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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?

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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.

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Electrically, they are similar devices. PHY tends to be associated with protocols.

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Talking specifically about USB, the terms "transceiver" and "PHY" are synonyms and are used interchangeably. In all cases, some aspects of the physical layer are implemented by the controller, not the PHY. This is the case for all PHYs: the full-speed ones with a serial USB interface, the high-speed ones with ULPI or UTMI, and super speed (USB 3_ with PIPE interface. In all instances, the link state changes are marshalled by the controller. The non-PIPE/UTMI/ULPI transceivers - those limited to full-speed USB (12MHz rate) - do not perform the serialization/deserialization either.

So the functionality is split roughly as follows:

  1. Controller - physical layer link state control.

  2. ULPI/UTMI/PIPE PHY or Controller when a serial FS transceiver is used - serialization/deserialization,.

  3. PHY - Logic-to-bus level translation, equalization, drive/receive.

In practice, it doesn't matter since you'd use a PHY that matches the interface provided by the USB controller. If the controller provides a ULPI or UTMI interface, you'd use an ULPI or UTMI PHY that does serdes. If the controller has a serial interface to the transceiver (only for LS/FS operation), you'd use a FS transceiver with a serial interface (literally a TXD and RXD pair).

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