TDA2: Termination Resistors on PCI-e TX and RX

Zach Barnes4

Part Number: TDA2

Is the a requirement for termination resistors on TX and RX for the PCI-e layout?

This PCI-e bus is chip to chip not to an off board device.

I see in the recommended layout details for differential and single ended trace impedance. The is detail on the clock termination 50 ohm to GND on P and N. Other resources for PCI-e show TX and RX termination resistors both to GND and across P and N.

over 6 years ago

0 Zach Barnes4 over 6 years ago

Intellectual 510 points

There is not much space to place the termination resistors to I am hoping I can get by without...

0 Stanislav Stilyanov over 6 years ago in reply to Zach Barnes4

TI__Genius 17980 points

Hi Zach,

PCIe data lanes are internally terminated. No need for external termination.

Note the need for AC-coupling capacitors on the Tx path however (on both PCIe devices).

Best regards,

Stan

0 Stanislav Stilyanov over 6 years ago in reply to Stanislav Stilyanov

TI__Genius 17980 points

Regarding TDA2 clock input (LJCB):

• If the clock is provided externally by a High-speed Current Steering Logic (HCSL) source, no termination at the TDA2 LJCB_CLKN and LJCB_CLKP differential input is needed, and the lines should be DC-coupled. The standard 50 Ohm single-ended termination that is inherent to HCSL should be implemented at the clock source.
• If the clock is provided externally by a LVDS source, the lines must be AC-coupled and a parallel (differential) termination of 100 Ohm might be required at the receiver. In this case, check that 100 nF capacitors are present on the clock pair lines, and that a 100 Ohm resistor is connected between the TDA2 LJCB_CLKN and LJCB_CLKP pads.

0 Zach Barnes4 over 6 years ago in reply to Stanislav Stilyanov

Intellectual 510 points

I have one TDA2 on one side and another TDA2 on the other side as well. Both have sides for the clock diff pair have 50 ohms to ground of P and N, and AC coupling caps. Do we need the 100 ohms parallel resistor in this case?

0 Stanislav Stilyanov over 6 years ago in reply to Zach Barnes4

TI__Genius 17980 points

Hi,

Can you please post a diagram of the clocking? Including the clock generator IC, clock path, clock inputs of TDA2s.

Thanks,

Stan

0 Zach Barnes4 over 6 years ago in reply to Stanislav Stilyanov

Intellectual 510 points

0 Stanislav Stilyanov over 6 years ago in reply to Zach Barnes4

TI__Genius 17980 points

Hi Zach,

This must be like bullet (1) from my previous post.

Assuming U1A is clock source and the other TDA2 is clock consumer, then you need only R301 and R302. Other components must be removed.

Regards,

Stan

0 Zach Barnes4 over 6 years ago in reply to Stanislav Stilyanov

Intellectual 510 points

Stan,

Ok. Remove the capacitors as well so not AC coupled as I read in the datasheet.

So my original question, the TX and RX signals. These do need to be AC coupled and don't need termination resistor just the impedance controlled trace?

Thanks,

Zach

0 Stanislav Stilyanov over 6 years ago in reply to Zach Barnes4

TI__Genius 17980 points

Please see summary below:

0 Zach Barnes4 over 6 years ago in reply to Stanislav Stilyanov

Intellectual 510 points

Great! Thank you.

Processors

Processors forum

TDA2: Termination Resistors on PCI-e TX and RX