My design uses an SN65DP159RSBR to interface with a SMARC standard SBC card. On the DP side, the following signals are passed to the CPU on the SBC.
1. DP0+/- (Pins 6,7)
2. DP1+/- (Pins4,5)
3. DP2+/- (Pins1,2)
4. DP3+/- (Pins9,10)
5. DP_AUX_+/- (Pins38,39)
6. HPD (Pin 3)
I compared what signals we feed to the SBC with what signals are fed to the DP J2 connector on the EVK. I noticed that all signals are there, except pins 13 and pin14 on J2. The EVM calls these pins CAD (GND) and DP_CEC (GND). By DP standard, they are CONFIG1 and 2 pins and should be connected to GND. However, I noticed that they are actually pulled up to 3V3 via 100k and 27k. Since my board does not send these signals to the DP source, I was wondering if the video transmission can work without these two signals.
To that end, I spliced a DP cable and soldered all signals together, as a control case. The transmission worked (ie: DP source was displayed on HDMI sink monitor).
I then desoldered just these two signals. The transmission did not work.
I then resoldered CAD (GND). The transmission worked.
I then desoldered CAD (GND) and resoldered DP_CEC (GND). The trasnmission failed again.
This tells that DP_CEC (GND) is not necessary to make transmission work, but CAD (GND) is necessary. What does the 100k on this signal do exactly? Why is it not connected to GND per DP spec?
Best regards,
Lu Tan
