
MT90500
146
Another consideration in the application of SRTS clock recovery using the MT90500 is the 5 RTS buffer in the
Receive SRTS circuitry. This puts an upper limit on the CDV (Cell Delay Variation) of the receive VC carrying
SRTS. Each RTS in the 5 RTS buffer is carried by one 8 cell sequence, and thus represents a fixed amount of
time dependent on the number of TDM channels being carried. Table 102 summarizes the maximum CDV
supported by the Receive SRTS circuitry at various numbers of TDM channels.
7.3.3
Free-running Clocks
It is usually possible to provide intelligible voice connections using a free-running clock, provided that the
crystal accuracy is constrained to a few parts per million. Free-running clocks are not recommended however,
as relatively frequent frame slips (and accompanying data errors) are inevitable, and this is generally not
acceptable for TDM data going to the public network.
Table 102 - Limits on CDV on Receive SRTS VC
Channels per VC carrying SRTS
Time represented by 5 RTS (msec)
Maximum CDV (msec)
1
2
234
117
14.7
9.87
7.32
3.66
2.44
117
58.6
7.32
4.84
3.66
1.83
1.22
16
24
32
64
96
N (General)
125
μ
sec X 375 / N X 5
= (234 / N) msec
125
μ
sec X 375 / N X 5 / 2
= (117 / N) msec
PHY
PHY
8 kHz
TDM
Reference
MCA1
TDM
Timing
Slave
MCA1
TDM
Timing
Master
PLL
16.384 MHz
TDM Bus
TDM Clocks
TDM Bus
R
Figure 70 - SRTS Clocking Application
ATM
Switch
Port 1
Port 2
CPU
or FPGA
f
nx
f
nx
1.
MCA1 compares
f
nx
to TDM clocks, and
generates SRTS.
4.
Network down-link carries ATM
network clock, and SRTS information.
6.
CPU or FPGA runs clock recovery
algorithm, based on SRTS difference.
SRTS Difference
5.
MCA1 generates local SRTS from
f
nx
and local TDM clocks, and calculates
SRTS difference.
Clock control
3.
The two physical ports must
have synchronous physical ATM
network clocks.
2.
Network up-link carries ATM net-
work clock in one direction, and
SRTS information in the other
direction.
UTOPIA Bus
UTOPIA Bus