zl50031 Zarlink Semiconductor, zl50031 Datasheet - Page 25

• Manufacturer Part Number: zl50031
• Description: Flexible 4 K X 2 K Channel Digital Switch With H.110 Interface And 2 K X 2 K Local Switch
• Manufacturer: Zarlink Semiconductor

16.6
The State Machine handles the reference selection. Depending on REF_SEL and LOS signals (selection between
primary reference failure and PRI_LOS and between secondary reference failure and SEC_LOS), the state
machine selects PRI_REF or SEC_REF as the current input reference and dictates the PLL Circuit mode: Normal
or Holdover Mode. In the Normal Mode, the DPLL output clocks are locked to the selected input reference
(PRI_REF or SEC_REF). In the Holdover Mode, the DPLL clocks retain the phase and frequency values they had
32 to 64 ms prior to moving from the Normal to the Holdover Mode. When going from the Holdover to the Normal
Mode, the State Machine activates the MTIE circuit and goes through the states MTIE PRI or MTIE SEC to prevent
a phase shift of the output clocks during the DPLL reference switch (from PRI_REF to SEC_REF and vice versa).
The state diagram is given in Figure 7, "State Machine Diagram" on page 25.
16.7
The DPLL can operate in two main modes: Normal and Holdover Mode. Each of these modes has two states:
primary or secondary state. The state depends on which reference is currently selected as the preferred reference
PRI_REF or SEC_REF.
16.7.1
Normal Mode is typically used when a clock source synchronized to the network is required.
In the Normal Mode, the DPLL provides timing (C32/64, CT_C8, C2M and C1M5o) and frame synchronization
(CT_FRAME) signals which are synchronized to one of two input references (PRI_REF or SEC_REF). The input
reference signal may have a nominal frequency of 8 kHz, 1.544 MHz, 2.048 MHz or 8.192 MHz.
From a device reset condition or after reference switch, the DPLL can take up to 50 seconds to phase lock the
output signals to the selected input reference signal.
State Machine Circuit
Modes of Operation
Normal Mode
RESET Pin = 0
and XX0
1XX or X01
Holdover
PRI
1X0 or X01
Normal
PRI
2
011
0X0 or
0
XXX = {LOS_PRI, LOS_SEC, REF_SEL}
100 or X01
Figure 7 - State Machine Diagram
0X0 or 011
PRI
MTIE
0X0 or
X11
Zarlink Semiconductor Inc.
ZL50031
3
25
100 or X01
0X0 or 011
SEC
MTIE
100 or
X01
7
Normal
SEC
Holdover
SEC
4
0X0 or X1X
RESET Pin = 0
and XX1
6
Data Sheet