For me the options for adding a screen to a Raspberry Pi have always come with a bit of dissatisfaction. This isn’t because of any intrinsic flaws in the designs. The Pi, having its own thickness which has contributed to solutions that have form factors that are not quite my preference. This started to change with the release of the Raspberry Pi Computer Modules. With the Raspberry Pi Compute Module 4 I see some satisfying solutions. One of the solutions has available a plan for a 3D printable case. Another comes already encased. I chose a solution that already has a case because I don’t have a 3D printer and I’ve had mixed results in using third-party printers. The solution that I selected is the Seeed Studio reTerminal.

Before speaking on it more, I want to point out that this case does not have a battery. If you are seeking a solution with a battery, then you may want to consider the solution with the 3D print designs and alter it to hold a battery.

The unit is sold with a Raspberry Pi Compute Module 4 (CM4) included. Right now, the unit is sold with the CM4 that has Wi-Fi, 4 gigs of RAM, and 32 Gigs of eMMC. This is great, as it is near impossible to get a CM4 by itself in present-day times. The packaging for the unit uses more flexible wording saying “Up to 8GB RAM/Up to 32GB EMMC” suggesting that at some point they may see the unit with other variants. The only indication of what CM4 module is in the box is a sticker with the barcode that spells out the CM4 version (CM4104032).

The display on the unit itself is 720×1280 pixels. It sounds like I said these dimensions reverse. I haven’t. Using the direction in which the pixels are refreshed, the first line of scan is the left area of the screen and it works its way to the right. This differs from conventional displays that start at the top and work their way down. Accessible through the case is gigabit Ethernet, 2 USB 2.0 ports, the Pi 40-pin header, and an industrial high-speed expansion interface. This unit was designed with industrial applications in mind. Though I won’t be paying attention to this industrial interface. The case also has a real-time clock, a cryptographic coprocessor, a few hardware buttons including a button to power the unit on, an accelerometer, and a light sensor. Out of the box the software needed for this additional hardware is preinstalled. Should you choose to reinstall the operating system yourself, you will need to install the software and drivers for the additional hardware manually.

The packaging for the unit contains extra screws, a screwdriver, and the reTerminal unit itself. On the lower side of the unit is a connector for a 1/4-inch screw. This is the same sized screw used by many camera tripods. I’m using one of the mini desktop tripods for my unit. To power the unit on all that is needed is to connect power to the USB-C connector on the left side of the reTerminal.

The unit does not ship with an on-screen keyboard installed. For initial setup, I will want to have at minimum a USB-C power supply and a keyboard. If you do not have a mouse, you could use the touch screen just fine.

reTerminal Specific Hardware

I’ve mentioned a number of hardware items contained within the reTerminal, such as the custom buttons. Accessing the additional hardware and interfaces is easier that I expected. The four buttons on the front have been mapped to the keyboard keys A, S, D, and F. If you would like to map these to different keys, they can be set through /boot/config.txt. Within that file is a line that looks similar to the following.

dtoverlay=reTerminal,key0=0x041,key1=0x042,key3=0x43,key3=0x044

The hex numbers are ASCII codes for the characters these keys will generate. You can change these as needed.

LEDs and Buzzer

There are four positions for LEDS at below the screen of the unit. Two of those positions have LEDs that are controllable through software. The positions are labeled STA and USR. USR has LED 0 (green). Position STA las LEDS 1 (red) and 2(green). Because of the two LEDs behind position STA, the perceived color of the position can range from green to yellow to red. Control of the LEDs is available through the file system. In the directory /sys/class/leds are the subdirectories usr_led0, usr_led1, and usr_led2. Writing a text string with a number between the range of 0 and 255 to a file named brightness will set the brightness of the LED with 0 being off and 255 being full brightness. Note that root access is needed for this to work.

According to documentation, the brightness of the LEDs is changed with this number. But in practice, each led appear to be binary. I don’t see any difference in brightness for a value of 1 and of 255.

