SN75LVPE3410: redriver placement suggestion

Hi Patrick,

I think we have worked with these questions through the TI FAE Cory Weng last week, you may check with him to align on previous feedback if you have not already.

Patrick Pan2 said:

1. Would you pls give me a comment could SN75LVPE3410 ok for this case ?

From what I see right now, the estimated total of loss between the CPU and data processor seems OK at 18.3 dB, but there are some details I think are not typical:

• The main board SN75LVPE3410 is shown with a complicated routing scheme to handle signals in the processor --> CPU direction
  •  
  • The SN75LVPE3410 is a unidirectional device (all 4 channels point in same direction) so I understand this is a workaround to make it operate like a bidirectional device
  • But the concern is that the channels for the RX direction could have different and worse SI characteristics than the channels in the TX direction
    • Trace length would be longer, the 180 degree turn in direction could lead to more reflections during sharp transitions in the trace angles, vias also add insertion loss and reflections
    • Redrivers are not designed to compensate for reflections
  • This could still work but the RX direction channels may need a different EQ setting from the TX direction channels, which can only be configured in I2C target mode and not in pin mode 

So for such a situation it would be very important to run simulations to see if the SN75LVPE3410 could support the RX direction or not and at what EQ setting, and then pay careful attention during PCB layout to minimize the negative effects of the routing scheme.

For comparison, this is a sketch of the typical method of using unidirectional redrivers across two different boards:

Also, what is the other device shown on the sub board?

Patrick Pan2 said:

2. If only one needs to be placed, could you suggest where it should be placed on which board?

This is hard to say, best to explore with simulations. Redrivers are more efficient at compensating for loss before the redriver than loss after the redriver. As I said, in the scheme of using separated unidirectional devices, one is placed on each board. A bidirectional device would normally be placed somewhere in the middle of the loss profile to average out the performance for both directions but since there is a connector cable here that is not really possible. Either the main board or sub board could be more suitable depending on the performance of the PCIe TX and RX in the CPU compared to the data processor.

Best,

Evan Su