I recently accidentally discovered a part of SparkFun that I hadn't encountered before - SparkX, their line of experimental products.
I purchased a MicroMod Big Display Carrier Board https://www.sparkfun.com/products/17718 to try out bitbanged DVI using the RP2040 RP2040 HDMI Display and didn't notice that I was buying a SparkX product. The big giveaway is that the PCB color is black rather than the red of their standard products. And it is clearly labeled.
I've had some issue getting my monitors to recognize this board as an HDMI source. A comment from Luke Wren caused me to look at the schematic and realize that there is a schematic error that causes the board not to properly source 5V to the monitor. It looks like they were expecting the 5V to come FROM the monitor.
I posted on the SparkFun forum and also raised an issue on their github page, but I noticed that my forum post got moved and also didn't get any responses. It finally occurred to me that this was not a standard product 
. Must be due to pandemic induced brain fog....
I actually like the idea of getting to try new things quickly, just didn't realize that this was an experimental product. I'll have to get out the DMM and verify which component is D5. The simplest fix is to short it out, but I probably should remove the component. The minimum spec for the HDMI 5V is 4.8V, but I think it should work if I leave D3 and the fuse in the path. I'll have to measure the voltage...
I'm sure I'll buy SparkX products in the future. I'll just expect that they may require additional effort.
EDIT 3/19/21: I received a response on github. See below.
The referenced link is https://www.ti.com/lit/an/snva412c/snva412c.pdf?ts=1615898507918 . Which includes a condition where two HDMI sources might be connected together (although it does use the word "accidentally").
So, SparkFun did intend to power this board over the HDMI 5V line. I was not aware of that capability in monitors or TVs, but apparently they are aware of use cases. So, the modification that I had suggested would be a bad idea if there were another source trying to drive the 5V line. And shorting out D5 would not be not compliant with the HDMI spec since shorting this line or connecting it to another source should do no damage. In case of a short, the fuse should work but there isn't any current limiting if it were connected to another source. I haven't had a chance yet to figure out whether it is the lack of 5V or the signal levels which are causing the problems with my monitors. In the meantime, be aware there are potential issues of doing what I suggested - although from Luke's schematic, he does put VBUS on the HDMI 5V pin.
Top Comments