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TMDS442: Register read/write works but no switch occurs

Nick Bolton
Prodigy 20 points
Part Number: TMDS442

Hello,

I'm able to successfully read and write to registers on the TMDS442 via I2C, but it seems that doing so has no effect on the device's behavior (in terms of switching). I've also tried GPIO directly, but it's the same effect. The same behavior happens on 5 different TMDS442.

[0x03] source status: sp=0 vEn=1 pwr4=0 pwr3=0 pwr2=0 pwr1=1
[0x01] sink port 1: pre=0 i2cEn=1 oe=0 source=0x00
[0x02] sink port 2: pre=0 i2cEn=1 oe=0 source=0x01

Use case is a PC connecting to a monitor via the TMDS442. When the PC is connected directly to a monitor and working correctly, TDMS (D2) signal min is 2.6 V and Vpp is 600mV. I2C normally goes high to 6.0 V after the data transmission.

I see that the TMDS442 5V_PWRn value in the source plug-in status register changes from 0 to 1 when I plug in a new connection to the source side, so from this I can tell that the TMDS442 is detecting the plugged-in cable and updating the source status register. The sink 1 register source port value is 0 (i.e. port 1, default value). OE is 0, I2C_EN is 1, and PRE is 0.

When plugging in a cable to source port 1 (cable already connected to sink port 1), the TDMS source signal goes from ~0 V (-80 mV, Vpp=120 mV) to 3.24 V and stays high. Source I2C stays high at 5.96 V and no data is transmitted. The source HPD is 0 V.

Same action (plug cable into source port 1, with sink port 1 cable already attached) observed from the sink side: there is a lot of activity on I2C, but the TDMS signal stays high at 3.2 V. Sink HPD is 2.92 V, and if I pull it low momentarily, I see data briefly on I2C.

Interestingly, when PC and monitor are connected via the TMDS442, the PC does actually detect a monitor (and shows it in the Windows display manager). Unplugging on the TMDS442 source side causes the monitor to disappear from Windows as expected.

When unplugging and plugging on the sink side of the TMDS442, the monitor attempts to sync with the signal but gives up after a few seconds.

Any ideas what might be going wrong with the TMDS442 implementation? I am happy to DM the schematic.

Thanks,
Nick