Altera CPRI IP Core - Manual (Page 205)

Appendix E: Delay Measurement and Calibration E–27
Multi-Hop Delay Measurement
December 2013 Altera Corporation CPRI MegaCore Function
User Guide
Multi-Hop Delay Measurement
In a multihop system, you must combine the delays between and through the
different CPRI masters and CPRI RE slaves to determine the round-trip delay.
Round-Trip Delay Calculation
The value in the rx_round_trip_delay field of the CPRI_ROUND_DELAY register is
meaningful only in CPRI REC and RE masters. It records the round-trip delay for the
current hop only, as shown in
Figure E–1 on page E–2 .
To determine the round-trip delay of a full multihop system, you must add together
the values in the
CPRI_ROUND_DELAY registers of the REC and RE masters in the system,
plus the delays through the external routers, and subtract the loopback delay from all
the hops except the final hop. Use the following calculation, based on the labels in
Figure E–2 on page E–2 :
Round-trip delay =  rx_round_trip_delay (hop i) +  (TBdelayUL + TBdelayDL)(j)
– n
where the REC and RE masters in the configuration are labeled i=0,1,...,n and the
routing layers in the configuration, and their uplink and downlink delays, are labeled
j=0,1,...,(n-1).
As the equation shows, you must omit the loopback delay of one cpri_clkout cycle
from the single-hop calculation for all but the final pair of CPRI link partners. The
loopback delay is only relevant at the turnaround point of the full multihop path.
Round-Trip Cable Delay Calculation
To determine the local round-trip cable delay at each hop, use the method described
in
“Round-Trip and Cable Delay Calculation Examples” , for the REC or RE master
and the RE slave at the current hop. Half of the resulting value is assumed to be the
cable delay in each direction at the current hop.
The round-trip cable delay is the sum of all the local round-trip cable delays in the
multihop path.