ACS8522AT datasheet(13/118 Pages) SEMTECH

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ACS8522A SETS LITE

ADVANCED COMMS & SENSING

FINAL

DATASHEET

Revision 1.00/September 2007 © Semtech Corp.

Page 13

www.semtech.com

In Revertive mode, if a re-validated (or newly validated)

source has a higher priority than the reference source

which is currently selected, a switch over will take place.

Many applications prefer to minimize the clock switching

events and choose Non-revertive mode.

In Non-revertive mode, when a re-validated (or newly

validated) source has a higher priority then the selected

source will be maintained. The re-validation of the

reference source will be flagged in the sts_sources_valid

an interrupt. Selection of the re-validated source can take

place under software control or if the currently selected

source fails.

To enable software control, the software should briefly

enable Revertive mode to effect a switch-over to the

higher priority source. When there is a reference available

with higher priority than the selected reference, there will

be NO change of reference source as long as the

Non-revertive mode remains on, and the currently

selected source is valid. A failure of the selected

reference will always trigger a switch-over regardless of

whether Revertive or Non-revertive mode has been

chosen.

Forced Control Selection

A configuration register, force_select_reference_source

Reg. 33, controls both the choice of automatic or forced

selection and the selection itself (when forced selection is

required). For Automatic choice of source selection, the

four LSB bit value is set to all zeros or all ones (default).

To force a particular input the bit value must be set as

follows: 0011 forces SEC1, 0100 forces SEC2, 1000

forces SEC3 and 1001 forces SEC4. Forced selection is

not the normal mode of operation, and the

force_select_reference_source variable is defaulted to

the all-one value on reset, thereby adopting the automatic

selection of the reference source.

Automatic Control Selection

When an automatic selection is required, the

force_select_reference_source register LSB four bits

must be set to all zeros or all ones. The configuration

registers, cnfg_ref_selection_priority (Reg. 19, 1B and

1C), hold 4-bit values which represents the desired

priority of that particular port. Unused ports should be

given the value 0000 in the relevant register to indicate

they are not to be included in the priority table. On

power-up, or following a reset, the whole of the

configuration file will be defaulted to the values defined

by Table 4. The selection priority values are all relative to

each other, with lower-valued numbers taking higher

priorities. Each reference source should be given a unique

number; the valid values are 1 to 15 (dec). A value of zero

disables the reference source. However if two or more

inputs are given the same priority number those inputs

will be selected on a first in, first out basis. If the first of

two same priority number sources goes invalid the second

will be switched in. If the first then becomes valid again, it

becomes the second source on the first in, first out basis,

and there will not be a switch. If a third source with the

same priority number as the other two becomes valid, it

joins the priority list on the same first in, first out basis.

There is no implied priority based on the channel

numbers. Revertive/Non-revertive mode has no effect on

sources with the same priority value.

Ultra Fast Switching

A reference source is normally disqualified after the Leaky

Bucket monitor thresholds have been crossed. An option

for a faster disqualification has been implemented,

whereby if Reg. 48 Bit 5 (ultra_fast_switch) is set, then a

loss of activity of just a few reference clock cycles will set

the main_ref_failed alarm and cause a reference switch.

This can be configured (see Reg. 06, Bit 6) to cause an

interrupt to occur instead of, or as well as, causing the

reference switch.

The sts_interrupts register Reg. 06 Bit 6 (main_ref_failed)

is used to flag inactivity on the reference that the device

is locked to much faster than the activity monitors can

support. If Reg. 48 Bit 6 of the cnfg_monitors register

(los_flag_on_TDO) is set, then the state of this bit is driven

onto the TDO pin of the device.

Note...The flagging of the loss of the main reference failure on

TDO is simply allowing the status of the sts_interrupts bit

main_ref_failed (Reg. 06

, Bit 6) to be reflected in the state of

the TDO output pin. The pin will, therefore, remain High until

the interrupt is cleared. This functionality is not enabled by

default so the usual JTAG functions can be used. When the

TDO output from the ACS8522A is connected to the TDI pin of

the next device in the JTAG scan chain, the implementation

should be such that a logic change caused by the action of the

interrupt on the TDI input should not effect the operation when

JTAG is not active.

Fast External Switching Mode-SRCSW pin

Fast External Switching mode allows fast switching

between inputs SEC1 and SEC2 only. The mode must first

be enabled before switching can take place, and then

switching is controlled via the SRCSW pin.

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