The Buzzer is treated like the LED, but only has a “brightness” level ranging from 0 (off) to 1. The name of the device directory for the buzzer is /sys/class/leds/usr_buzzer. Like with the LEDs, write to a file named brightness.

Real Time Clock

The real-time clock is connected to the I2C interface of the CM4. The command line utility hwclock works with the clock.

Light Sensor

The light sensor is exposed through the file path /sys/bus/iio/devices/iio:device0. Reading from this file will expose the brightness value.

Accelerometer

The accelerometer in the unit is a ST Microelectronics LIS3DHTR. This hardware can be used to automatically change the screen orientation, or for other applications. To see it in action, you can use the evtest tool that Seeed Studio preinstalled on the device. Running evtest and selecting the index for the accelerometer hardware will result in it displaying the readings for each axis.

My Setup

As per my usual, after I had the Pi up and running there were a few other changes that I wanted to apply.

• Installed Visual Studio Code
• Installed .Net Core
• Installed Cmake (sudo apt-get install cmake -y)

Testing the Hardware

For testing much of the above-mentioned hardware, root access is needed. I would prefer to avoid using root access. I first tried to grant permission on the needed files to the user pi. Ultimately, this doesn’t work as planned. The file paths are sysfs paths. This is part of a virtual file system used for accessing hardware. It gets recreated on each reboot. Changes made do not persist. But if you wanted to grant permissions that are available until the next reboot, you could use the following. Otherwise, you’ll need to run your applications that use this additional hardware as root.

#enter interactive root session
sudo -i
#navigate to the folder for LEDs and the buzzer
/sys/class/leds/
#grant permission to the pi user for the brightness folder
chown pi usr_led0/brightness
chown pi usr_led1/brightness
chown pi usr_led2/brightness
chown pi usr_buzzer/brightness

#grant permission to the light sensor
chown pi /sys/bus/iio/devices/iio:device0

#exit the root session
exit

Some of the hardware uses the SPI and I2C interfaces. Using the Raspberry Pi Config tool, make sure that these interfaces are enabled.

Install the tool for input event viewing. The tool is named evtest.

sudo apt-get install evtest -y

Once installed, run evtest. Note that even if you are using SSH to enter commands into your terminal that this tool still works. The tool will list the input devices and prompt you to select one.

 $ evtest
No device specified, trying to scan all of /dev/input/event*
Not running as root, no devices may be available.
Available devices:
/dev/input/event0:      Logitech K400
/dev/input/event1:      Logitech K400 Plus
/dev/input/event2:      Logitech M570
/dev/input/event3:      gpio_keys
/dev/input/event4:      ST LIS3LV02DL Accelerometer
/dev/input/event5:      seeed-tp
/dev/input/event6:      Logitech K750
/dev/input/event7:      vc4
/dev/input/event8:      vc4
Select the device event number [0-8]: 3

The actual order and presence of your options may vary. In my case, you can see the devices associated with a Logitech Unify receiver that is connected to the device. The hardware buttons that are on the device are represented by the device gpio keys. For me, this is option 3. After selecting three, as I press or release any of these buttons, the events print in the output. Remember that by default these buttons are mapped to the keys A, S, D, and F. This is reflected in the output.

