EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: wanton789 on August 21, 2013, 10:47:14 pm
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I hesitated to open another post so quickly, but this is a different topic and figured it was better than tagging onto the other one (https://www.eevblog.com/forum/projects/hdmi-signal-integrity/). The other post will get bumped down.
This question has to do with terminations on an HDMI line - specifically the four TDMS (Transition-minimized differential signaling) (http://en.wikipedia.org/wiki/Transition-minimized_differential_signaling) lines:
1. clock
2. data 0
3. data 1
4. data 2
This post looks long, but if you follow the instructions closely it should be quick and easy to navigate. Thanks in advance.
I am using a Pericom PI3HDMI101 (http://www.pericom.com/products/signal-switch-multiplexers/?protocol=HDMI%2B%252F%2BDVI%2B%252F%2BMHL#tab-finder). This IC is a "HDMI/DVI Buffer + I2C Buffer for Source Applications".
I will simply be using it as a switch or relay for some automation of testing by controlling the Output Enable. It will sit on a PCB between two HDMI connectors.
On the very first page of this application note (http://www.pericom.com/assets/App-Note-Files/AN216.pdf), the first two paragraphs talk about different termination resistors for the TDMS signals. Using an input pin (RX_SENSE), one can toggle the termination on the CLOCK lines between 50 ohms and 250k ohms.
The datasheet says it is possible to interchange the clock and data lines in which ever way the designer likes if necessary (e.g. for layout reasons).
My question is:
Why does the clock have options for termination resistors?
If the designer decides to interchange lines, the RX_SENSE pin should be toggled to set the termination at 50 ohms (so that they match the data lines - data lines have 50 ohm pull ups by default).
If it is possible to interchange the lines and maintain functional operation - again, why does the clock have different termination options?
Further, refer to the 2nd half of page 2 on this datasheet (http://www.pericom.com/assets/Datasheets/PI3HDMI101.pdf) for the PI3HDMI101.
NOTE: there are two versions of this chip - the PI3HDMI101 and PI3HDMI101 B <---
They are nearly the exact same as far as pin out and operation.
2nd half of page 2 further illustrates the operation of the RX_SENSE pin. It again mentions that the lines can be interchanged.. blah blah..
Scroll down to page 5 for an example schematic of the terminations.
I believe these terminations are all internal. But then check out that first application note (http://www.pericom.com/assets/App-Note-Files/AN216.pdf) again, looking at page 2 and 3 - the two Source Application Schematics.
-The first schematic is without RX_SENSE function - RX_SENSE is tied to VDD - setting termination to 50 ohms
-The second schematic is with RX_SENSE function - RX_SENSE is... going to a voltage divider tied to the output of the CLK_OUT-
-also CLK_OUT+ is terminated via 20k to GND, which it was not in the first schematic
-CLK_IN+ is 1.5k to gnd, which it WAS in the first schematic
-CLK_IN- is 22k to gnd, which it was not in the first schematic
Finally, in the datasheet for the B (http://www.pericom.com/assets/Datasheets/PI3HDMI101-B.pdf) version - (see page 2, the TMDS Receiver Block paragraph).
...it says the termination is done automatically.
And now I'm lost. If I had slightly more knowledge on terminations and t line theory...