AD9956SLASHPCB datasheet(27/32 Pages) AD

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AD9956

Rev. 0 | Page 27 of 32

CONTROL FUNCTION REGISTER DESCRIPTIONS

Control Function Register 1 (CFR1)

This control register is comprised of four bytes, all of which

must be written during a write operation involving CFR1. CFR1

is used to control various functions, features, and operating

modes of the AD9956. The functionality of each bit(s) is

described below. In general, the bit is named for the function it

serves when the bit is set.

CFR1<31:25> Open. Unused locations. Write a Logic 0

CFR1<24> PLL Lock Error (Read-Only)

When the device is operating in automatic synchronization

mode or hardware manual synchronization mode (see below),

the PLL_LOCK/ SYNC_IN pin behaves as the SYNC_IN. To

determine whether or not the PLL has become unlocked while

in synchronization mode, this bit serves as a flag to indicate that

an unlocked condition has occurred within the phase frequency

detector. Once set, the flag stays high until it is cleared by a

readback of the value even though the loop might have

relocked. Readback of the CFR1 register clears this bit.

CFR1<24> = 0 indicates that the loop has maintained lock since

the last readback.

CFR1<24> = 1 indicates that the loop became unlocked at some

point since the last readback of this bit.

CFR1<23> Load Sweep Ramp Rate at I/O_UPDATE, also

known as Load SRR @ I/O_UPDATE

The sweep ramp rate is set by entering a value to a down

counter that is clocked by the SYNC_CLK. Each time a new step

is taken in the linear sweep algorithm, the ramp rate value is

passed from the linear sweep ramp rate register to this down

counter. When set, CFR1<23>, enables the user to force the part

to restart the countdown sequence for the current linear sweep

step by toggling the I/O_UPDATE pin.

CFR1<23> = 0 (default). The linear sweep ramp rate countdown

value is loaded only upon completion of a countdown sequence.

CFR1<23> = 1. The linear sweep ramp rate countdown value is

reloaded, if an I/O_UPDATE signal is sent to the part during a

sweep.

CFR1<22> Auto-Clear Frequency Accumulator

This bit enables the auto-clear function for the frequency accu-

mulator. The auto-clear function serves as a clear and release func-

tion for the frequency accumulator (which performs the linear

sweep operation), which then begins sweeping from a known value

of FTW0.

CFR1 <22> = 0 (default). Issuing an I/O_UPDATE has no effect

on the current state of the frequency accumulator.

CFR1 <22> = 1. Issuing an I/O_UPDATE signal to the part

clears the current contents of the frequency accumulator for

one sync-clock period.

CFR1 <21> Auto-Clear Phase Accumulator

This bit enables the auto-clear function for the phase accumula-

tor. The auto-clear function serves as a reset function for the

phase accumulator, which then begins accumulating from a

known phase value of 0.

CFR1<21> = 0 (default). Issuing an I/O_UPDATE has no effect

on the current state of the phase accumulator.

CFR1<21> = 1. Issuing an I/O_UPDATE clears the current con-

tents of the phase accumulator for one SYNC_CLK period.

CFR1 <20> Enable Sine Output

Two different trigonometric functions can be used to convert

the phase angle to an amplitude value, cosine or sine. This bit

selects the function used.

CFR1<20> = 0 (default). The phase-to-amplitude conversion

block uses a cosine function.

CFR1<20> = 1. The phase-to-amplitude conversion block uses a

sine function.

CFR1 <19> Clear Frequency Accumulator

This bit serves as a static-clear or a clear-and-hold bit for the

frequency accumulator. It prevents the frequency accumulator

from incrementing the value as long as it is set.

CFR1 <19> = 0 (default). The frequency accumulator operates

normally.

CFR1 <19> = 1. The frequency accumulator is cleared and held

at a value of 0.

CFR1 <18> Clear Phase Accumulator

This bit serves as a static-clear or a clear-and-hold it for the

phase accumulator. It prevents the phase accumulator from

incrementing the value as long as it is set.

CFR1 <18> = 0 (default). The phase accumulator operates

normally.

CFR1 <18> = 1. The phase accumulator is cleared and held at a

value of 0.

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