ZL30402
Data Sheet
5.1.4 Reference Switching (RefSel): NORMAL --> HOLDOVER --> NORMAL
The NORMAL to HOLDOVER to NORMAL mode switching is usually performed when:
•
•
A reference clock is available but its frequency drifts beyond some specified limit. In a Network Element with
stratum 3 internal clocks, the reference failure is declared when its frequency drifts more than ±12 ppm
beyond its nominal frequency.
During routine maintenance of equipment when orderly switching of reference clocks is possible. This may
happen when synchronization references must be rearranged or when a faulty line card must be replaced.
MS2, MS1 == 01 OR
RefSel change
Ref: FAIL --> OK &
MS2, MS1 == 00 &
AHRD=1 &
NORMAL
(LOCKED)
00
MHR= 0-->1 then 1-->0
{MANUAL}
Ref: OK &
Ref: OK --> FAIL &
MS2, MS1 == 00
{AUTO}
Ref: FAIL --> OK &
MS2, MS1 == 00 &
AHRD=0
MS2, MS1 == 00
{AUTO}
MS2, MS1! = 10
RESET == 1
{AUTO}
AUTO
HOLD-
OVER
RefSel Change
FREE-
RUN
10
HOLD-
OVER
01
RESET
MS2, MS1 == 10 forces
unconditional return from
any state to Free-run
Figure 10 - Manual Reference Switching
Two types of transitions are possible:
•
Semi-automatic transition, which involves changing RefSel input to select a secondary reference clock
without changing the mode select inputs MS2, MS1 = 00 (Normal mode). This forces ZL30402 to
momentarily transition through the Holdover state and automatically return to Normal mode after
synchronizing to a secondary reference clock.
•
Manual transition, which involves switching into Holdover mode (MS2, MS1 = 01), changing references with
RefSel, and manual return to the Normal mode (MS2, MS1 = 00).
In both cases, the change of references provides “hitless” switching.
5.2 Programming Master Clock Oscillator Frequency Calibration Register
The Master Crystal Oscillator and its programmable Master Clock Frequency Calibration register (see Table 18,
Table 19, Table 20, and Table 21) have been described in Section 3.0 "Master Clock Frequency Calibration Circuit",
on page 16. Programming of this register should be done after system has been powered long enough for the
Master Crystal Oscillator to reach a steady operating temperature. When the temperature stabilizes the crystal
oscillator frequency should be measured with an accurate frequency meter. The frequency measurement should be
substituted for the foffset variable in the following equation.
MCFC = 45036 * ( - foffset
)
where foffset is the crystal oscillator frequency offset from the nominal 20 000 000 Hz frequency expressed in Hz.
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Zarlink Semiconductor Inc.
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