MT9044
Input Impairment Monitor
This circuit monitors the input signal to the DPLL and
automatically
enables
the
Holdover
Mode
(Auto-Holdover) when the frequency of the incoming
signal is outside the auto-holdover capture range
(See AC Electrical Characteristics - Performance).
This includes a complete loss of incoming signal, or
a large frequency shift in the incoming signal. When
the incoming signal returns to normal, the DPLL is
returned to Normal Mode with the output signal
locked to the input signal. The holdover output
signal is based on the incoming signal 30ms
minimum to 60ms prior to entering the Holdover
Mode. The amount of phase drift while in holdover is
negligible because the Holdover Mode is very
accurate (e.g.
±0.05ppm).
The the Auto-Holdover
circuit does not use TIE correction. Consequently,
the phase delay between the input and output after
switching back to Normal Mode is preserved (is the
same as just prior to the switch to Auto-Holdover).
Automatic/Manual Control State Machine
The Automatic/Manual Control State Machine allows
the MT9044 to be controlled automatically (i.e.
LOS1, LOS2 and GTi signals) or controlled manually
(i.e. MS1, MS2, GTi and RSEL signals). With
manual control a single mode of operation (i.e.
Normal, Holdover and Freerun) is selected. Under
automatic control the state of the LOS1, LOS2 and
GTi signals determines the sequence of modes that
the MT9044 will follow.
As shown in Figure 1, this state machine controls the
Reference Select MUX, the TIE Corrector Circuit, the
DPLL and the Guard Time Circuit. Control is based
on the logic levels at the control inputs LOS1, LOS2,
RSEL, MS1, MS2 and GTi of the Guard Time Circuit
(See Figure 6).
All state machine changes occur synchronously on
the rising edge of F8o. See the Controls and Modes
of Operation section for full details on Automatic
Control and Manual Control.
To
Reference
Select MUX
To TIE
Corrector
Enable
To DPLL
State
Select
Advance Information
Guard Time Circuit
The GTi pin is used by the Automatic/Manual Control
State Machine in the MT9044 under either Manual or
Automatic control. The logic level at the GTi pin
performs two functions, it enables and disables the
TIE Corrector Circuit (Manual and Automatic), and it
selects which mode change takes place (Automatic
only). See the Applications - Guard Time section.
For both Manual and Automatic control, when
switching from Primary Holdover to Primary Normal,
the TIE Corrector Circuit is enabled when GTi=1, and
disabled when GTi=0.
Under Automatic control and in Primary Normal
Mode, two state changes are possible (not counting
Auto-Holdover). These are state changes to Primary
Holdover or to Secondary Normal. The logic level at
the GTi pin determines which state change occurs.
When GTi=0, the state change is to Primary
Holdover. When GTi=1, the state change is to
Secondary Normal.
Master Clock
The MT9044 can use either a clock or crystal as the
master timing source. For recommended master
timing circuits, see the Applications - Master Clock
section.
Control and Modes of Operation
The MT9044 can operate either in Manual or
Automatic Control. Each control method has three
possible modes of operation, Normal, Holdover and
Freerun.
As shown in Table 3, Mode/Control Select pins MS2
and MS1 select the mode and method of control.
Control
MANUAL
RSEL
0
1
AUTO
0
1
Input Reference
PRI
SEC
State Machine Control
Reserved
RSEL
LOS1
LOS2
Automatic/Manual Control
State Machine
To and From
Guard Time
Circuit
Table 2 - Input Reference Selection
MS1
MS2
Figure 6 - Automatic/Manual Control State
Machine Block Diagram
8
MITEL [ MITEL NETWORKS CORPORATION ]
