M1033/34 Datasheet Rev 1.0

7 of 14

Revised 07Apr2005

M1033/34

Prod uct Data Sh eet

Integrated

Circuit

Systems, Inc.

Using the AutoSwitch Feature

See also Table 6, Example AutoSwitch Sequence.

In application, the system must be powered up with the

device in Manual Select mode (AUTO pin is set low).

The activity monitor output (LOR) should then be polled

to verify that the input clock reference is valid.

REF_SEL should be set to select the desired input

clock reference. This selection determines the

reference clock to be used in Manual Select mode and

the initial reference clock used in AutoSwitch mode.

Sufficient time must be allocated for the PLL to acquire

lock to the selected input 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. The REF_SEL input state

must be maintained when switching to AutoSwitch

mode (AUTO pin high) and in addition must still be

maintained until a reference fault occurs. If a reference

fault occurs on the selected reference input, the LOR

output goes high and the input reference is

automatically reselected. The REF_ACK output always

indicates the reference selection status and the LOR

output always indicated the selected input reference

clock status. A successful automatic reselection is

indicated by a change of state of the REF_ACK output.

If an automatic reselection is made to a non-active

reference clock input, the REF_ACK output will

change state and both LOR outputs will remain high.

No further automatic reselection is made by the device;

only one reselection is made each time the AutoSwitch

mode is armed by the system. AutoSwitch mode is

re-armed by the system 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

unintentional reference reselection, AutoSwitch mode

must not be re-enabled until the desired state of the

REF_SEL pin is set and the LOR 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.

Example AutoSwitch Sequence

0 = Low; 1 = High. Example with REF_SEL initially set to 0 (i.e., DIF_REF0 selected)

Clock Input

REF_ACK AUTO

LOR

Conditions

Input

Output

Input

Output

Initialization

0

DIF_REF0

0

0

0

Device power-up. Manual Select mode. DIF_REF0 input selected as the working reference.

Both input references should be active.

0

DIF_REF0

0

-1-

0

AUTO set to 1: Device placed in AutoSwitch mode (with DIF_REF0 as working reference

clock).

Operation & Activation

0

DIF_REF0

0

1

0

Normal operation with AutoSwitch mode armed, with DIF_REF0 as the working reference

clock; DIF_REF1 is the protection reference clock. Both input references should be active.

0

DIF_REF0

0

1

-1-

Due to loss of reference at DIF_REF0 input (clock fault), the LOR output asserts high, then

device immediately goes to the following stage below.

0

-DIF_REF1-

-1-

1

-0-

Device initiates an automatic reselection to DIF_REF1 (indicated by REF_ACK pin), and then

the LOR output asserts low, indicating an active reference on DIF_REF1.

Re-initialization

-1-

DIF_REF1

1

1

-0-

When operation of DIF_REF0 is restored, the device can be prepared once again for

AutoSwitch. Preparation begins by setting the REF_SEL pin to 1, which will maintain the

current reference input selection when entering Manual Select mode.

1

DIF_REF1

1

-0-

0

AUTO set to 0: Manual Select mode entered briefly, manually selecting DIF_REF1 as the

working reference.

1

DIF_REF1

1

-1-

0

AUTO set to 1: Device is now placed in AutoSwitch mode, re-initializing AutoSwitch with

DIF_REF1 now specified as the working reference clock.

Table 6: Example AutoSwitch Sequence