Input driver version is 1.0.1
Input device ID: bus 0x19 vendor 0x1 product 0x1 version 0x100
Input device name: "gpio_keys"
Supported events:
  Event type 0 (EV_SYN)
  Event type 1 (EV_KEY)
    Event code 30 (KEY_A)
    Event code 31 (KEY_S)
    Event code 32 (KEY_D)
    Event code 33 (KEY_F)
    Event code 142 (KEY_SLEEP)
Properties:
Testing ... (interrupt to exit)
Event: time 1651703749.722810, type 1 (EV_KEY), code 30 (KEY_A), value 1
Event: time 1651703749.722810, -------------- SYN_REPORT ------------
Event: time 1651703750.122811, type 1 (EV_KEY), code 30 (KEY_A), value 0
Event: time 1651703750.122811, -------------- SYN_REPORT ------------
Event: time 1651703750.832809, type 1 (EV_KEY), code 31 (KEY_S), value 1
Event: time 1651703750.832809, -------------- SYN_REPORT ------------
Event: time 1651703751.402797, type 1 (EV_KEY), code 31 (KEY_S), value 0
Event: time 1651703751.402797, -------------- SYN_REPORT ------------
Event: time 1651703751.962817, type 1 (EV_KEY), code 32 (KEY_D), value 1
Event: time 1651703751.962817, -------------- SYN_REPORT ------------
Event: time 1651703752.402812, type 1 (EV_KEY), code 32 (KEY_D), value 0
Event: time 1651703752.402812, -------------- SYN_REPORT ------------
Event: time 1651703753.132807, type 1 (EV_KEY), code 33 (KEY_F), value 1
Event: time 1651703753.132807, -------------- SYN_REPORT ------------
Event: time 1651703753.552818, type 1 (EV_KEY), code 33 (KEY_F), value 0
Event: time 1651703753.552818, -------------- SYN_REPORT ------------

Since we are speaking of evtest, exit it with CTRL-C and run it again. This time select the accelerometer. A data stream of the accelerometer values will fly by. These are hard to visually track in the console. But if you reorient the device, if you are able to track one of the readings, you will see it change accordingly.

Event: time 1651705644.013288, -------------- SYN_REPORT ------------
Event: time 1651705644.073140, type 3 (EV_ABS), code 1 (ABS_Y), value -18
Event: time 1651705644.073140, type 3 (EV_ABS), code 2 (ABS_Z), value -432
Event: time 1651705644.073140, -------------- SYN_REPORT ------------
Event: time 1651705644.133259, type 3 (EV_ABS), code 1 (ABS_Y), value 18
Event: time 1651705644.133259, type 3 (EV_ABS), code 2 (ABS_Z), value -423
Event: time 1651705644.133259, -------------- SYN_REPORT ------------
Event: time 1651705644.193161, type 3 (EV_ABS), code 0 (ABS_X), value 1062
Event: time 1651705644.193161, type 3 (EV_ABS), code 1 (ABS_Y), value 0
Event: time 1651705644.193161, type 3 (EV_ABS), code 2 (ABS_Z), value -409
Event: time 1651705644.193161, -------------- SYN_REPORT ------------
Event: time 1651705644.253290, type 3 (EV_ABS), code 0 (ABS_X), value 1098
Event: time 1651705644.253290, type 3 (EV_ABS), code 2 (ABS_Z), value -405
Event: time 1651705644.253290, -------------- SYN_REPORT ------------

Light Sensor

Getting a value from the light sensor is as simple as reading a file. From the terminal, you can read the contents of a file to get the luminance value.

cat /sys/bus/iio/devices/iio:device0/in_illuminance_input

HDMI and Screen Orientation

I earlier described the screen as having a resolution of 1280×720. That isn’t quite correct. It is 720×1280. It might look like I just reversed the numbers. I did. Typically screens refresh from top to bottom. This screen refreshes from left to right. You can check this for yourself by grabbing a window and moving it around rapidly. Some screen tearing will occur exposing the way in which the screen is rendering. If you were to make a fresh install of Raspbian or Ubuntu, this will be more apparent because the screen will be oriented such that the left edge is the top of the screen and the right edge is the bottom. If you would like to manage the orientation of the display and external displays that the reTerminal is connected to, there is a screen orientation utility to install for managing the layout.

sudo apt-get install arandr -y

Future Expansion

I don’t give much weight to plans for future products in general since there is no guarantee that they will materialize. But I’ll reference what Seeed Studio has published. The “Industrial High-Speed Interface” connects to a number of interfaces on the CM4. This includes the interfaces for PCIe, USB 3.0, and SDIO 3.0. Seeed Studio says that it plans to make modules available for connecting to this interface, such as a camera module, speaker and mic array. PoE, 5G/4G modems, so on.

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