A PCI PHY (Physical Layer) is the hardware interface responsible for the physical transmission and reception of data in a PCI (Peripheral Component Interconnect) system. Specifically, it handles the low-level signaling, encoding, and decoding of data, bridging the gap between the digital logic and the physical medium.
Key Functions of PCI PHY
The PCI PHY is a crucial component in the PCI communication process and executes a number of functions as follows:
- Data Encoding:
- The PHY encodes data for transmission. This often involves techniques such as 8B/10B encoding.
- 8B/10B encoding maps 8-bit data values to 10-bit symbols. This ensures sufficient signal transitions and DC balance for reliable transmission.
- Byte Striping:
- PHY also performs byte striping, which involves dividing the data stream into parallel lanes for high-speed transmission.
- Scrambling:
- Data is scrambled to avoid repetitive patterns. This reduces electromagnetic interference (EMI).
- Disparity Functionality:
- The transmitter ensures disparity (balance between 1s and 0s) is properly managed in the transmitted signal, maintaining signal integrity.
- Physical Interface:
- It is the physical interface for signal transmission.
- Receiver Functions:
- The receiver side performs the reverse operations, decoding, de-scrambling, and byte combining the received data.
PCI PHY vs MAC
It is important to understand that the PHY is distinct from the MAC (Media Access Control). The MAC is a logical sub-block that sits between the DLL (Data Link Layer) and the PHY.
| Feature | PCI PHY | MAC |
|---|---|---|
| Function | Physical interface for transmission and reception of data | Logical sub-block for data link management |
| Layer | Physical Layer | Data Link Layer |
| Operations | 8B/10B encoding, byte striping, scrambling, disparity handling, signal conversion | Addressing, error detection/correction, data flow control |
| Location | Interfaces directly with the physical medium | Sits between the DLL and the PHY |
Example Scenario
Imagine sending data over a PCI Express (PCIe) link:
- The MAC prepares the data and sends it to the PHY.
- The PHY encodes the data using 8B/10B encoding.
- The PHY performs byte striping, scrambling and manages disparity.
- The PHY converts the digital data into electrical signals and transmits them over the PCIe bus.
- On the receiving end, the PHY receives the electrical signals.
- The PHY performs the reverse operations (de-scrambling, byte combining and decoding) to recover the original data and passes it to the MAC.
- The MAC processes the received data.
