AD9957SLASHPCBZ datasheet(39/62 Pages) AD

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AD9957

Data Sheet

Rev. E | Page 38 of 61

Table 9. OSK Amplitude Step Size

ASF<1:0>

Amplitude Step Size

As mentioned earlier, the step interval is controlled by a 16-bit

programmable timer. Normally, this timer is loaded with the

programmed timing value whenever the timer expires, thus

initiating a new timing cycle. However, three events cause the

timer to have its timing value reloaded prior to the timer expiring.

One such event is when the Select Auto-OSK bit is transitioned

from a Logic 0 state to a Logic 1 state followed by an I/O update. A

second such event is a change of state in the OSK pin. The third

event is dependent on the status of the Load ARR @ I/O Update

bit. If this bit is Logic 0, no action occurs; otherwise, when the

I/O_UPDATE pin is asserted (or a profile change occurs), the

timer resets to its initial starting point.

PROFILES

Each of the three operating modes of the AD9957 support the

use of profiles, which consist of a group of registers containing

pertinent operating parameters for a particular operating mode.

Profiles enable rapid switching between parameter sets. Profile

parameters are programmed via the serial I/O port. Once pro-

grammed, a specific profile is activated by means of three

external pins (PROFILE<2:0>). A particular profile is activated

by providing the appropriate logic levels to the profile control

pins per the settings listed in Table 10.

Table 10. Profile Control Pins

PROFILE<2:0>

Active Profile

000

001

010

011

100

101

110

111

Consider an application of basic two-tone frequency shift

keying (FSK) where binary data is transmitted by selecting

between two different frequencies: a mark frequency (Logic 1)

and a space frequency (Logic 0). To accommodate FSK, the

Profile 0 register is programmed with the appropriate frequency

tuning word for a space, and the Profile 1 register is programmed

with the appropriate frequency tuning word for a mark. Then,

with the PROFILE1 and PROFILE2 pins tied to Logic 0, the

PROFILE0 pin is used to transmit the data bits. The logic state

of the PROFILE0 pin causes the appropriate mark and space

frequencies to be generated.

I/O_UPDATE PIN

By default, the I/O_UPDATE pin is an input that serves as a

strobe signal to allow synchronous update of the device operating

parameters. For example, frequency, phase, and amplitude con-

trol words for the DDS can be programmed using the serial I/O

port. However, the serial I/O port is an asynchronous interface;

consequently, programming of the device operating parameters

using the I/O port is not synchronized with the internal timing.

Using the pin, I/O_UPDATE, the user can synchronize the

application of certain programmed operating parameters with

external circuitry when new parameters are programmed into

the I/O registers. A rising edge on I/O_UPDATE initiates transfer

of the register contents to the internal workings of the device.

The transfer of programmed data from the programming

registers to the internal hardware is also accomplished by

changing the state of the profile pins.

AUTOMATIC I/O UPDATE

The AD9957 offers an option whereby the I/O update function

is asserted automatically rather than relying on an external

signal supplied by the user. This feature is enabled by setting the

Internal I/O Update Active bit in CFR2.

When this feature is active, the I/O_UPDATE pin becomes an

output pin. It generates an active high pulse each time an inter-

nal I/O update occurs. The duration of the pulse is approximately

12 cycles of SYSCLK. This I/O update strobe can be used to

notify an external controller that the device has generated an

I/O update internally.

The repetition rate of the internal I/O update is programmed

via the serial I/O port. Two parameters control the repetition

rate. The first parameter consists of the two I/O update rate

control bits in CFR2. The second parameter is the 32-bit word

in the I/O update rate register that sets the range of an internal

counter.

The I/O update rate control bits establish a divide by 1, 2, 4, or 8

of a clock signal that runs at ¼ fSYSCLK. The output of the divider

clocks the aforementioned 32-bit internal counter. The repetition

rate of the I/O update is given by

SYSCLK

UPDATE

where:

A is the value of the 2-bit word comprising the I/O update rate

control bits.

B is the value of the 32-bit word stored in the I/O update rate

If B is programmed to 0x0003 or less, the I/O_UPDATE pin no

longer pulses, but assumes a static Logic 1 state.

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