Advance Information
Normal Mode
MS2
0
0
1
1
MS1
0
1
0
1
Control
MANUAL
MANUAL
MANUAL
AUTO
Mode
NORMAL
HOLDOVER
FREERUN
State Machine Control
MT9044
Normal Mode is typically used when a slave clock
source, synchronized to the network is required.
In Normal Mode, the MT9044 provides timing (C1.5o,
C2o, C3o, C4o, C8o, C16o, and C19) and frame
synchronization (F0o, F8o, F16o, RSP, TSP) signals,
which are synchronized to one of two reference
inputs (PRI or SEC). The input reference signal may
have a nominal frequency of 8kHz, 1.544MHz or
2.048MHz.
From a reset condition, the MT9044 will take up to 25
seconds for the output signal to be phase locked to
the selected reference.
The selection of input references is control
dependent as shown in State Tables 4 and 5. The
reference frequencies are selected by the frequency
control pins FS2 and FS1 as shown in Table 1.
Holdover Mode
Holdover Mode is typically used for short durations
(e.g. 2 seconds) while network synchronization is
temporarily disrupted.
In Holdover Mode, the MT9044 provides timing and
synchronization signals, which are not locked to an
external reference signal, but are based on storage
techniques. The storage value is determined while
the device is in Normal Mode and locked to an
external reference signal.
When in Normal Mode, and locked to the input
reference signal, a numerical value corresponding to
the MT9044 output frequency is stored alternately in
two memory locations every 30ms. When the device
is switched into Holdover Mode, the value in memory
from between 30ms and 60ms is used to set the
output frequency of the device.
The frequency accuracy of Holdover Mode is
±0.05ppm,
which translates to a worst case 35 frame
(125us) slips in 24 hours. This meets the Bellcore
GR-1244-CORE Stratum 3 requirement of
±0.37ppm
(255 frame slips per 24 hours).
Two factors affect the accuracy of Holdover Mode.
One is drift on the Master Clock while in Holdover
Mode, drift on the Master Clock directly affects the
Holdover Mode accuracy. Note that the absolute
Master Clock (OSCi) accuracy does not affect
Holdover accuracy, only the change in OSCi
accuracy while in Holdover. For example, a
±32ppm
9
Table 3 - Operating Modes and States
Manual Control
Manual Control should be used when either very
simple MT9044 control is required, or when complex
control is required which is not accommodated by
Automatic Control. For example, very simple control
could include operation in a system which only
requires Normal Mode with reference switching
using only a single input stimulus (RSEL). Very
simple control would require no external circuitry.
Complex control could include a system which
requires state changes between Normal, Holdover
and Freerun Modes based on numerous input
stimuli. Complex control would require external
circuitry, typically a microcontroller.
Under Manual Control, one of the three modes is
selected by mode/control select pins MS2 and MS1.
The active reference input (PRI or SEC) is selected
by the RSEL pin as shown in Table 2. Refer to Table
4 and Figure 7 for details of the state change
sequences.
Automatic Control
Automatic Control should be used when simple
MT9044 control is required, which is more complex
than the very simple control provide by Manual
Control with no external circuitry, but not as complex
as Manual Control with a microcontroller.
For
example, simple control could include operation in a
system which can be accommodated by the
Automatic Control State Diagram shown in Figure 8.
Automatic Control is also selected by mode/control
pins MS2 and MS1. However, the mode and active
reference source is selected automatically by the
internal Automatic State Machine (See Figure 6).
The mode and reference changes are based on the
logic levels on the LOS1, LOS2 and GTi control pins.
Refer to Table 5 and Figure 8 for details of the state
change sequences.
MITEL [ MITEL NETWORKS CORPORATION ]
