AD9954SLASHPCB datasheet(29/36 Pages) AD

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AD9954

Rev. 0 | Page 29 of 36

External Shaped On-Off Keying Mode Operation

The external Shaped On-Off Keying mode is enabled by writing

CFR1<25> to a logic 1 AND writing CFR1<24> to a Logic 0.

When configured for external Shaped On-Off Keying, the

content of the ASFR becomes the scale factor for the data path.

The scale factors are synchronized to SYNC_CLK via the I/O

UPDATE functionality.

Synchronization; Register Updates (I/O UPDATE)

Functionality of the SYNC_CLK and I/O UPDATE

Data into the AD9954 is synchronous to the SYNC_CLK signal

(supplied externally to the user on the SYNC_CLK pin). The

I/O UPDATE pin is sampled on the rising edge of the

SYNC_CLK.

Internally, SYSCLK is fed to a divide-by-4 frequency divider to

produce the SYNC_CLK signal. The SYNC_CLK signal is pro-

vided to the user on the SYNC_CLK pin. This enables synchro-

nization of external hardware with the device’s internal clocks.

This is accomplished by forcing any external hardware to obtain

its timing from SYNC_CLK. The I/O UPDATE signal coupled

with SYNC_CLK is used to transfer internal buffer contents

into the control registers of the device. The combination of the

SYNC_CLK and I/O UPDATE pins provide the user with

constant latency relative to SYSCLK and also ensures phase

continuity of the analog output signal when a new tuning word

or phase offset value is asserted. Figure 23 demonstrates an I/O

UPDATE timing cycle and synchronization.

Notes to synchronization logic:

The I/O UPDATE signal is edge detected to generate a

single rising edge clock signal that drives the register bank

flops. The I/O UPDATE signal has no constraints on duty

cycle. The minimum low time on I/O UPDATE is one

SYNC_CLK clock cycle.

The I/O UPDATE pin is set up and held around the rising

edge of SYNC_CLK and has zero hold time and 4 ns setup

time.

SYSCLK

SDI

SYNC_CLK

DISABLE

SCLK

TO CORE LOGIC

OSK

PROFILE<1:0>

I/O UPDATE

÷ 4

SYNC_CLK

GATING

EDGE

DETECTION

LOGIC

MEMORY

I/O BUFFER

LATCHES

Figure 23. I/O Synchronization Block Diagram

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