Serial TTY device tree configuration

1. Activation of a USART or UART instance

Information

Some UART pins are available on GPIO expansion and Arduino connectors (depending on the connectors available on the board). STM32MP157C-EV1 Evaluation board GPIO expansion connector STM32MP157X-DKX Discovery kit GPIO expansion connector

To communicate with a UART instance, an RS232 card must be plugged on the UART pins.

The example below shows how to configure and enable a UART instance at board level, based on STM32MP157C-EV1 board USART3 example.
Note: For STM32 boards, the configuration is already defined in the device tree. Only the device activation is needed.

To activate a UART instance, please follow steps below:

• Define the instance pin configuration (ex: stm32mp157-pinctrl.dtsi ).

usart3_pins_a: usart3-0 {
	pins1 {
		/* USART3 TX and RTS pins activation for default mode */
		pinmux = <STM32_PINMUX('B', 10, AF7)>, /* USART3_TX */
			 <STM32_PINMUX('G', 8, AF8)>; /* USART3_RTS */
		bias-disable;
		drive-push-pull;
		slew-rate = <0>;
	};
	pins2 {
		/* USART3 RX and CTS_NSS pins activation for default mode */
		pinmux = <STM32_PINMUX('B', 12, AF8)>, /* USART3_RX */
			 <STM32_PINMUX('I', 10, AF8)>; /* USART3_CTS_NSS */
		bias-disable;
	};
};

usart3_idle_pins_a: usart3-idle-0 {
	pins1 {
		/* USART3 TX, RTS, and CTS_NSS pins deactivation for sleep mode */
		pinmux = <STM32_PINMUX('B', 10, ANALOG)>, /* USART3_TX */
			 <STM32_PINMUX('G', 8, ANALOG)>, /* USART3_RTS */
			 <STM32_PINMUX('I', 10, ANALOG)>; /* USART3_CTS_NSS */      
	};
	pins2 {
		/* USART3_RX pin still active for wake up */
		pinmux = <STM32_PINMUX('B', 12, AF8)>; /* USART3_RX */
		bias-disable;
	};
};

usart3_sleep_pins_a: usart3-sleep-0 {
	pins {
		/* USART3_TX, RTS, CTS_NSS, and RX pins deactivation for sleep mode */
		pinmux = <STM32_PINMUX('B', 10, ANALOG)>, /* USART3_TX */
			 <STM32_PINMUX('G', 8, ANALOG)>, /* USART3_RTS */
			 <STM32_PINMUX('I', 10, ANALOG)>, /* USART3_CTS_NSS */
			 <STM32_PINMUX('B', 12, ANALOG)>; /* USART3_RX */
	};
};

• Define the serial alias for this instance at board level (ex: stm32mp157c-ev1.dts ).

aliases {
	/* Serial1 alias (ie ttySTM1) assigned to usart3 */
	serial1 = &usart3;
	ethernet0 = &ethernet0;
};

• Configure and activate the instance at board level (ex: stm32mp157c-ev1.dts ).

&usart3 {
	pinctrl-names = "default", "sleep", "idle";	/* pin configurations definition */
	pinctrl-0 = <&usart3_pins_a>;			/* default pin configuration selection */
	pinctrl-1 = <&usart3_sleep_pins_a>;		/* sleep pin configuration selection */
	pinctrl-2 = <&usart3_idle_pins_a>;		/* idle pin configuration selection */
	status = "okay";				/* device activation */
};

Note: The pin configuration selected has to be aligned with the pin configuration described in the board datasheet.

Universal Synchronous/Asynchronous Receiver/Transmitter

Universal Asynchronous Receiver/Transmitter

General-Purpose Input/Output (A realization of open ended transmission between devices on an embedded level. These pins available on a processor can be programmed to be used to either accept input or provide output to external devices depending on user desires and applications requirements.)

Transmit

Receive

Compatibility Test Suite (Android specific) or Clear to send (in UART context)