MIPI-HDMI Bridge RE

Here is the breakout board that came with my unit:

This consists of a specialized OLED driver chip, a 2.8v LDO, and two OLED connectors that utilize only 1 MIPI data pair on each channel:
I was unable to find a datasheet for my specific displays, but I determined the pinout for each display is roughly:

As explained in previous logs, the MCU is the brain of the board. It configures the HDMI bridge chip, and handles power-sequencing of the display.

Out of curiousity, I connected an STLink to the SWD pads on the PCB, and behold, the chip is unlocked!

This allowed me to extract the firmware. I plan to reverse-engineer the firmware to determine how it configures the system, and how it could be configured via USB.

GPIO map:

(Ignore the labels column - I am using it within my reverse engineering to track the signals)

Flash Layout

The connector provides two independent MIPI channels, each with 4 data pairs & 1 clock pair. The 5V rail is always powered, but the other rails can be individually sequenced and controlled by the MCU. The GPIOs can be used to control additional regulators or enable control signals on the display board. They are output only.

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I do not know what chip "ET7" and "MH 23 22" are, but they are obviously some kind of buck regulator. A/B/C/D are the GPIO control signals from the MCU (output only).
E6 enables the 3.3v rail, and supplies the 1.8v regulator. IOC/D are powered even if this rail is not.
E4 enables the 1.8v rail to the display connector, but only if E6 is also enabled (for 3.3v). IOA/B are pulled-up to 1.8v if this is enabled.
The 5V rail is always powered.
I am not sure what the deal is with the chip at the top that is fed directly from +5v.

TFT Backlight Driver

For TFTs, there is a dedicated backlight driver:
V3 is +V for the backlight, and either FB1/2 can probably be chosen for -V depending on the drive strength that is desired.
E9 is obviously an analog feedback signal to the MCU for controlling the drive current, E5 is the main supply enable, and E8 is probably a PWM control signal for defining the drive current.

OLED Bias Voltage

E7 enables the bias circuit.

E10 is probably a PWM drive signal for controlling display brightness.

On my board, the following signals come up in this order when it is powered on:
A: Slow Flashing Blue LED / Initial power-up

You can see that on my board, the LED backlight & OLED bias supplies are never turned on. Instead the dsplay board has it's own regulators which are controlled by the IOA/B/C/D GPIOs.

To start reverse-engineering the circuit behind this board, I started by taking photos of the bottom and top layers, and inking over with my Surface:

MIPI-HDMI Bridge Chip

You can see that the connector on the right does indeed support two MIPI channels, each consisting of 4 data pairs & 1 clock pair. The MIPI bridge chip is connected to the MCU via I2C, and there is minimal support circuitry (only a crystal, a few buck regulators, and some passives for the PLL).

Main Buck Regulators

On the bottom-right you can see the main buck regulators which supply power to the MIPI bridge chip. 5.0V is fed onwards to the display buck regulators elsewhere on the PCB.

E1 controls the 1.1v supply for the MIPI bridge (VDDC11, VDD11_HDMI)
E2 controls the 1.2v supply (VDD12_MIPI)
E3 controls the 3.3v supply for the MIPI bridge (VDDIO33, VDD33_HDMI), and the MCU & Audio circuit. I believe this defaults to on, and allows the MCU to power down the system.
The Display & IO has an always-on 3.3V supply (DISP33)

OLED Supplies

On the upper-left you can see that things get a bit more complicated. A lot of this is to support many different kinds of displays, and provides various voltages for backlighting, OLED biasing, etc. It also supples two sets of bi-directional GPIOs, one at 1.8V, and one at 3.3V.

For more information on how this works, see the next project log.

MCU

Finally we have the "STM32" MCU which is the brains of the board:
Those four pads on the right look interesting... could they be SWD perhaps?
(VDD, GND, PA13/SWDIO, PA14/SWCLK)