Datasheet

AD9558 Data Sheet
Rev. B | Page 30 of 104
REFERENCE CLOCK INPUTS
Four pairs of pins provide access to the reference clock receivers.
To accommodate input signals with slow rising and falling edges,
both the differential and single-ended input receivers employ
hysteresis. Hysteresis also ensures that a disconnected or
floating input does not cause the receiver to oscillate.
When configured for differential operation, the input receivers
accommodate either ac- or dc-coupled input signals. The input
receivers are capable of accepting dc-coupled LVDS and 2.5 V
and 3.3 V LVPECL signals. The receiver is internally dc biased
to handle ac-coupled operation, but there is no internal 50 Ω or
100 Ω termination.
When configured for single-ended operation, the input
receivers exhibit a pull-down load of 45 kΩ (typical). Three
user-programmable threshold voltage ranges are available for
each single-ended receiver.
REFERENCE MONITORS
The accuracy of the input reference monitors depends on a
known and accurate system clock period. Therefore, the
functioning of the reference monitors is not operable until the
system clock is stable.
Reference Period Monitor
Each reference input has a dedicated monitor that repeatedly
measures the reference period. The AD9558 uses the reference
period measurements to determine the validity of the reference
based on a set of user-provided parameters in the profile
register area of the register map.
The monitor works by comparing the measured period of a
particular reference input with the parameters stored in the
profile register assigned to that same reference input. The
parameters include the reference period, an inner tolerance,
and an outer tolerance. A 40-bit number defines the reference
period in units of femtoseconds (fs). The 40-bit range allows for
a reference period entry of up to 1.1 ms. A 20-bit number defines
the inner and outer tolerances. The value stored in the register
is the reciprocal of the tolerance specification. For example, a
tolerance specification of 50 ppm yields a register value of
1/(50 ppm) = 1/0.000050 = 20,000 (0x04E20).
The use of two tolerance values provides hysteresis for the
monitor decision logic. The inner tolerance applies to a
previously faulted reference and specifies the largest period
tolerance that a previously faulted reference can exhibit before it
qualifies as nonfaulted. The outer tolerance applies to an already
nonfaulted reference. It specifies the largest period tolerance
that a nonfaulted reference can exhibit before being faulted.
To produce decision hysteresis, the inner tolerance must be less
than the outer tolerance. That is, a faulted reference must meet
tighter requirements to become nonfaulted than a nonfaulted
reference must meet to become faulted.
Reference Validation Timer
Each reference input has a dedicated validation timer. The
validation timer establishes the amount of time that a previously
faulted reference must remain unfaulted before the AD9558
declares it valid. The timeout period of the validation timer is
programmable via a 16-bit register. The 16-bit number stored
in the validation register represents units of milliseconds (ms),
which yields a maximum timeout period of 65,535 ms.
It is possible to disable the validation timer by programming the
validation timer to 0b. With the validation timer disabled, the
user must validate a reference manually via the manual reference
validation override controls register (Address 0x0A0B).
Reference Validation Override Control
The user also has the ability to override the reference validation
logic and can either force an invalid reference to be treated as
valid, or force a valid reference to be treated as an invalid
reference. These controls are in Register 0x0A0B to Register
0x0A0D.
REFERENCE PROFILES
The AD9558 has an independent profile for each reference input.
A profile consists of a set of device parameters such as the R divider
and N divider, among others The profiles allow the user to
prescribe the specific device functionality that should take effect
when one of the input references becomes the active reference.
The AD9558 evaluation software includes a frequency planning
wizard that can configure the profile parameters, given the
input and output frequencies.
The user should not change a profile that is currently in use
because unpredictable behavior may result. The user can either
select free run or holdover mode or invalidate the reference
input prior to changing it.
REFERENCE SWITCHOVER
An attractive feature of the AD9558 is its versatile reference
switchover capability. The flexibility of the reference switchover
functionality resides in a sophisticated prioritization algorithm
that is coupled with register-based controls. This scheme
provides the user with maximum control over the state machine
that handles reference switchover.
The main reference switchover control resides in the loop
mode register (Address 0x0A01). The REF switchover mode
bits (Register 0x0A01, Bits[4:2]) allow the user to select one of
the five operating modes of the reference switchover state machine,
as follows:
• Automatic revertive mode
• Automatic non-revertive mode
• Manual with automatic fallback mode
• Manual with holdover mode
• Full manual mode (without auto-holdover)