AD9546SLASHPCBZ datasheet(101/205 Pages) AD

101 / 205 page

Data Sheet

AD9546

Rev. 0 | Page 101 of 205

Each Q divider has a dedicated pair of generators: one for

generating randomized clock signals (PRBS) and another for

supporting burst or gapped clock applications (N shot), as

shown by the expanded detail of N shot/PRBS Generator A in

Figure 68. Two bits are associated with each Q divider with one

bit for the N shot function and one bit for the PRBS function

per Table 68.

Table 68. Register Address and Bit Assignments for the

N Shot and PRBS Functions

Q Divider

Address

N Shot

Function Bit

PRBS

Function Bit

Q0A

0x10D4

Q0AA

0x10D4

Q0B

0x10D4

Q0BB

0x10D4

Q0C

0x10D5

Q0CC

0x10D5

Q1A

0x14D4

Q1AA

0x14D4

Q1B

0x14D4

Q1BB

0x14D4

Table 69 shows how the PRBS and N shot function bit values

select different output clock functions.

Table 69. N Shot/PRBS Output Clock Function Selections

PRBS Function

N Shot Function

Output Clock Function

Normal clock (Q divider)

Burst or gapped clock

Randomized clock

Not applicable

Although it is physically possible to set both function bits to

Logic 1, the user must avoid doing so because Logic 1 is an

undefined mode and may result in unspecified device behavior.

RANDOMIZED CLOCK (PRBS)

When the randomized clock function is in effect, the output

clock signal is a pseudorandom sequence of high and low

output levels generated at the rate of the associated Q divider.

Figure 69 shows a representation of the output clock signal.

The spectrum of a normal clock signal (square wave) is a series

of line spectra consisting of the fundamental clock frequency

and its associated harmonics. The main purpose of a PRBS

clock (or spread spectrum clock) is to broaden the spectral line

of the fundamental while at the same time reducing the

magnitude of the harmonic spectral lines.

Because the user can program more than one output of the

AD9546 as a spread spectrum clock, each of the 10 PRBS

generators produces a unique PRBS sequence, which prevents

the possibility of correlated crosstalk between outputs enabled

for PRBS clock generation. All 10 PRBS generators use

17th-order generator polynomials, which means the PRBS

sequence is inherently periodic over a period of 217 − 1 (131,071)

Q divider output cycles.

N SHOT (JESD204B AND GAPPED CLOCKING)

N Shot Overview

The ability of the AD9546 to generate JESD204B clock signals

and gapped clock signals derives from the N shot generators.

Each N shot generator operates at the rate of the Q divider

output. The user has control over whether the N shot generator

produces a periodic output clock pattern or a single-burst

pattern. In either case, triggering a pattern is via Bit 0 of

Alternatively, an external signal can serve as the N shot trigger

via an Mx pin (see the Status and Control Pins section).

As shown in Figure 68, the N shot parameters (trigger, mode,

gap, and N) apply to all N shot generators in a PLL channel. As

such, the N shot parameters are common to all outputs in a

channel that have burst or gapped clock functionality enabled (per

Table 69).

Using the N shot generators imposes a minimum divide ratio of

8 on Q dividers configured for N shot functionality.

Q DIVIDER

OUTPUT

PRBS

SEQUENCE

CLOCK

OUTPUT

Q DIVIDER PERIOD BASED ON ITS INPUT CLOCK AND DIVIDE RATIO

TIME

Figure 69. PRBS Clock Signal

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