Add HDMI monitor

Have the 7" touchscreen display with Raspiberry pi 4. Fixed the upside display. Works great. Wondering if I can add a HDMI monitor. If so, how?

0 shabaz over 3 years ago

Hi,

I think that's not a supported combination, these were some comments I wrote a few years back - the situation may have changed, or it may not have changed:

I think there are some difficulties in doing this, there is some information in this thread How to run HDMI and Official Raspberry Pi Display (DSI) simultaneously?

According the raspberrypi.org link referenced off that, and also this link https://www.raspberrypi.org/forums/viewtopic.php?f=108&t=129211&p=880255 , only certain apps can do it, due to the fact that there is only one /dev/fb that gets created regardless if HDMI and the screen are present. So the situation seems stuck until someone with driver knowledge (and possibly GPU knowledge - not sure) can move things forward.
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0 ralphjy over 3 years ago in reply to shabaz

I haven't tried this myself, but according to a comment in this thread https://forums.raspberrypi.com/viewtopic.php?f=63&t=246384#p1504551

"The Pi4 does support HDMI0 + DSI out of the box, but not HDMI1 + DSI due to internal plumbing within the SoC. Please ensure you use the right HDMI connector."

I don't have a DSI LCD, so I can't try it. The link for the SPI display seems broken, but maybe I'll search for it if I want to try it in the future.
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0 dritguy over 3 years ago

You are right. HDMI0 does not work for dual monitor. HDMI1 does. I did have to turn on SPI. However, in the beginning before with only the 7" display, I had to invert the viewing. When I plugged the monitor into HDMI1, the 7" got inverted. I simply figure out to re-invert. That I have overlay where I can move the mouse between screens. Interesting. for now, I will leave it that way so I can continue software development and when done, switched back to the 7" to get the menu back to the screen. Thank you again for the reply. I am glad I picked Element14 to acquire the devices.
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