The i.MX93 System-On-Chip has a lot of functionality but a limited number of pins (or pads). Even though a single pin can only perform one function at a time, they can be configured internally to perform different functions. This is called pin multiplexing.

The ConnectCore 93 Hardware Reference Manual contains a Module pinout section that identifies all module pads with their corresponding signal name used in the reference boards schematics, and maps them to the corresponding i.MX pad name.

Pad multiplexing

The NXP i.MX93 Reference Manual uses pad names such as ENET2_RD0 to refer to the System-On-Chip pads. This pad name typically corresponds to the first pad functionality.

Electrical parameters of a pad

Every pad also has a PAD configuration control register where you can set the pad’s electrical parameters. See the NXP Influence of Pin Setting on System Function and Performance application note for details about setting functional and electrical parameters.

The i.MX93 GPIOs are configurable and can work at 1.8 V or 3.3 V depending on the power domain of the pad they are on. To determine the working voltage of a given GPIO, see GPIO pads power domains.

Use the device tree to configure pin IOMUX and pad control

Digi Embedded Yocto uses the Linux kernel device tree to configure the pad multiplexing and electrical characteristics for each of the reference boards. Customers designing their own boards will create a device tree matching their new board design and may need to configure the pads differently.

See Device tree files and overlays for an explanation of the Digi Embedded Yocto device tree structure.

The following example shows pad selection and IOMUX setting for a particular device: XBee. On the Linux kernel source, you can find the XBee device tree node in arch/arm64/boot/dts/digi/ccimx93-dvk.dts

ConnectCore 93 Development Kit device tree
/* XBee interface */
&lpuart4 {
	pinctrl-names = "default";
	pinctrl-0 = <&pinctrl_uart4>, <&pinctrl_xbee_rf_gpios>;
	fsl,uart-has-rtscts;
	status = "okay";
};

The pin configuration is defined on the pinctrl-0 property, assigned the default name and set to pinctrl_uart4 and pinctrl_xbee_rf_gpios:

ConnectCore 93 Development Kit device tree
	/* XBee uart */
	pinctrl_uart4: uart4grp {
	fsl,pins = <
			MX93_PAD_ENET2_RD0__LPUART4_RX			0x31e
			MX93_PAD_ENET2_TD0__LPUART4_TX			0x31e
			MX93_PAD_ENET2_RD2__LPUART4_CTS_B		0x31e
			MX93_PAD_ENET2_TD1__LPUART4_RTS_B		0x31e
		>;
	};

	/* XBee RF IO lines */
	pinctrl_xbee_rf_gpios: xbeerfgrp {
		fsl,pins = <
			/* XBEE_RESET_N */
			MX93_PAD_GPIO_IO19__GPIO2_IO19			0x31e
		>;
	};

The device tree pinctrl documentation binding explains the fsl,pins entry as consisting of six integers representing the IOMUX selection and electrical settings of the pin.

You can look closely at the macro to discern how the pin name for the IC is connected to the desired pad functionality.

Breakdown of pin function macro

From left to right, the first part of the macro after the MX93_PAD_ prefix is the PAD/PIN name in the IC. In this example, it is ENET2_RD0. The second part (everything to the right of the PAD/PIN name) represents the functionality you would like to assign to that pad. In this example, it is LPUART4_RX.

The MX93_PAD_ENET2_RD0__LPUART4_RX macro defined in arch/arm64/boot/dts/freescale/imx93-pinfunc.h contains five integers:

#define MX93_PAD_ENET2_RD0__LPUART4_RX	0x00F8 0x02A8 0x0424 0x1 0x1

These five integers are:

  • IOMUX register offset (0x00F8)

  • Pad configuration register offset (0x02A8)

  • Select input daisy chain register offset (0x0424)

  • IOMUX configuration setting (0x1)

  • Select input daisy chain setting (0x1)

The sixth integer, 0x31e, corresponds to the configuration for the PAD control register. This number defines the low-level physical settings of the pin. You can build this integer using the information found in the device tree pinctrl documentation binding. You can also copy and then modify another pin definition that has similar functionality.

Configure independent pin IOMUX and pad control

You may want to configure an IOMUX setting that is not associated with a specific device driver. An example of this would be a GPIO connected to a push button, LED, or control lines of devices that do not have a binding on their drivers. For that purpose, you can define the generic pinctrl-0 property inside the iomuxc node:

&iomuxc {
	pinctrl-names = "default";
	pinctrl-0 = <&pinctrl_gpio>;

	[...]

	/* GPIOs for User Leds, Buttons, ... */
	pinctrl_gpio: gpiogrp {
		fsl,pins = <
			/* ULED/BUTTON1 */
			MX93_PAD_SD2_RESET_B__GPIO3_IO07			0x31e
			/* MIKROBUS_INT */
			MX93_PAD_GPIO_IO11__GPIO2_IO11				0x31e
		>;
	};

	[...]
};
Do not configure the same pad twice in the device tree. IOMUX configurations are set by the drivers in the order the kernel probes the configured device. If the same pad is configured differently by two drivers, the settings associated with the last-probed driver will fail to apply and the driver will not be brought up.