AD9523 datasheet(25/60 Pages) AD

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Data Sheet

AD9523

Rev. D | Page 25 of 60

When using the sync dividers bit, the user first sets and then

clears the bit. The synchronization event is the clearing operation

(that is, the Logic 1 to Logic 0 transition of the bit). The dividers

are all automatically synchronized to each other when PLL2 is

ready. The dividers support programmable phase offsets from 0

to 63 steps, in half periods of the input clock (for example, the

VCO divider output clock). The phase offsets are incorporated

in the dividers through a preset for the first output clock period

of each divider. Phase offsets are sup-ported only by programming

the initial phase and divide value and then issuing a sync to the

distribution (automatically at startup or manually, if desired).

When using the SYNC pin (Pin 17), there are 11 VCO divider

output pipe line delays plus one period of the clock from the

rising edge of SYNC to the clock output. There is at least one

extra VCO divider period of uncertainty because the SYNC

signal and the VCO divider output are asynchronous.

In normal operation, the phase offsets are already programmed

through the EEPROM or the SPI/I2C port before the AD9523

starts to provide outputs. Although the user cannot adjust the

phase offsets while the dividers are operating, it is possible to

adjust the phase of all the outputs together without powering

down PLL1 and PLL2. This is accomplished by programming

the new phase offset, using Register 0x192[7:2] (see Table 51)

and then issuing a divider sync signal by using the SYNC pin

or the sync dividers bit (Register 0x232[0]). All outputs that are

not programmed to ignore the sync are disabled temporarily

while the sync is active. Note that, if an output is used for the

zero delay path, it also disappears momentarily. However, this is

desirable because it ensures that all the synchronized outputs

have a deterministic phase relationship with respect to the zero

delay output and, therefore, also with respect to the input.

FAN OUT

VCO OUTPUT DIVIDER

SYNC (PIN 17)

SYNC

SYNC DIVIDERS BIT

DIVIDER

DRIVER

OUTx

OUT

SYNC

PHASE

DIVIDE

Figure 27. Clock Output Synchronization Block Diagram

DIVIDE = 2, PHASE = 0

DIVIDE = 2, PHASE = 6

VCO DIVIDER OUTPUT CLOCK

SYNC

CONTROL

6 × 0.5 PERIODS

Figure 28. Clock Output Synchronization Timing Diagram

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