Data Sheet
AD9548
Rev. E | Page 43 of 112
MULTIFUNCTION PIN
SYNC SOURCE
0
1
REGISTER
0402[5]
SYSCLK/4
DPLL
FEEDBACK
EDGE
REGISTER
0402[4]
REF A
REF AA
REF D
REF DD
RESET
EDGE
DETECT
EDGE
DETECT
ARM
EDGE
DETECT
ARM
DIRECT SYNC
DPLL EDGE SYNC
ACTIVE REFERENCE SYNC
EEPROM SYNC
SOURCE
PRIMARY
SYNCHRONIZATION
SIGNAL
DIRECT SYNC
SOURCE
(REGISTER 0A02[1])
AUTOMATIC SYNC
SOURCE
(REGISTER 0403)
TO CLOCK
DISTRIBUTION
SYNCHRONIZATION
CONTROL
STALL
DIVIDERS
SYNC OUTPUT
DISTRIBUTION
08
02
2-
02
3
TO MULTIFUNCTION
PIN STATUS LOGIC
Figure 49. Output Synchronization Block Diagram
The deterministic delay, expressed as tLATENCY in the following
equation is a function of the frequency division factor (Qn) of
the channel divider associated with the zero-delay channel.
tLATENCY = (Qn + 4) × tCLK_IN
or
tLATENCY = (Qn + 5) × tCLK_IN
In addition to deterministic delay, there is random delay (tPROP)
associated with the propagation of the reference signal through
the input reference receiver, as well as the propagation of the
clock signal through the clock distribution logic. The total delay is
tDELAY = tLATENCY + tPROP
The user can compensate for tDELAY by using the phase offset
controls of the device to move the edge timing of the
distribution output signal relative to the input reference edge.
One method is to use the open-loop phase offset registers
(Address 030D to Address 030E) for timing adjustment.
However, be sure to use sufficiently small phase increments to
make the adjustment. Too large a phase step can result in the
clock distribution logic missing a CLKINx edge, thus ruining the
edge alignment process. The appropriate phase increment
depends on the transient response of any external circuitry
connected between the DACOUTx and CLKINx pins.
The other method is to use the closed-loop phase offset registers
(Address 0x030F to Address 0x0315) for timing adjustment.
However, be sure to use a sufficiently small phase vs. time profile.
Changing the phase too quickly can cause the DPLL to lose
lock, thus ruining the edge alignment process. Th
e AD9548phase slew limit register (Address 0x0316 to Address 0x0317) can
be used to limit the rate of change of phase automatically, thereby
mitigating the potential loss-of-lock problem.
To guarantee synchronization of the output dividers, it is
important to make any edge timing adjustments after the
synchronization event. Furthermore, when making timing
adjustments, the distribution outputs can be disabled and then
enabled after the adjustment is complete. This prevents the
device from generating output clock signals during the timing
adjustment process.
Note that the form of zero-delay synchronization described here
does not track propagation time variations within the distribution
clock input path or the reference input path (on or off chip)
over temperature, supply, and so on. It is strictly a one-time
synchronization event.
Synchronization Mask
Each output channel has dedicated synchronization mask bits
(Register 0x0402, Bits[3:0]). When the mask bit associated with
a particular channel is set, then that channel does not respond
to the synchronization signal. This allows the device to operate
with the masked channels active and the unmasked channels
stalled while they wait for a synchronization pulse.