GPIO HIGH during transmit (enables driver)
+	 *   - Set GPIO LOW during receive (enables receiver)
+	 *
+	 * Option 2: Separate DE and RE control
+	 * -------------------------------------
+	 * If your transceiver has separate DE and RE pins, specify both:
+	 *
+	 *   de-gpios = <&porta 22 GPIO_ACTIVE_HIGH>;
+	 *   re-gpios = <&porta 23 GPIO_ACTIVE_LOW>;
+	 *
+	 * Note: RE is typically active-low (inverted), so when the driver
+	 * "enables RX", it sets the GPIO to 1, which pulls RE low.
+	 */
+	de-gpios = <&porta 22 GPIO_ACTIVE_HIGH>;
+
+	pinctrl-0 = <&sercom0_uart_rs485>;
+	pinctrl-names = "default";
+};
+
+/* Override the chosen UART for this sample */
+/ {
+	chosen {
+		zephyr,shell-uart = &sercom0;
+	};
+};
diff --git a/samples/drivers/uart/echo_bot_rs485/prj.conf b/samples/drivers/uart/echo_bot_rs485/prj.conf
@@ -0,0 +1,8 @@
+CONFIG_SERIAL=y
+CONFIG_UART_INTERRUPT_DRIVEN=y
+
+# Enable RS485 support
+CONFIG_UART_USE_RUNTIME_CONFIGURE=y
+
+# Enable GPIO for DE/RE control
+CONFIG_GPIO=y
diff --git a/samples/drivers/uart/echo_bot_rs485/sample.yaml b/samples/drivers/uart/echo_bot_rs485/sample.yaml
@@ -0,0 +1,18 @@
+sample:
+  name: UART RS485 Echo Bot
+  description: RS485 echo bot sample using UART with transceiver control
+tests:
+  sample.drivers.uart.rs485:
+    platform_allow:
+      - same54_xpro
+    integration_platforms:
+      - same54_xpro
+    tags:
+      - serial
+      - uart
+      - rs485
+    filter: CONFIG_SERIAL and
+            CONFIG_UART_INTERRUPT_DRIVEN and
+            CONFIG_GPIO and
+            dt_chosen_enabled("zephyr,shell-uart")
+    harness: keyboard
diff --git a/samples/drivers/uart/echo_bot_rs485/src/main.c b/samples/drivers/uart/echo_bot_rs485/src/main.c
@@ -0,0 +1,123 @@
+/*
+ * Copyright (c) 2022 Libre Solar Technologies GmbH
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+#include <zephyr/device.h>
+#include <zephyr/drivers/uart.h>
+
+#include <string.h>
+
+/* change this to any other UART peripheral if desired */
+#define UART_DEVICE_NODE DT_CHOSEN(zephyr_shell_uart)
+
+#define MSG_SIZE 32
+
+/* queue to store up to 10 messages (aligned to 4-byte boundary) */
+K_MSGQ_DEFINE(uart_msgq, MSG_SIZE, 10, 4);
+
+static const struct device *const uart_dev = DEVICE_DT_GET(UART_DEVICE_NODE);
+
+/* receive buffer used in UART ISR callback */
+static char rx_buf[MSG_SIZE];
+static int rx_buf_pos;
+
+/*
+ * Read characters from UART until line end is detected. Afterwards push the
+ * data to the message queue.
+ */
+void serial_cb(const struct device *dev, void *user_data)
+{
+	uint8_t c;
+
+	if (!uart_irq_update(uart_dev)) {
+		return;
+	}
+
+	if (!uart_irq_rx_ready(uart_dev)) {
+		return;
+	}
+
+	/* read until FIFO empty */
+	while (uart_fifo_read(uart_dev, &c, 1) == 1) {
+		if ((c == '\n' || c == '\r') && rx_buf_pos > 0) {
+			/* terminate string */
+			rx_buf[rx_buf_pos] = '\0';
+
+			/* if queue is full, message is silently dropped */
+			k_msgq_put(&uart_msgq, &rx_buf, K_NO_WAIT);
+
+			/* reset the buffer (it was copied to the msgq) */
+			rx_buf_pos = 0;
+		} else if (rx_buf_pos < (sizeof(rx_buf) - 1)) {
+			rx_buf[rx_buf_pos++] = c;
+		}
+		/* else: characters beyond buffer size are dropped */
+	}
+}
+
+/*
+ * Print a null-terminated string character by character to the UART interface
+ */
+void print_uart(char *buf)
+{
+	int msg_len = strlen(buf);
+
+	for (int i = 0; i < msg_len; i++) {
+		uart_poll_out(uart_dev, buf[i]);
+	}
+}
+
+int main(void)
+{
+	char tx_buf[MSG_SIZE];
+
+	if (!device_is_ready(uart_dev)) {
+		printk("UART device not found!");
+		return 0;
+	}
+
+	/* Configure UART for RS485 mode */
+	struct uart_config uart_cfg;
+	int ret = uart_config_get(uart_dev, &uart_cfg);
+
+	if (ret == 0) {
+		uart_cfg.flow_ctrl = UART_CFG_FLOW_CTRL_RS485;
+		ret = uart_configure(uart_dev, &uart_cfg);
+		if (ret < 0) {
+			printk("Failed to configure RS485 mode: %d\n", ret);
+			return 0;
+		}
+		printk("RS485 mode enabled\n");
+	} else {
+		printk("Failed to get UART config: %d\n", ret);
+	}
+
+	/* configure interrupt and callback to receive data */
+	ret = uart_irq_callback_user_data_set(uart_dev, serial_cb, NULL);
+
+	if (ret < 0) {
+		if (ret == -ENOTSUP) {
+			printk("Interrupt-driven UART API support not enabled\n");
+		} else if (ret == -ENOSYS) {
+			printk("UART device does not support interrupt-driven API\n");
+		} else {
+			printk("Error setting UART callback: %d\n", ret);
+		}
+		return 0;
+	}
+	uart_irq_rx_enable(uart_dev);
+
+	print_uart("Hello! I'm your echo bot.\r\n");
+	print_uart("Tell me something and press enter:\r\n");
+
+	/* indefinitely wait for input from the user */
+	while (k_msgq_get(&uart_msgq, &tx_buf, K_FOREVER) == 0) {
+		print_uart("Echo: ");
+		print_uart(tx_buf);
+		print_uart("\r\n");
+	}
+	return 0;
+}
