Hello,
I have a four-chip four-layer board wi the following:
tfp401a for hdmi->ttl video
thc63lvd823 for ttl video -> lvds video (2ch)
tps54620 for 3.3v dc supply (12v input)
24c02 for edid/ddc function
The purpose of this boar dis to take hdmi video input and convert it to lvds on a pixel-per-pixel level, with no video scaling, osd, or other functions.
I have various lvds lcd modules to test with as well as a video source that is completely programmable with respect to video parameters (pc graphics card with "powerstrip" application installed). here is the situation:
When I attempt to send video which has a pixel clock of less than 75MHz, the attached lcd displays the expected images without visual degradation, everything appears to work fine.
When I increase pixel clock above 75MHz, I get visual defects on th escreen which tend to occur at similar places along a gradient (eg: if I were to display a picture of a rainbow or other image which has smooth gradients across colors on it, I would get visual defects along certain arcs within the rainbow).
I have toggled the PIXS input of the tfp401 to change the clock rate of the transmitting side of the the system and can confirm that even when PIXS is set to 2 pixel/clock mode, the pixel clock limit appears at the same 75MHz frequency. As changing PIXS halves the tfp401 to thc63lvd823 chip communication speed without changing the failure point of the system, I belive the problem must be in the hdmi-recieving side of the system. The inter-chip communication as well as the lvds transmitter should be OK.
Changing the drive strength or the output clock edge select of the tfp401 outputs did not change the situation.
That leaves the high-speed side of the tfp401a at the top of my list of suspects. What do you recommend ?
I'm fairly certain that my PCB follows the decoupling and layout guidelines, but I can't think of anything else that would cause such issues.
I can supply full gerber files, schematic, output video pictures, etc. if it will help. Just let me know what you want to see.
Thanks,
Zis
