cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 
rtfinch
Adventurer
Adventurer
333 Views
Registered: ‎01-06-2016

TMDS_33 drivers

Can TMDS_33 signals be driven through regular LVC33 buffers like the 74LVC245? What would be the issues with doing this? This is for a point-to-point FPGA connect via a backplane. Is it possible to omit the 50ohm pullups on the inputs?

Tags (1)
0 Kudos
2 Replies
klumsde
Moderator
Moderator
273 Views
Registered: ‎04-18-2011

Hi @rtfinch

I'm not familiar with this level shifter, but it seems to be for single ended signals whereas tmds is differential.

TMDS also relies on a pull up at the inputs to function. So I'm not sure how this should work with a signal you're trying to level shift.

You should try sketch a diagram of this circuit you are proposing, try to understand the level shifter and how it might treat a differential signal like this. My guess is that this level n shift is not suitable here since its intended for use with single ended signals 

-------------------------------------------------------------------------
Don’t forget to reply, kudo, and accept as solution.
-------------------------------------------------------------------------
0 Kudos
rtfinch
Adventurer
Adventurer
259 Views
Registered: ‎01-06-2016

I think all the differential signaling means is that the ‘P’ and ‘N’ signals are at opposite logic levels. When one of the pair is at a logic 1 the other of the pair is at a logic 0. Since this is just 3v3 logic I think a pair of single-ended drivers may be used. Since this is for distances measured in inches and I am thinking ordinary buffers may be adequate. Since the drivers max out about 200 MHz a somewhat slower interface will be used. Normally TMDS input buffers have open collector outputs meaning a pull-up resistor is required. But since the ‘245s are actively driving the bus I think the resistors may be omitted. I am not sure about actively driving an open collector input. Why the ‘245’s? It is easier to wire the signals for prototyping and buffers can be made bi-directional. I would like to have the bus driven using the ‘245 buffers as that may enable multiple modes of operation for the bus just by reprogramming the FPGA. If I cannot get the differential serial transceiver interface working I would still be able to use the board as a single-ended parallel bus without having to scrap the board.

0 Kudos