TMDS181: OE pin not reseting registers

Martin Walton97

Part Number: TMDS181

Hello

I'm trying to use the OE pin to reset the I2C registers and force the chip into powerdown mode. I have a 220nF capacitor connected between OE and GND, and also a BSS138 FET, so an external device can force the TMDS181 into reset. Electrically, this appears to be working, OE is usually at 2.5V (which other posts mention is normal), and when I turn the FET on, it goes to 0V.

According to the manual, this should reset all the I2C registers and put the chip into power down mode, but if I write a register like 0x0C (the VSWING settings), with non-default values, and then pull OE down for 200mS and then release it, the previous value is still in the register. If I read the register 0x20 which contains the power and standby flags, it always reads 0x4A regardless of the state of OE. I set HPD_AUTO_PWRDWN_DISABLE during this test, so the status of HPD doesn't affect this.

Why isn't this working?

Thanks

Martin

over 7 years ago

0 Francisco Javier Zamudio over 7 years ago

TI__Expert 5455 points

Hello Martin

Could you share your schematic and can you test the other methods (HPD and register)?

Regards
Francisco

0 Martin Walton97 over 7 years ago in reply to Francisco Javier Zamudio

Prodigy 200 points

Please see attached schematic as to how I'm controlling the OE pin.

In answer to your question, when HPD goes low, register 0x20 shows 0x8A indicating power-down mode. This also happens when I set PD_EN in register 0x09, but in neither case does the VSWING register get reset to its default value.

So the questions are:

* Why doesn't the OE pin cause a power down condition.

* What registers (if any) actually get reset to default during a power-down condition.

Thanks

Martin

0 JMMN over 7 years ago in reply to Martin Walton97

TI__Guru 60870 points

Hi Martin,

Can you confirm that at the pin that OE is actually going low? I've tested this in the lab and I always see the registers clear after OE is set low. Is your device in I2C mode? How are you reading the registers?

Thanks,

JMMN

0 Martin Walton97 over 7 years ago in reply to JMMN

Prodigy 200 points

Hello

I've checked with a scope and OE is going low (less than 0.1V) when I expect it to.

The device is in I2C mode - pin 10 is pulled high, and I can reliably read the registers. The software reads the 8 byte DEVICE_ID field on startup and verifies it.

Thanks

Martin

0 Francisco Javier Zamudio over 7 years ago in reply to Martin Walton97

TI__Expert 5455 points

Hello Martin

Can you re-upload it? I can't see anything.

Regards
Francisco

0 JMMN over 7 years ago in reply to Francisco Javier Zamudio

TI__Guru 60870 points

Hi Martin,

After rereading the support thread, I think you are seeing the expected device operation but our documentation is not very clear. I will work with the team to clarify the datasheet.

Device registers will only be cleared when OE goes low. The device registers will stay loaded if PD_EN is set or HPD_SNK goes low. This is done so that the registers do not need to be reprogrammed every time a sink is disconnected / reconnected.

While OE is low, the device registers are not able to be read so you would not see a power-down status bit get set.

Can you confirm this matches the behavior you are seeing?

Regards,
JMMN

0 Martin Walton97 over 7 years ago in reply to JMMN

Prodigy 200 points

Hello

Confirmed - this is what I'm now seeing.

Thanks for clearing this up.

Regards

Martin

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TMDS181: OE pin not reseting registers