Auto Switching DVI with TMDS251

Robert Rockel

Other Parts Discussed in Thread: TMDS251, TMDS351

We are trying to develop a auto switching circuit for two DVI inputs using the TMDS251. Here is our circuit

When we energize it, the negative side of the differential input is pulled down and we cannot figure out what we did wrong.

Thanks

Rock

over 10 years ago

0 Elias Villegas M. over 10 years ago

TI__Guru* 76130 points

Hello,
A couple of observations to your schematic:

Terminal EQ is not 5V tolerant, it must be connected to 3.3V
The pull-ups on SCL/SDA_SINK must be connected to 5V
Terminals SDA1 and SDA2 require a pull-up to 5V as well.

Regards

0 AJ Johnson over 10 years ago in reply to Elias Villegas M.

Prodigy 60 points

Hello Elias,

Thanks for the help. I made your suggested changes and also added a pullup to the HPD J4 output. It is my understanding the monitor needs to see a high there to receive video.

After all of that, we're still not getting any video through the circuit.

0 AJ Johnson over 10 years ago in reply to AJ Johnson

Prodigy 60 points

ttt

0 Elias Villegas M. over 10 years ago in reply to AJ Johnson

TI__Guru* 76130 points

Just to be sure,
Are you connecting J4 to the Video sink?
Can you measure the voltage levels on the power rails?
Can you remove the auto-switching section and just hardwire one of the options?
Regards

0 AJ Johnson over 10 years ago in reply to Elias Villegas M.

Prodigy 60 points

J4 is an HDMI connector. DDC_CLK_Y and DDC_Y are connected to SCL_SINK and SDA_SINK respectively.

Voltage rails are exactly 3.3V and 5.0V.

I confirmed the logic levels at S1/S2 and they are correct per the logic table in the datasheet.

0 John Tyler10 over 10 years ago

Prodigy 20 points

So far we're not getting anywhere. Let's try this: Do you have a known working schematic that uses this TMDS251 and can you send it to us? We have an application that this device would be perfect for but we can't seem to get it to work.

0 Elias Villegas M. over 10 years ago in reply to John Tyler10

TI__Guru* 76130 points

Here is a reference schematic, it is for the TMDS351 but it works as a reference for your device.

Regards

TMDS351 referance circuit.pdf

0 AJ Johnson over 10 years ago in reply to Elias Villegas M.

Prodigy 60 points

Thanks Elias!

The TMDS351 reference schematic shows an EEPROM for each input. Because we want the sources to talk directly to the complete monitor we're using, we want the communications to go straight through our switch like it's not there. We're not building this switch into a monitor. Because of that, we don't need the EEPROMs, correct?

There is added circuitry around the HPD inputs. Is that something we need considering our application?

0 AJ Johnson over 10 years ago in reply to Elias Villegas M.

Prodigy 60 points

Hi Elias,

We noticed the schematic you sent us is sheet 1 of 16; do any of the other sheets have application specifics regarding the TMDS?

0 Elias Villegas M. over 10 years ago in reply to AJ Johnson

TI__Guru* 76130 points

Hello,

The EEPROM is not a must have, you can switch the SCL/SDA directly from sink to source.
The circuitry around HPD is not a must have, you can connect HPD directly to the TMDS251.
Please provide an email address and I can send you the schematic of our evaluation module which is complete.

Regards

0 AJ Johnson over 10 years ago in reply to Elias Villegas M.

Prodigy 60 points

rjohnson@seacorp.com

0 John Tyler10 over 10 years ago in reply to Elias Villegas M.

Prodigy 20 points

You provided us with the schematic of your evaluation module which is complete. Thank You! We would like to purchase that module with all the documentation that goes with it such as application notes, datasheets, Circuit Board layout notes, etc.. How can we go about purchasing this module as soon as possible?

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Auto Switching DVI with TMDS251