i.MX7: Raspberry Pi HAT Support with PEB-D-RPI Expansion Board

Introduction

The PEB-D-RPI expansion board comes equipped with a 40-pin header that allows for emulation of some of the Raspberry Pi single-board computer functionality. One interesting function supported by this 40-pin header is the use of Raspberry Pi HATs on PHYTEC's phyBOARD-Zeta (i.MX7) platform. The following sections will use the Raspberry Pi Sense HAT as an example for connecting Raspberry Pi HATs and running demos on the phyBOARD-Zeta. Additional sections will help you enable the use of more GPIOs through the 40-pin header. These instructions assume that you have properly connected the expansion board to the carrier board and can boot into a supported environment (U-Boot and/or Linux) using BSP version PD18.2.0 or newer.

Connecting the Sense HAT to the PEB-D-RPI Expansion Board

Start by ensuring that your phyBOARD-Zeta (i.MX7) is powered off. Do not connect or disconnect a Raspberry Pi HAT to or from the PEB-D-RPI expansion board while it is being powered. Next, connect the Sense HAT to your PEB-D-RPI expansion board via the 40-pin header (X11) as shown below:

Power on the phyBOARD-Zeta and boot into Linux and log in. Run the following command:

dmesg | grep phytec-hat

You should see output similar to the following lines. This output indicates that the Sense HAT was detected and the correct pin muxing and device tree overlay was loaded:

phytec-hat 3-0050: HAT Vendor:           Raspberry Pi
phytec-hat 3-0050: HAT Product:          Sense HAT
phytec-hat 3-0050: overlay loaded:       imx7-peb-d-sense.dtb

Demos for the Sense HAT have already been installed to the BSP file system. You can find the demos for the Sense HAT in the '/usr/share/phytec-rpihat-examples/sense-hat' directory. To run a demo that showcases the LED matrix and framebuffer functionality of the Sense HAT, run the following commands:

cd /usr/share/phytec-rpihat-examples/sense-hat
python rainbow.py

You should now see a slow-moving rainbow shift across the LED matrix on the Sense HAT. There are many other Python demos to try for the Sense HAT in the same directory. You can run any of them in the same manner as above by executing 'python <demoname.py>' on the target.

Enabling Additional GPIOs to 40-pin Header

By default, the GPIOs that are muxed to the 40-pin header on the PEB-D-RPI evaluation board are those that only have GPIO functionality. This facilitates the use of device tree overlays that may have pin mux conflicts with additional GPIOs. To enable additional GPIOs, you must modify the device tree file 'imx7-peb-d-rpi.dtsi' as follows:

Under the &i2c4 entry in 'imx7-peb-d-rpi.dtsi', you will find a node called 'rpi_hat_eeprom'. This node contains the pin control definitions that are loaded on boot and when the 'phytec-hat' driver detects a supported HAT. The pin control entry pinctrl-0, or "default", points to the 'pinctrl_pebdrpi' pin control group. You can add an arbitrary set of pin control groups to the pin control pinctrl-0 entry which will then be loaded on boot. For example, if you wanted to enable GPIOs on the pins normally used by the UART3 interface, you would change the pin control entry pinctrl-0 to the following:

pinctrl-0 = <&pinctrl_pebdrpi &pinctrl_uart3_gpio>;

This table contains the names of the pin control groups to add to the pin control entry pinctrl-0 if you need the use of GPIOs on specific pins:

The following table contains the alternative pin control group and pin number pairing if you de-solder and re-solder these jumpers in their alternative position: J5, J6, J7, J8, J9, J10, J11, and J12.