Recently, I have been conducting a role-playing session called
where we interview customers and make device selections.
The following conversation took place during the process.
---------------- conversation ------------------
I ｢ Will you be using a transceiver? ｣
The other party ｢ Yes, we will be using three PCI Express (PCIe) gen3 x8 systems. ｣
I ｢ Is PCIe a transceiver? ｣
The other party ｢ ... ｣
--------------------------------------
I asked about transceivers and the answer was PCIe gen3 x8.
In this article, I would like to go step by step until I understand the meaning of the answer "PCIe gen3 x8".
1. What is a transceiver?
2. What is PCIe gen3?
3. What is x8?
1. What is a transceiver?
A transceiver is a circuit for transmitting and receiving high-speed serial data.
（Transceivers are dedicated circuits in the FPGA that are separate from the logic elements.)
Some Altera devices have built-in transceivers and some do not.
Please refer to the Overview tab of the following URL to see which devices have built-in transceivers.
https://www.intel.com/content/www/us/en/programmable/solutions/technology/transceiver/protocols.html
Another important metric when discussing transceivers is the data rate.
The data rate is the amount of data transmitted and received.
The unit is bps (bits per second).
The latest Altera device, Stratix® 10, has a data rate of up to 56 Gbps, which is very fast.
2. What is PCIe gen3?
PCIe is a protocol (*1).
*1: A protocol is a communication standard, and when connecting to an external device, the sending and receiving protocols must be the same.
So when you asked me about transceivers, we suddenly jumped to protocols.
So what is gen3?
gen stands for generation, meaning generation.
As the generation increases, performance improves.
The table below shows the data transfer rate (bidirectional) per lane (*2).
*2: Lanes will be explained later.
So this time gen 3 meant the third generation!
To find out which protocols Altera supports, please see the protocols tab at the following URL as before.
https://www.intel.com/content/www/us/en/programmable/solutions/technology/transceiver/protocols.html
What is 3, x8?
One last question.
At a quick glance, it seems to mean that something has 8 lanes.
I looked it up and it seems to mean 8 lanes.
A lane is a bundle of circuits for Tx, meaning the transmitter side, and Rx, meaning the receiver side.
The more lanes, the wider the bandwidth of data that can be transferred at once, and the faster the transfer rate.
From the above, we know what "PCIe gen3 x8" means!
Devices that support PCIe gen3 were on the protocol list earlier, but it's hard to narrow it down to just one since there are several.
There are still a lot of hearings to be done!
Conclusion
･A transceiver is a circuit used to send and receive high-speed serial data.
･PCIe is a communication protocol.
･ x8 means 8 lanes.
[Extra]
I would like to explain more about the lanes introduced in the text.
The inside of a lane is shown in the figure below.
Both Tx (transmitter side) and Rx (receiver side) are differential signals, so there are two wires for each, for a total of four wires in one lane.
(For more information on differential signals, please refer to the past article "Beware of LVDS Amplitude")
Whenever you are confused, everyone, please try to visualize the above diagram to clear your mind.
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