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While I wait for my audio board and breakout boards to show up, I started working on the video output side of things.
The Raspberry Pi CM4 has four ways to output video:
- 2x HDMI
- 2x MIPI DSI
- 1x DPI
- 1x Composite
I need to convert these into three different formats (in order of priority):
- LVDS (FPD-Link or OLDI maybe) for the LP097X02-SLA1 built-in display.
- DisplayPort for powering 30-pin video dongles such as the Digital AV Adapter (HDMI) or the 30-pin to VGA Adapter.
- Composite and/or S-Video for the 30-pin connector
I'm not sure why Apple went with DisplayPort internally. I guess Macbook Pros of that era already had Mini DisplayPort ports. HDMI is roughly as complicated as DisplayPort, though, and would have simplified their HDMI dongle without adding much complication to their VGA dongle, in my opinion.
Here are the various options for converter ICs I found:
| DisplayPort | LVDS | |
|---|---|---|
| HDMI x2 | STDP2600 (0.8mm BGA - doable?) | TFP401 (HTQFP) -> SN65LVDS93 (TSSOP) ADV7613 (0.8mm BGA) RTD2668 (massive QFP) |
| MIPI DSI x2 | SN65DSI86 (tiny BGA) SN65DSI86-Q1 (HTQFP) PS8640 (tiny BGA) TC358766X (tiny BGA) ANX7805 (tiny BGA) | SN65DSI83 (tiny BGA) SN65DSI83-Q1 (HTQFP) |
| DPI x1 (Parallel RGB) | ANX9807 (0.8mm BGA) STDP4028 (0.8mm BGA) | SN65LVDS93 (TSSOP) |
I think the SN65DSI86-Q1 and SN65DSI83-Q1 are my best bets:
- For some reason, MIPI seems like a simpler on-board video format than HDMI.
- TI didn't have any single-chip solutions for HDMI -> DP or HDMI -> LVDS, and going with another manufacturer would ruin the TI hegemony I've found on my boards. (But actually, TI has pretty good datasheets and training videos and will sell you things in small quantities.)
- QFP packages are pretty small, but will avoid the difficulties of tiny BGA packages.
- DPI would use up a lot of the GPIO pins.
It's not nearly done yet, but I've started making progress on the board for both LVDS and DisplayPort (as of right now, I've mostly wired up the SN65DSI83-Q1 but haven't finished the LED backlight driver nor started on the DisplayPort side of things.)