adv7480: failed to detect hdmi signal

Question

Hi, 
 
 I am trying to use adv748x linux driver (kernel 5.10.35). On top of the kernel we have modified a bit to enable configuring tmds as per the recommended settings. This driver was earlier used for adv7482 chip and works fine for the hdmi detection. 
 I am observing that the regs written are not the same if I try to do readback (reg 5A, 9C), but it is unclear as what these reg means from the hardware manual rev.PrB 
 
 It seems to fail to read hdmi_rx[87]:[2]. As per the "the Poll Reg 0x87[2] in HDMI Rx Map until value read back is 0b1 or timeout occurs. Timeout should occur after at least 100 ms." 
 
 Is there any config diff for register compared to ADV7482 and ADV7480? 
 
 Thanks,

PoornimaSubramani · Answer

Hi, 
 Please look at the HDMI specification to see how HDMI TMDS are terminated. 
 HDMI receivers must provide proper termination on the TMDS lines. Normally this is done internally so you can turn off the termination to save power and indicate to the transmitter the receiver is not set up yet. 
 Usually transmitters will not transmit until they see the correct termination. Transmitters monitor the dc voltage on the TMDS clock line for something greater then 2V ( All TMDS lines when operating properly should be at ~3V with a 300mVpp signal on them) and if it see this then it assumes it is properly terminated and allows you to turn on the transmitter drivers. Of course you can override this and force it to transmit. 
 
 As per expert comment the optimal solution would be, 
 1) Design for a 50 Ohm board (55 +- 10% can be done easily at most board fabricators) 
 2) Use 22-47 series resistors in the control and pixel buses. If the traces are short enough you may not need the series resistors and control the integrity with the drive settings only. 
 3) Do signal integrity analysts using the IBIS files and tweaking the series resistor and drive strength as needed for best signal integrity. 
 We are in the High Performance Media group, are designing boards based on 50 Ohm trace impedances. This better matches design requirements for HDMI, USB, DDRs, SDRAMs and DSPs we use on our boards. For the pixel bus we generally use 33 Ohms and tweak the output drive strengths to get good signal integrity. 
 Note: Normally receivers have on-die termination resistors. For schematics you can do point to point routing as long as you maintain the 50 Ohms, 100 Ohm differential layout for the TMDS pairs 
 
 Thanks, 
 Poornima

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