TX1 PCI-E: What does "2.5GHz Half-rate architecture" in the OEM Design Guide mean?

I’m a bit stumped: every other document on the X1 I’ve read indicates PCI-E 5GT/s support. However, page 25 of the OEM Design Guide(table 23), has the note “2.5GHz, half-rate architecture” for PCI-E data rate.

I have yet to find an explanation that sits well with me. Options I’ve considered:

  • Does that mean the PCI-E lanes transmit two bits per clock? I haven't been able to find a PCI-E specification that allows two bits per clock.
  • Is the controller limited to half-data rate on the number of transactions per second? That doesn't seem likely since TX and RX are at least conceptually independent and decoupled.
  • Is the controller really limited to PCI-E 1.0 data rates? Everywhere else says it's not, and other forums posts claim PCI-E performance beyond 1.0 limits.
  • Perhaps a misprint? It doesn't seem to be mentioned anywhere else.

Anyone have any insight as to what the OEM Design Guide means?

Thanks!

Hi ahaase,

5GT/s is data rate, 2.5GHz is the clock frequency, as you know the differential signal is sampled on both rising edge and falling edge, so the data rate is double as clock.