Deterministic Latency Protocols—CPRI and OBSAI
A deterministic latency option is available for use in high-speed serial interfaces such as the Common Public
Radio Interface (CPRI) and OBSAI Reference Point 3 (OBSAI RP3). Both CPRI and OBSAI RP3 protocols
place stringent requirements on the amount of latency variation that is permissible through a link that
implements these protocols.
Figure 4-28: Transceiver Datapath in Deterministic Latency Mode
Byte
Deserializer
Byte Serializer
8B/10B
Decoder
8B/10B Encoder
Rate
Match
FIFO
Receiver Channel PCS
Receiver Channel
PMA
Deskew
FIFO
Word
Aligner
rx_datain
Deserializer
CDR
Transmitter Channel PCS
Transmitter Channel
PMA
tx_dataout
Serializer
wrclk
wrclk
rdclk
rdclk
PCIe
hard
IP
FPGA
Fabric
PIPE
Interface
Transmitter Channel Datapath
Receiver Channel Datapath
TX Phase
Compensation
FIFO
Byte
Ordering
RX
Phase
Compensation
FIFO
Latency Uncertainty Removal with the Phase Compensation FIFO in Register Mode
To remove the latency uncertainty through the receiver's phase compensation FIFO, the receiver and
transmitter phase compensation FIFOs are always set to register mode. In register mode, the phase
compensation FIFO acts as a register and thereby removes the uncertainty in latency. The latency through
the transmitter and receiver phase compensation FIFO in register mode is one clock cycle.
The following options are available:
• Single-width mode with 8-bit channel width and 8B/10B encoder enabled or 10-bit channel width with
8B/10B disabled
• Double-width mode with 16-bit channel width and 8B/10B encoder enabled or 20-bit channel width with
8B/10B disabled
Channel PLL Feedback for Deterministic Relationship
To implement the deterministic latency functional mode, the phase relationship between the low-speed
parallel clock and channel PLL input reference clock must be deterministic. A feedback path is enabled to
ensure a deterministic relationship between the low-speed parallel clock and channel PLL input reference
clock.
To achieve deterministic latency through the transceiver, the reference clock to the channel PLL must be
the same as the low-speed parallel clock. For example, if you need to implement a data rate of 1.2288 Gbps
for the CPRI protocol, which places stringent requirements on the amount of latency variation, you must
choose a reference clock of 122.88 MHz to allow the usage of a feedback path from the channel PLL. This
feedback path reduces the variations in latency.
Altera Corporation
Transceiver Protocol Configurations in Cyclone V Devices
4-29
Deterministic Latency Protocols—CPRI and OBSAI
CV-53004
2013.10.17