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MColl.3
MColl.3
Associate
October 28, 2021
Question

Bidirectional Line Termination - will just one resistor work?

  • October 28, 2021
  • 2 replies
  • 2178 views

I am designing a PCB for an ST micro MCU with external SDRAM.

On the demo board, ST is using one series resistor termination for the bidirectional data lines. ST places this resistor at about 30/70% split of line length, where the resistor is 30 % of line length away from the MCU.

My understanding from research is to use two (2) resistors on bidirectional lines, one at each end, very close to the ICs connected to the line. The resistor values would match the IC output impedance to the line impedance.

What is the general recommended way to terminate a bidirectional data line?

Can a single resistor perform termination on a bidirectional line?

This topic has been closed for replies.

2 replies

TDK
TDK
October 28, 2021

The most correct way is to terminate it at each end. The least correct way is to have no resistors, but if the traces are short enough, it will likely work just fine unterminated. If both chips are on the same board and relatively close, I doubt any termination is necessary. One resistor is somewhere in the middle. Better than none, worse than two.

If you need it to work and can spare the space, put in two resistors. If traces are short and you want to see if you can get away with one or no terminations, try it and see.

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MColl.3
MColl.3Author
Associate
November 1, 2021

I will use two resistors to terminate each end of the line. Space is not a constraint for this design. Tesla23 at StackExchange simulated termination options and terminating at both end is the best way for signal integrity.

Thanks for response.

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