MT8941 CMOS
Besides the improved jitter performance, the
MT8941 differs from the MT8940 in three other
areas:
Applications
The following figures illustrates how the MT8941 can
be used in a minimum component count approach in
providing the timing and synchro-nization signals for
the Mitel T1 or CEPT interfaces, and the ST-BUS.
The hardware selectable modes and the
independent control over each PLL adds flexibility to
the interface circuits. It can be easily reconfigured to
provide the timing and control signals for both the
master and slave ends of the link.
1. Input pins on the MT8941 do not incorporate
internal pull-up or pull-down resistors.
In
addition, the output configuration of the
bidirectional C8Kb pin has been converted from
an open drain output to a Totem-pole output.
2. The MT8941 includes a no-correction window to
filter out low frequency jitter and wander as
illustrated in Figure 4. Consequently, there is no
constant phase relationship between reference
signal F0i of DPLL # 1 or C8Kb of DPLL #2 and
the output clocks of DPLL #1 or DPLL #2.
Figure 4 shows the new phase relationship
between C8Kb and the DPLL #2 output clocks.
Figure 8 illustrates an application where the
MT8941 cannot replace the MT8940 and
suggests an alternative solution.
Synchronization and Timing Signals for the T1
Transmission Link
Figures 9 and 10 show examples of how to generate
the timing signals for the master and slave ends of a
T1 link. At the master end of the link (Figure 9),
DPLL #2 is the source of the ST-BUS signals derived
from the crystal clock. The frame pulse output is
looped back to DPLL #1 (in NORMAL mode), which
locks to it to generate the T1 line clock. The timing
relationship between the 1.544 MHz T1 clock and
3. The MT8941 must be reset after power-up in
order to guarantee proper operation, which is not
the case for the MT8940.
the 2.048 MHz ST-BUS clock
meets the
requirements of the MH89760/760B. The crystal
clock at 12.352 MHz is used by DPLL #1 to generate
the 1.544 MHz clock, while DPLL #2 (in FREE-RUN
mode) uses the 16.384 MHz crystal oscillator to
generate the ST-BUS clocks for system timing. The
generated ST-BUS signals can be used to
synchronize the system and the switching equipment
at the master end.
4. For the MT8941, DPLL #2 locks to the falling
edge of the C8Kb reference signal. DPLL#2 of
the MT8940 locks on to the rising edge of C8Kb.
5. While the MT8940 is available only in a 24 pin
plastic DIP, the MT8941 has an additional 28 pin
PLCC package option.
MT8980/81
Crystal Clock
MT8941
(12.352 MHz)
VDD
CVb
MS0
MS1
MS2
MS3
ST-BUS
SWITCH
MH89760B
C1.5i
DSTi
C2i
F0i
DSTo
F0i
C12i
C4b
C2o
F0b
CSTi
ENCV
C8Kb
C16i
CSTo
TxT
TxR
RxT
RxR
TRANSMIT
RECEIVE
T1
ENC4o
ENC2o
LINK
(1.544 Mbps)
Crystal Clock
(16.384 MHz)
VSS
RST
Mode of Operation for the MT8941
DPLL #1 - NORMAL (MS0 = X; MS1 = 0)
VDD
DPLL #2 - FREE-RUN (MS0=1; MS2=1; MS3=1)
R
C
Figure 9 - Synchronization at the Master End of the T1 Transmission Link
3-52
MITEL [ MITEL NETWORKS CORPORATION ]
