# PCIe internal clock speeds - Gen3

#### Electronic_Maniac

Joined Oct 26, 2017
253
Hi all,

"First of all, the 100MHz you refer to is the reference clock frequency. The reference clock is either shared between the motherboard and devices, or created from a local oscillator. This is not part of the throughput calculation.

To get the transceiver clock frequencies (the frequency of the high speed TX and RX lines), a Phase-locked loop (PLL) device is used to step this up the reference clock frequency to a higher value. The clock rates are 1.25GHz (2.5 Giga-transfers per second (GTps)) for PCIe Gen 1, 2.5GHz (5GTps) for PCIe Gen 2, or 4GHz (8GTps) for PCIe Gen 3."

Can you tell me why the clock is not doubled in gen3? And how is the clock speed decided to 4GHz? Is there any calculation? Please explain

Thanks

#### kubeek

Joined Sep 20, 2005
5,753
Who says it needs to be doubled? USB1.1 to USB2 went from 12Mbit to 480Mbit.

From gen1 to gen2 it was doubled, because that amount was the frequency that could be achieved with present technology and a few years later when the standard gets to the customers the end product would not cost too much. For gen3 someone did the calculations, and found that 5GHz clock is too hard to achieve and would need to cost the customer too much, so they went with 4GHz.

#### Electronic_Maniac

Joined Oct 26, 2017
253
Who says it needs to be doubled? USB1.1 to USB2 went from 12Mbit to 480Mbit.

From gen1 to gen2 it was doubled, because that amount was the frequency that could be achieved with present technology and a few years later when the standard gets to the customers the end product would not cost too much. For gen3 someone did the calculations, and found that 5GHz clock is too hard to achieve and would need to cost the customer too much, so they went with 4GHz.
Hi,

If you check the link below, there is a table towards the end, where we see the clock for gen3 is 8GHz. But you say it is 4GHz. Can you explain how?

#### kubeek

Joined Sep 20, 2005
5,753
Frankly I have no idea, I was just quoting the numbers you provided.