M1040 datasheet(6/12 Pages) ICST

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M1040 Datasheet Rev 0.1

6 of 12

Revised 11Nov2003

Integr ate d Circuit Systems , In c. ● Comm unication s Modules ● www.icst.com ● tel (508) 852-5400

Integrated

Circuit

Systems, Inc.

M1040

VCSO BASED CLOCK PLL WITH AUTOSWITCH

Preliminar y In f o r m atio n

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 5, 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

DIF_REF0

Device power-up. Manual Select mode. DIF_REF0 input selected reference, not yet locked to.

DIF_REF0

-0-

LOL

to 0: Device locked to reference (may get intermittent LOL pulses until fully locked).

DIF_REF0

-1-

AUTO

set to 1: Device placed in AutoSwitch mode (with DIF_REF0 as initial reference clock).

Operation & Activation

DIF_REF0

Normal operation with AutoSwitch mode armed, with DIF_REF0 as initial reference clock.

DIF_REF0

-1-

LOL

to 1: Clock fault on DIF_REF0, loss of lock indicated by LOL pin, ...

-DIF_REF1-

-1-

... and immediate automatic reselection to DIF_REF1 (indicated by REF_ACK pin).

DIF_REF1

-0-

LOL

to 0: Device locks to DIF_REF1 (assuming valid clock on DIF_REF1).

Re-initialization

-1-

DIF_REF1

REF_SEL

set to 1: Prepares for Manual Selection of DIF_REF1 before then re-arming AutoSwitch.

DIF_REF1

-0-

AUTO

set to 0: Manual Select mode entered briefly, manually selecting DIF_REF1 as reference.

DIF_REF1

-1-

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 5: Example AutoSwitch Sequence

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