Electronic – In USB, what is the difference between a PHY and a transceiver


I understand PHY to be the physical layer of the transceiver protocol stack. At the same time I understand that transceivers are devices that transmit serial data at a very high speed i.e >100Mbps.

What I am not sure about is ICs that have PHY in their title and others that have Transceiver in their title. Here are 2 examples. One is a USB PHY while the other is USB transceiver.

e.g http://www.smsc.com/Products/USB/USB_Transceivers/USB3280/Download and http://www.smsc.com/Products/USB/USB_Transceivers/USB333x/Download

What is the difference between them?

Best Answer

The terminology is vague at best. There are no hard and fast rules, and nobody playing police when people don't use the term correctly.

Usually, but not always, a transceiver is a simple buffer that has different signal standards on the inputs and outputs. For example, an RS-232 transceiver usually interfaces TTL signals to RS-232 signals. Also, transceivers are similar to "drivers" and "receivers" except that a transceiver usually does both driver and receiver functions in the same chip.

A Phy is similar to a transceiver in that there is usually different signal standards on "both sides of the chip". With Ethernet it is MII/GMII/etc on one side and, well, Ethernet on the other. There are other Phy's that talk USB, PCIe, and many others. Phys usually incorporate some sort of SERDES (SERializer-DESerializer) function and line encoding. More sophisticated PHYs contain a mini-DSP in them to do all sorts of crazy communication things like advanced baseband wander correction.

A transceiver does not have to be associated with a serial data-- they can be used for parallel data as well. A PHY usually involves some sort of serial data stream. But this is more of a convention than a rule, and I am sure there are chips out there that are exceptions to this.