Monday, May 11, 2015

Carputer 2015 - System Design

The carputer design is coming together. I've collected a decent set of system components, cabling, connectors, and tools that will be useful for prototyping. System design has been a bit up in the air as I'm coming to understand the limited selection of components and their constraints. In particular, finding acceptable display panels and controllers has been a problem.

The following is my current iteration on the overall design.


Car outline clip art image by Cliparts.co. Diagram produced using Inkscape.
  1. Vehicle battery.
    Note the new cable run through the engine firewall to the rear of the vehicle. This will allow maintenance of a low-power sleep mode to quickly bring the master carputer online when the vehicle is turned on or a remote network query is made.
  2. Vehicle system bus and accessory power distribution module.
    This represents a magical (to me) black box that provides 12V accessory power and vehicle systems signaling.
  3. Vehicle A/V control unit.This module processes input and output for non-essential vehicle systems, such as climate control and audio. This is designed to interface with a display unit that will be entirely replaced by new components for this project.
  4. Flat panel touchscreen display and controller module.
    The existing LCD panel is 8 years old and the backlight is weak. Readily available panels in the 7" size seem to go from 800x480 to 1280x800. For this application, panel visibility in a variety of lighting conditions is much more important than resolution.
  5. Legacy display signal conversion and digital capture module.
    The A/V control unit generates what appears to be a 15kHz RGBs video output (like an old arcade game). This will be converted to composite video and then digitally captured with a USB dongle. The master carputer needs to be able to process and display these video frames.
  6. Legacy wiring interface microcontroller.
    This is a controller like an Arduino that interfaces signals from the A/V wiring harness to USB. The master carputer will then be able to determine things like vehicle speed and reverse state.
  7. Powered USB hub.
    This supplies the power and data bus for components in the front and center consoles of the vehicle. The uplink is wired to the master carputer host at the rear of the vehicle.
  8. Switchable DC to DC power conversion module.
    The master carputer requires a stable DC power supply, possibly with multiple voltages and possibly with battery backup support.
  9. Master carputer.
    This is the main computer that handles all USB I/O devices and generates audio/video output to the front display over HDMI. The carputer setup is as yet unspecified. This could be a repurposed coreboot plus Linux Chromebook, an embedded PC, or a Raspberry Pi. Potentially the frontend (audio/video) and backend (input processing and control) could be separate computers.
  10. Rear-view camera module.
    This vehicle did not originally come with a backup camera. A camera will be added to provide rear-facing video to the master carputer for display or recording.
Notice that the long cable runs between the front and rear of the vehicle use digital signaling. I had originally planned to run composite video from the rear camera and analog audio from the master carputer to the front of the vehicle. The flatscreen display module I'm most strongly considering now only has HDMI input and breaks out HDMI audio to a D/A converter. Display controllers often have a board with switches to select between inputs and configure the display like a computer monitor. In the carputer case, fewer switches might be a good thing. I also like the idea of using only digital signals over distance. Solving glitchy or noisy analog audio/video problems is no fun at all.