
Chapter 11: Deterministic Latency PHY IP Core 11–3
Achieving Deterministic Latency
November 2012 Altera Corporation Altera Transceiver PHY IP Core
User Guide
Achieving Deterministic Latency
Figure 11–2 illustrates the TX and RX channels when configured as a wireless
basestation communicating to a remote radio head (RRH) using a CPRI or OBSAI
interface. Figure 11–2 also provides an overview of the calculations that guarantee
deterministic delay. As this figure illustrates, you can use a general-purpose PLL to
generate the clock that drives the TX CMU PLL or an external reference clock input
pin.
There are two ways to control the total latency through the datapath:
■ Use TX PLL feedback to align the TX core clock with the source to the TX PLL
which is the RX recovered clock.
1 If you use this method, Altera recommends that you drive
rx_clkout
to an
external voltage controlled crystal oscillator (VCXO) to remove clock jitter
unless you have determined that the clock jitter is not excessive. The input
from the VCXO is
pll_ref_clk
which drives the PLL for the TX datapath.
Refer to Using TX PLL Feedback to Align the TX Core Clock with the RX
Core Clock for a block diagram illustrating this approach.
■ Use sampling techniques in a delay estimate FIFO to measure the phase difference
between the
tx_clkout
and
rx_clkout
, and the clock output of the PLL
(Figure 11–2) and ensure the delay through the FIFO to a certain accuracy.
Figure 11–2. Achieving Deterministic Latency for the TX and RX Datapaths
(1)
Note to Figure11–2:
(1) The TX and RX Phase Compensation FIFOs always operate in register mode.
TX Data
RX Data
bitslipboundaryselect (from RX Word Aligner)
TX PMA
tx_dataout
DQ
DQ
Serializer
RX PMA
De-
serializer
CMU
PLL
GPLL
CDR
refclk
refclk
(from On- or
Off-Chip PLL)
<n>
8B/10B
rx_datain
RX PCS
TX PCS
Achieving Deterministic Latency for the TX & RX Datapaths
RX Phase
Comp
FIFO
TX Phase
Comp
FIFO
tx_clkout
TX PLL refclk
or External refclk Pin
TX Feedback (for Remote Radio Head Only)
rx_clkout
8B/10B
Word
Aligner
Bit Slip
Remote
Radio
Head
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