ACS8582 Semtech Corporation, ACS8582 Datasheet - Page 14

• Manufacturer Part Number: ACS8582
• Manufacturer: Semtech Corporation

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
register (Reg. 0E and 0F) and, if not masked, will generate
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 and 0100 forces SEC2. 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
register, cnfg_ref_selection_priority (Reg. 19,), holds 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
values are all relative to each other, with lower-valued
Revision 1.00/March 2008 ©Semtech Corp.
ADVANCED COMMS & SENSING
Table
4. The selection priority
CONFIDENTIAL
FINAL
Page 14
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.
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
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 ACS8582 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.
There are two ways to enable Fast External Switching
mode:
,
Bit 6) to be reflected in the state of
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