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I'm designing an interface board to connect a MIPI DSI LCD screen to an RPi. The screen is a KWH070KQ40-C08 (link to pdf), but the datasheet isn't quite clear and I don't get an answer from the company. I tried googling, but found no topic talking about interfacing MIPI DSI screens.

These are the pins with their recommended voltages. The datasheet only talks about current draw on the LED+/LED- pin (which is around 180mA).

  • LED+: 10V (Anode LED blacklight)
  • LED-: 0V (Kathode LED blacklight)
  • LCD_VGH: 18V (Gate ON Voltage)
  • LCD_VGL: -6V (Gate OFF Voltage)
  • AVDD: 9.6V (9.4V - 9.8V) (Power for analog circuit)
  • VDD: 3.3V (Power for digital circuit (1.8V))
  • VCOM: 3.15V (2.85V - 3.45V) (Common Voltage)

The board will be powered by a 5V USB connection and has a 3.3V voltage regulator. (not on schematic)

For making the 10V (LED+), I plan on using a DC-DC switching converter (MIC2288), which may or may not be an overkill. Can I connect LED- with the ground or do I use a 0R resister inbetween?

For VCOM and AVDD I was thinking on using respectively +3.3V and +10V and drop the voltage with a Schottky diode or voltage divider with buffer. But since I don't know the current draw I'm not sure what to choose.

Same goes for LCD_VGH and LCD_VGL, I could use a second MIC2288, make 24V and shift it down somehow by 6V. Or use 2 voltage converters for making -6V and +18V.

enter image description here

Besides the power pins, we have the 4 datapairs and clockpair. These I will connect directly. (not on schematic)

Lastly, the 4 signal lines. UPDN, SHLR, STBYB and LRSTB, as there is no further information about these pins, I made room for a pullup and pulldown resistor so I test which one it is. (not on schematic)

pin description

supply

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    \$\begingroup\$ Just a note on power supervision here. All of these supplies should be "good" to allow the screen to operate. And is 28/29 really 1.8v? Yet another supply? Because this conflicts with VDD listed at bottom as 3.0v to 3.6v. Your only choice may be to either experiment, or select a different display with a better datasheet. \$\endgroup\$
    – rdtsc
    Mar 11, 2020 at 14:11
  • \$\begingroup\$ @rdtsc added datasheet. And I had the same confusion as you. They say VDD should be 3.3V but then they add 1.8V in brackets. I think they regulate the voltage themself to 1.8V, -as I don't see other VDD pins mentionned in the pinout. \$\endgroup\$
    – Swedgin
    Mar 11, 2020 at 14:22
  • \$\begingroup\$ It seems that the MIPI specification is a well-guarded secret. That is why the data sheets are so unclear. Not good for hobbyists. Did you make any progress? There is a youtube video series about interfacing the IPad Nano 6 display. It describes how this differential data stream is pushed. But it's not a tutorial. \$\endgroup\$
    – Gunther Schadow
    Aug 26, 2020 at 20:59
  • \$\begingroup\$ @GuntherSchadow Not with this particular screen. I've designed the circuit and am waiting for the PCB's to arrive. Luck would have it, that I had to design an interface board between a PIC32 and an LVDS screen for work. There I also used the TPS65150 and it works. Although we send the data in RGB format and use a SN75LVDS83B to convert it to LVDS signal. Do you have problems with something or need extra info? I know now what UPDN, SHLR are and how to use the reset pins. I'll have to test to see if they're active low or high though. \$\endgroup\$
    – Swedgin
    Aug 27, 2020 at 7:51
  • \$\begingroup\$ @Swedgin thanks for asking. I would really want to see a spec or tutorial because I want to hard-wire an interface to a cellphone display. I don't want to use an pre-fabricated interface adapter or FPGA, I want to drive the wires straight from breadboards. That is how I really understand how stuff works. \$\endgroup\$
    – Gunther Schadow
    Aug 28, 2020 at 4:23

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In the end, I ditched the seperate supplies and went with an all-in chip that provided the necessary voltages, namely the TPS65150 (Universal, compact LCD bias Power Supply with integrated VCOM Buffer). With a few external components I have my needed voltages generated from a 5V signal.

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