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M1025 Datasheet(PDF) 7 Page - Integrated Circuit Systems

No. de Pieza. M1025
Descripción  VCSO BASED CLOCK PLL WITH AUTOSWITCH
Descarga  14 Pages
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Fabricante  ICST [Integrated Circuit Systems]
Página de inicio  http://www.icst.com
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M1025 Datasheet(HTML) 7 Page - Integrated Circuit Systems

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M1025/26 Datasheet Rev 1.0
7 of 14
Revised 28Jul2004
Integr ated Circuit Systems , Inc. ● Netw o r ki ng & C o mmun ica t io ns ● ww w. icst.com ● tel (5 08) 85 2-54 00
M1025/26
VCSO BASED CLOCK PLL WITH AUTOSWITCH
Prod uct Data Sh eet
Integrated
Circuit
Systems, Inc.
AutoSwitch (AUTO) Reference Clock Reselection
This device offers an automatic reference clock
reselection feature for switching input reference clocks
upon a reference clock failure. With the AUTO input pin
set to high and the LOL output low, the device is placed
into automatic reselection (AutoSwitch) mode. Once in
AutoSwitch mode, when LOL then goes high (due to a
reference clock fault), the input clock reference is
automatically reselected internally, as indicated by the
state change of the REF_ACK output. Automatic clock
reselection is made only once (it is non-revertive).
Re-arming of automatic mode requires placing the
device into manual selection (Manual Select) mode
(AUTO pin low) before returning to AutoSwitch mode
(AUTO pin high).
Using the AutoSwitch Feature
See alsoTable 6, Example AutoSwitch Sequence.
In application, the system is powered up with the device
in Manual Select mode (AUTO pin is set low), allowing
sufficient time for the reference clock and device PLL to
settle. The REF_SEL input selects the reference clock to
be used in Manual Select mode and the initial reference
clock used in AutoSwitch mode. The REF_SEL input state
must be maintained when switching to AutoSwitch
mode (AUTO pin high) and must still be maintained until a
reference fault occurs.
Once a reference fault occurs, the LOL output goes high
and the input reference is automatically reselected. The
REF_ACK
output always indicates the reference selection
status and the LOL output always indicates the PLL lock
status.
A successful automatic reselection is indicated by a
change of state of the REF_ACK output and a momentary
level high of the LOL output (minimum high time is 10
ns).
If an automatic reselection is made to a non-valid
reference clock (one to which the PLL cannot lock),
the REF_ACK output will change state but the LOL
output will remain high.
No further automatic reselection is made; only one
reselection is made each time the AutoSwitch mode is
armed. AutoSwitch mode is re-armed by placing the
device into Manual Select mode (AUTO pin low) and then
into AutoSwitch mode again (AUTO pin high).
Following an automatic reselection and prior to
selecting Manual Select mode (AUTO pin low), the
REF_SEL
pin has no control of reference selection.
To prevent an unintential reference reselection,
AutoSwitch
mode must not be re-enabled until the
desired state of the REF_SEL pin is set and the LOL output
is low. It is recommended to delay the re-arming of
AutoSwitch
mode, following an automatic reselection,
to ensure the PLL is fully locked on the new reference.
In most system configurations, where loop bandwidth is
in the range of 100-1000 Hz and damping factor below
10, a delay of 500 ms should be sufficient. Until the PLL
is fully locked intermittent LOL pulses may occur.
Example AutoSwitch Sequence
0 = Low; 1 = High. Example with REF_SEL initially set to 0 (i.e., DIF_REF0 selected)
REF_SEL Selected
Clock Input
REF_ACK AUTO LOL
Conditions
Input
Output
Input
Output
Initialization
0
DIF_REF0
0
0
1
Device power-up. Manual Select mode. DIF_REF0 input selected reference, not yet locked to.
0
DIF_REF0
0
0
-0-
LOL
to 0: Device locked to reference (may get intermittent LOL pulses until fully locked).
0
DIF_REF0
0
-1-
0
AUTO
set to 1: Device placed in AutoSwitch mode (with DIF_REF0 as initial reference clock).
Operation & Activation
0
DIF_REF0
0
1
0
Normal operation with AutoSwitch mode armed, with DIF_REF0 as initial reference clock.
0
DIF_REF0
0
1
-1-
LOL
to 1: Clock fault on DIF_REF0, loss of lock indicated by LOL pin, ...
0
-DIF_REF1-
-1-
1
1
... and immediate automatic reselection to DIF_REF1 (indicated by REF_ACK pin).
0
DIF_REF1
1
1
-0-
LOL
to 0: Device locks to DIF_REF1 (assuming valid clock on DIF_REF1).
Re-initialization
-1-
DIF_REF1
1
1
0
REF_SEL
set to 1: Prepares for Manual Selection of DIF_REF1 before then re-arming AutoSwitch.
1
DIF_REF1
1
-0-
0
AUTO
set to 0: Manual Select mode entered briefly, manually selecting DIF_REF1 as reference.
1
DIF_REF1
1
-1-
0
AUTO
set to 1: Device is placed in AutoSwitch mode (delay recommended to ensure device fully
locked), re-initializing AutoSwitch with DIF_REF1 now specified as the initial reference clock.
Table 6: Example AutoSwitch Sequence


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