AD9953_17 datasheet(23/32 Pages) AD

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Data Sheet

AD9953

Rev. B | Page 23 of 32

OSK Ramp Rate Timer

The OSK ramp rate timer is a loadable down counter, which

generates the clock signal to the 14-bit counter that generates

the internal scale factor. The ramp rate timer is loaded with the

value of the ASFR every time the counter reaches 1 (decimal).

This load and countdown operation continues for as long as the

timer is enabled, unless the timer is forced to load before

reaching a count of 1.

If the load OSK timer bit (CFR1<26>) is set, the ramp rate

timer is loaded upon an I/O UPDATE or upon reaching a value

of 1. The ramp timer can be loaded before reaching a count of 1

by three methods.

The first method of loading is by changing the OSK input pin.

When the OSK input pin changes state, the ASFR value is

loaded into the ramp rate timer, which then proceeds to count

down as normal.

The second method in which the sweep ramp rate timer can be

loaded before reaching a count of 1 is if the load OSK timer bit

(CFR1<26>) is set and an I/O UPDATE is issued.

The third method in which the sweep ramp rate timer can be

loaded before reaching a count of 1 is when going from the

inactive auto shaped on-off keying mode to the active auto

shaped on-off keying mode; that is, when the sweep enable bit is

being set.

OSK PIN

LOAD OSK TIMER

CFR1<26>

SYNC_CLK

AUTO DESK

ENABLE

CFR1<24>

TO DAC

AUTO SCALE

FACTOR GENERATOR

RAMP RATE TIMER

CLOCK

DDS CORE

OSK ENABLE

CFR<25>

AMPLITUDE SCALE

FACTOR REGISTER

(ASF)

HOLD

INC/DEC ENABLE

OUT

COS(X)

AMPLITUDE RAMP

RATE REGISTER

(ASF)

UP/DN

DATA

LOAD

Figure 20. On-Off Shaped Keying Block Diagram

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