4.4.3. Supported Features for PCIe Gen3
In Stratix V Hard IP for PCI Express, selecting PCIe Base Specification Version 3.0 or PCI Express Base Specification Version 2.1 enables a 32-bit wide PIPE 3.0-like interface for Gen1, Gen2, and Gen3 operations.
In PHY IP Core for PCI Express, selecting Gen3 enables the 32-bit wide PIPE 3.0-like interface and selecting Gen1 or Gen2 enables the 16-bit/8-bit wide PIPE 2.0 interface for Gen1 and Gen2 operation.
Block Synchronization (Word Aligner)
The block synchronizer aligns the recovered serial data coming from the CDR to 130-bit word boundaries. The block synchronizer delineates the word boundaries by searching and identifying the Electrical IDLE Exit Sequence Ordered Set (EIEOS) or the Last FTS OS and SKP ordered set to correctly identify the word boundary from the incoming serial data stream. The block synchronizer continues to realign to a new block boundary following the receipt of an SKP ordered set because of varying word lengths.
Gen3 Rate Match FIFO
To accommodate PCIe protocol requirements and to compensate for clock frequency differences of up to ±300 ppm between source and termination equipment, receiver channels have a rate match FIFO. The rate match FIFO adds or deletes four SKP characters (32 bits) to keep the FIFO from becoming empty or full. It monitors the block synchronizer for a skip_found signal. If the rate match FIFO is almost full, the FIFO deletes four SKP characters. If the rate match FIFO is nearly empty, the FIFO inserts an SKP character at the start of the next available SKP ordered set.
Unlike PCIe Gen1 and Gen2, the PCIe Gen3 encoder/decoder does not use 8B/10B encoding. The PCIe Gen3 encoder/decoder uses a 2-bit sync header and a 128-bit data word. The PCS encoder appends the two sync header bits to every 128 bits of data and enables scrambling for the data packets except for ordered set packets and the first symbol of a TS1/TS2 ordered set. The encoder/decoder continuously enables or disables scrambling, based on whether the payload being processed is an ordered set or a data packet. If an Electrical IDLE Exit Ordered Set or a Fast Training Sequence Ordered Set is received, the scrambler is reset to the initial seed value. The encoder/decoder also monitors the data stream for ordered set and sync header bit violations.
Gen3 Gear Box
The PCIe 3.0 base specification requires a block size of 130 bits with the exception of SKP ordered sets, which can be 66, 98, 130, 162, or 194 bits in length. The 130-bit block of data generated by the 128B/130B encoder and variable length SKP characters must be reordered in 32-bit parallel data segments that the PMA serializer can accept. The transceivers employ a gear box to accommodate this fractional bit difference between the 130-bit data word and a fixed 32-bit serialization PMA factor for Gen3.
Scrambling and descrambling are used during PCIe Gen3 operation to guarantee adequate transitions for the receiver in order to correctly regenerate the recovered clock. The 2-bit sync header bit, ordered set, and the first symbol of the TS1/TS2 ordered set are never scrambled.
PIPE 3.0-Like Gen3 Interface
PCIe Gen3 is a new feature added to the transceivers. The PCS supports PCI Express 3.0 base specification. The PIPE interface has been expanded to a 32-bit wide PIPE 3.0-like interface. The PIPE interface controls PHY functions such as transmission of electrical idle, receiver detection, and speed negotiation and control. In summary, the Gen3 PIPE 3.0-like interface block performs the following:
- Dynamic clock selection between Gen1, Gen2, and Gen3 speeds
- Gen3 auto speed negotiation (ASN)
- Controlling the 128B/130B encoder/decoder
- Gen3 Electrical Idle Entry and Exit detections/CDR Control Block
- Dynamic Gen3 and Gen2/Gen1 PCS data rate Auto Speed Negotiation
- Dynamic transceiver PMA data rate and PLL switching
Auto-Speed Negotiation Block
PCIe Gen3 mode enables ASN (auto-speed negotiation) between Gen1 (2.5 Gbps), Gen2 (5.0 Gbps), and Gen3 (8.0 Gbps) signaling data rates. The signaling rate switch is accomplished through frequency scaling and configuration of the PMA and PCS blocks using a fixed 32-bit wide PIPE 3.0-like Interface.
The PMA switches clocks between Gen1, Gen2, and Gen3 data rates in a glitch-free manner. For a non-bonded x1 channel, an ASN module facilitates speed negotiation in that channel. For bonded x2, x4, and x8 channels, the ASN module selects the master channel to control the rate switch. The master channel distributes the speed change request to the other PMA and PCS channels.
|PCIe Gen3 Capability Mode Enabled||Gen1||Gen2||Gen3|
|Lane data rate||2.5G||5G||8G|
|PCS clock frequency||250 MHz||500 MHz||250 MHz|
|FPGA Core IP clock frequency||62.5 MHz||125 MHz||250 MHz|
|PIPE interface width||32-bit||32-bit||32-bit|
The PCIe Gen3 speed negotiation process is initiated by writing a 1 to bit 5 of the Link Control register of the root port, causing a PIPE rate signal change from the hard IP. The ASN then places the PCS in reset, dynamically shuts down the clock paths to disengage the current active state PCS (either Standard PCS or Gen3 PCS). If a switch to or from Gen3 is requested, the ASN automatically selects the correct PCS clock paths and datapath selection in the multiplexers. The ASN block then sends a request to the PMA block to switch the data rate change and waits for a rate change done signal for confirmation. When the PMA completes the rate change and sends confirmation to the ASN block, ASN enables the clock paths to engage the new PCS block and releases the PCS reset. Successful completion of this process is indicated by assertion of the pipe_phystatus signal by the ASN block to the hard IP block.
Transmitter Electrical IDLE Generation
The PIPE 3.0-like interface under the control of the hard IP block in Hard IP for PCIe or the user Core IP in PHY IP Core for PCIe may place the transmitter in electrical idle during low power states and the ASN process. Before the transmitter enters electrical idle, the HIP sends an electrical idle order set (EIOS) to the PHY. For Gen1 and Gen2, the order set format is COM, IDL, IDL, IDL. For Gen3, the order set format consists of 16 symbols with value 0x66.
During electrical idle, the transmitter differential and common mode voltage levels are compliant to the PCIe Base Specification 3.0.
Receiver Electrical IDLE Inference
If there is no activity on the link for a period of time or during the ASN process, the Inferring Electrical Idle condition is detected by the receiver PHY. These conditions are specified according to Table 4-11 of the PCI Express Base Specification, Rev 3.0.
Gen3 Power State Management
The PCIe base specification defines low power states for PHY layer devices to minimize power consumption. The Gen3 PCS does not implement these power saving measures, except when placing the transmitter driver in electrical idle state in the low power states. In P2 low power state, the transceivers do not disable the PIPE block clock.
CDR Control Block
The CDR control block controls the PMA CDR to obtain bit and symbol alignment and deskew within the allocated time, and generates status signals for other PCS blocks. The PCIe base specification requires that the receiver L0s power state exit time be a maximum of 4 ms for Gen1, 2 ms for Gen2, and 4 ms for Gen3 signaling rates. The transceivers have an improved CDR control block to accommodate fast lock times when the CDR must relock to the new multiplier/divider settings when entering or exiting Gen3 speeds.
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