Trouble Driving TFT LCD with a custom TFP401 implementation - Page 1
Forum section: electronics / projects

author: dario-cns
author_rank: Newbie
timestamp: February 25, 2023, 02:42:49 am
content: Hi everyone, hope this is the right place to post this but this has been driving me absolutely nuts for the last few days. If anyone has worked with this part or seen an issue like this help would be greatly appreciated!--- Check out the update ----On to the the issue description:I just designed and got assembled a custom board using a TFP401A to decode an HDMI signal into a 24 bit parellel RGB signal that is then sent to a standard 50-pin TFT display module. The module itself uses a 24-bit parallel RGB bus. After configuring an EDID with the correct resolution, the image shows up with the correct resolution but the completely wrong colors. I've tried a couple of different things and have double checked the pin-out but have been unable to root cause the issue so far.Link to to images and schematichttps://www.figma.com/file/TUZusg8Pi2v0H3nc0oSPi4/TFP401-Issue?node-id=0%3A1&t=OByFgF71Z4KnRJcj-0]https://www.figma.com/file/TUZusg8Pi2v0H3nc0oSPi4/TFP401-Issue?node-id=0%3A1&t=OByFgF71Z4KnRJcj-0] https://www.figma.com/file/TUZusg8Pi2v0H3nc0oSPi4/TFP401-Issue?node-id=0%3A1&t=OByFgF71Z4KnRJcj-0Here's a summary of what I've done:TFP401 Configuration:See attached schematic page for more info.DFO: set to TFT (low)ST: set to high (33 ohm external resistor in series with each RGB line)PIXS: set to 1px/clk mode (low)STAG: set to high (unstaggered)OCK_INV: set to falling edge (low)Observations/Issue Description:1. Red and Black appear inverted. This is odd as it does not seem like a simple R/B or R/G swap.2. Image shows some "ghosting" that tracks the image on the display. It looks like two frames are overlapping each other with one frame being vertically offset from the other.3. HSYNC and VSYNC signals are generated at ~38Khz and ~60hz as expected for display resolution with no obvious Jitter4. Clock shows up at expected 51Mhz frequency.Solutions Attempted:1. Tested a different TFT panel. Same issue occurs2. Tested the TFT panel with the board supplied by the manufacturer. This worked as expected and had no color issues. Their implementation used an LT8619C from Lontium semiconductor to perform the decoding.3. Checked all the bias voltages and made sure they matched the manufacturer demo board.4. Tried changing clock edge from high to low. This resulted in a worse image showing up with the same color issue.5. Tried changing from TFT mode to DSTN mode. This offset the image on the display but did not fix any issues.6. Change the EDID color space from 8-bit RGB to Y'CbCr 4:4:4 or other color-spaces. This shifted the colors that showed up on the display but did not actually resolve the issue.7. Change the EDID frame rate from 60hz to 30hz. This appeared to do nothing------------ Update ---------Turns out the color issue was caused by the red channel being inverted. This was caused by a swapped D2 pair coming from the HDMI line. After swapping this pair the color issue went away but I am still seeing the second "Ghosting". Any clue on what could be causing this issue? Image posted on the link above.If anyone has any ideas on what could be wrong help would be greatly appreciated!If anyone has any ideas on what could be wrong help would be greatly appreciated!

author: darkspr1te
author_rank: Frequent Contributor
timestamp: February 25, 2023, 08:02:56 am
content: I have seen this before ,I had 3 lcd media players where two stopped working and one lcd got broke but still worked, despite looking identical i found that the two that had failed had inverted bits on the red lines, so bits 0-7 were read as 7-0 causing the inversion, to confirm this i removed the top 4 bits and wired them to bottom 4 bits inverted and it worked.I've also had a issue with sending two 8 bit numbers per transfer instead of one 8 bithope this ideas helpdarkspr1te

author: dario-cns
author_rank: Newbie
timestamp: February 27, 2023, 01:13:36 am
content: Turns out one of the diff pairs was hooked up backwords...Still having an issue where there seems to be an overlapping frame happening every so often which causes a "ghost" image to move around with items on the screen. Not sure what options to change in the config to do that.