1.1. QPI Overview
The QPI is a point-to-point connection protocol developed by Intel to replace the front-side-bus (FSB). It was designed to transfer data between the processors and IO hubs. Compared to a parallel bus, the QPI can achieve higher performance.
QPI is a serial bus technology similar to other point-to-point interconnects. All channels work at the differential IO standard. The physical data rates for QPI can be 4.8, 6.4, or 8 Gbps. The channel links can be defined as full-width for 20 channels, and half-width for 10 channels.
QPI has the following special features, which may differ from other serial interface protocols:
- DC Coupling Mode
The transmitter (TX) and receiver (RX) use DC coupling mode. Per the QPI spec, the connected transceiver pair are DC-coupled.
Stratix V devices have been characterized to be fully compatible with DC mode for QPI interfaces. From the transmitter, the output common mode DC voltage can vary between 0.23 to 0.27 V.
Figure 1. DC Mode
- Bonded TX Channels
All the TX channels must be configured in bonded mode to reduce the channel-to-channel skew. For example, full-width link design will bond all 20 TX channels together.
Stratix V devices can support TX channel bonding for QPI with PLL feedback compensation mode.
To bond all channels using the PLL feedback compensation path, the input reference clock frequency used by the TX PLL must be the same as the parallel clock that clocks the PCS of the same channel. For example, the reference clock is 250 MHz for an 8 Gbps data rate and 200 MHz for 6.4 Gbps.
Figure 2. Bonded Channels
- QPI Link Detection
The Intel QPI physical layer uses a TX based detect scheme. Each TX lane contains a link detect circuit on each TXP and TXN.
During QPI link detecting, the transmitter driver will be tri-stated to avoid interference.
Stratix V devices support the PCIe RX detector and QPI link detector in the transmitter buffer. An assignment setting is used to select QPI mode or PCIe mode. When enabling QPI link detection, the QPI link detector will assert rx_found if a remote RX exists with a 42.5Ω termination.
Figure 3. Link Detection