This guide will show you how to control the mikroBUS GPIOs, SPI, I2C, and UART from the RZ/G2LC HummingBoard Linux userspace.

Here is a microBUS connector pinout:

Choosing the pin function

According to the SOC specification, most pins can be used as GPIO or alternative function pins.

• The pin P45_2 can be used as GPIO.

• The pins P44_3, P44_0, P44_2, and P44_1 can be used as GPIOs by default. Enable the spi1 node in the devise tree to use them as SPI.

• By default, the pins P41_0, P41_1, P40_1, and P40_0 are used as uart ttySC1. Disable the scif1 node in the devise tree to use them as GPIOs.

• The pins P40_1, and P40_0 Can be used as CANFD RX/TX. Disable the scif1 node and enable canfd node in the devise tree to use them as CANFD.

• I2C0_SCL and I2C0_SDA can’t be used as GPIOs, only as I2C pins.

GPIO

To use GPIO in Linux, either gpiod or sysfs can be used.

Both methods need the GPIO number that Linux assigned during the enumeration. This number is dynamically assigned to the GPIO based on the number of pins, GPIO controllers, etc. Usually, this number is stable, but it might change during the BSP updates. The gpioinfo utility gives us information about the number of the GPIO we need to use according to its label:

$ gpioinfo | grep P45_2
        line 362:      "P45_2"       unused   input  active-high

In this example, the GPIO P45_2 has the number 362.

Setting GPIO using gpiod:

gpioset -m wait gpiochip0 362=1

Reading GPIO using gpiod:

gpioget gpiochip0 362

Setting GPIO using sysfs:

The export number of the GPIO can be calculated as the GPIO controller base plus the GPIO number.

$ ls /sys/class/gpio/
export       gpiochip120  unexport

The GPIO controller base is 120, and the GPIO number is 362.

The export number is 120 + 362 = 482

echo 482 > /sys/class/gpio/export 
echo out > /sys/class/gpio/P45_2/direction 
echo 1 > /sys/class/gpio/P45_2/value 

UART

UART is available in Linux as a standard serial device /dev/ttySC1.

To use it, you can use the default utilities:

# stty -F /dev/ttySC1 9600
# echo "test string" > /dev/ttySC1

You can also use Picocom, minicom, screen, etc.

CANFD

The CANFD RX/TX pins are available on P40_1 and P40_0. These pins conflict with SCIF1 RX/TX. To enable CANFD, the scif1 node needs to be disabled, and the canfd node needs to be enabled in the dts.

&canfd {
	status = "okay";
};

&scif1 {
	status = "disabled";
};

The CANFD will be available with the /dev/can0 interface.

Basic CAN usage:

# ip link set can0 type can bitrate 1000000 dbitrate 4000000 fd on
# ip link set up can0
# candump can0

I2C

The I2C0_SCL and I2C0_SDA pins can be utilized as an I2C0 connection and are available via /dev/i2c-0.

New devices can be detected via the i2cdetect utility:

$ i2cdetect -y 0
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- UU -- -- -- UU UU -- UU 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: UU -- -- -- -- -- -- UU -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- UU -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --       

Options to control the device:

• i2c-tools utilities

• Add device to i2c0 node in the dts

• Also possible that the kernel will bind the driver automatically during the i2c bus scan

SPI

The mikroBUS connector's SPI pins are connected to the SOC's SPI1 interface. To use them, configure the spi1 node in the dts first.

Depending on the task, the spi1 node can be configured as a specific or dummy SPI device.

Here is an example of the dummy device configuration:

&spi1 {
	status = "okay";
	spidevice@0 {
		compatible = "linux,spidev";
		reg = <0>;
		spi-max-frequency = <12000000>;
	};
};

After this configuration, the SPI would be accessible via the /dev/spidev0.0 device and can be used with the spidev_test utility or tools that can work with spidev (for example, flashrom).

To fully use SPI, replace the spidev node with a specific device node and use a particular driver.