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Raspberry Pi CM4 Сarrier with Hi-Res MIPI Display

An interface breakout board for Raspberry Pi Compute Module 4, permanently attached to a 7" IPS touchscreen display

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Back in 2019 we developed a carrier board with Raspberry Pi CM3+ and a high-resolution touchscreen. We have been anxiously waiting for the CM4 ever since Raspberry Pi 4 became available. It's finally here! Now we're ready to develop a CM4-based module and make it available as an open-source project.

The board will have:

  • standard RPi 40-pin header for peripherals and integration
  • connector with 4-lane MIPI DSI interface for display
  • connector with I2C interface for the touchscreen
  • micro-SD card holder
  • USB 2.0 host connector
  • PCM audio amplifier with 3W speaker output
  • USB-C power connector
  • micro-USB connector for flashing the on-board eMMC
  • general purpose push button
  • power and activity LEDs
  • mounting standoffs

All components are surface-mounted on one side, making the resulting assembly slim and good for wall-mount applications like a smart home dashboard.

There is a challenge to overcome: we need to hack our way through the undocumented MIPI 
DSI peripheral to make the interface work. With the CM3+ module, we used an HDMI-DSI bridge, which is an expensive and hard to source component. This time we want to do things differently and connect the interface directly to the Raspberry Pi Compute Module. Broadcom and Raspberry Pi Foundation are hesitant to release the documentation for the peripheral and reserve this interface for official displays. However, the source code for the driver is available, giving us a good starting point. We may need to adapt it to our display and switch the interface from 2-lane to 4-lane mode to support the higher resolution.

Let's start hacking.

PYXIS-CM4-BD-1.png

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Portable Network Graphics (PNG) - 82.31 kB - 01/05/2021 at 18:27

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  • 1 × Display
  • 1 × Raspberry Pi Compute Module 4
  • 1 × Custom board

  • Week 7. Ready for bring-up.

    Igor Shturmaa day ago 1 comment

    Assembled the board, the next challenge is software. 

    Read more »

  • Week 6. Getting ready to put the hardware together.

    Igor Shturma01/11/2021 at 05:09 0 comments

    Everything has now arrived: PCBs, displays, compute modules, components. Time to get hands dirty with the solder paste.

  • Week 5. PCBs ordered.

    Igor Shturma01/05/2021 at 19:36 1 comment

    An exciting milestone in the project was achieved right before New Year's Eve: the design is finished for the first PCB revision, time to order the boards.

    Read more »

  • Week 4. The board architecture is finalized.

    Igor Shturma12/29/2020 at 03:21 2 comments

    We are now done with the architecture, BOM, and most of the schematic.

    Read more »

  • Week 3. Ordering the display samples.

    Igor Shturma12/25/2020 at 01:24 1 comment

    Last week we got in touch with nearly 20 display suppliers from China listed on Alibaba. A few of them could offer us our display panel bound with a capacitive touch digitizer. If we order 100 pcs shortly, they are going to cost us around $35 each. We will get five samples in a few weeks. Those are going to be complete modules, with optically clear adhesive bound. No need to buy touch sensors separately anymore. We are also getting an HDMI-MIPI board that we can use to test the displays initially and as a reference, if something doesn't work right.

  • ​Week 2.2 Applied for Seeed sponsorship program

    Sasha Shturma12/11/2020 at 20:30 0 comments

    In October Seeed announced sponsorship of projects that use Raspberry Pi CM4. For those approved, they give $500 towards PCB assembly services. More details on their blog

    We haven’t worked with Seeed before, but this seems like a good opportunity to try manufacturing with them.

  • Week 2.1 Choice of the touchscreen.

    Sasha Shturma12/08/2020 at 02:57 0 comments

    We further narrowed down specs for the touchscreen:

    1. Should use capacitive, multi-point touch technology.
    2. The touchscreen driver IC should be cost-effective, widely available, and have good software support. A GT911 IC from Goodix Technology is a popular touchscreen driver found in many consumer electronic products. It connects over I2C and is supported by mainline Linux

    Most display panels are sold without touchscreen because touchscreen glass and framing are customized for specific products. There are plenty of suppliers in China that offer customization and assembly of screen parts together. However, they usually prefer to work with high volume orders. We'll most likely buy a touchscreen for initial testing and glue it with a display panel ourselves. Ideally, this should be done in a cleanroom at a factory, but our simplified approach works for fast evaluation.

  • Week1. Choice of the display.

    Sasha Shturma12/03/2020 at 23:25 0 comments

    Criteria:

    • High-resolution, 
    • 7" diagonal,
    • Bright - 400+ cd/m2,
    • MIPI interface,
    • Costs less than $50,
    • Available off-the-shelf from multiple suppliers.

    We found display panel LT070ME05000 used in Asus Google Nexus 7". Specs look great, so we need to find a supplier with this panel available in a final assembly with hardened glass and touch input.

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Manny wrote 01/07/2021 at 06:45 point

Seems like a great initiative. Looking forward to your updates as you progress toward a first release. 

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