參數(shù)資料
型號(hào)： M1026-1Z-168.0400
英文描述： VCSO BASED CLOCK PLL WITH AUTOSWITCH
中文描述： 才能開發(fā)出復(fù)雜基于時(shí)鐘鎖相環(huán)AUTOSWITCH
文件頁(yè)數(shù)： 7/14頁(yè)
文件大?。?/td> 322K
代理商： M1026-1Z-168.0400
M1025/26 Datasheet Rev 1.0
7
of 14
Revised 28Jul2004
Integrated Circuit Systems, Inc.
●
Networking & Communications
●
www.icst.com
●
tel (508) 852-5400
M1025/26
VCSO B
ASED
C
LOCK
PLL
WITH
A
UTO
S
WITCH
P r o d u c t D a t a S h e e t
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
ClockInput
REF_ACK AUTO LOL
Conditions
Input
Output
Input
Output
Initialization
Device power-up. Manual Select mode
. DIF_REF0
input selected reference, not yet locked to.
LOL
to
0
: Device locked to reference (may get intermittent LOL pulses until fully locked).
AUTO
set to
1
: Device placed in AutoSwitch mode
(
with
DIF_REF0
as initial reference clock).
0
DIF_REF0
0
0
1
0
DIF_REF0
0
0
-
0
-
0
DIF_REF0
0
-
1
-
0
Operation & Activation
Normal operation with AutoSwitch mode armed
,
with
DIF_REF0
as initial reference clock.
LOL
to
1
: Clock fault on
DIF_REF0
, loss of lock indicated by
LOL
pin, ...
... and immediate automatic reselection to
DIF_REF1 (
indicated
by REF_ACK
pin
)
.
LOL
to
0
: Device locks to
DIF_REF1
(assuming valid clock on
DIF_REF1).
0
DIF_REF0
0
1
0
0
0
0
DIF_REF0
-DIF_REF1-
0
1
1
1
-
1
-
1
-
0
-
-
1
-
1
DIF_REF1
Re-initialization
REF_SEL
set to
1
:
P
repares for Manual Selection of
DIF_REF1
before then re-arming AutoSwitch.
AUTO
set to
0
: Manual Select mode entered briefly, manually selecting
DIF_REF1
as reference.
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.
-
1
-
DIF_REF1
1
1
0
1
DIF_REF1
1
-
0
-
0
1
DIF_REF1
1
-
1
-
0
Table 6: Example AutoSwitch Sequence
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