SIT3521 datasheet(21/44 Pages) SITIME

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Rev 0.90

Page 21 of 44

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SiT3521 1 to 340 MHz Elite™ I2C/SPI Programmable Oscillator

PRELIMINARY

8.2. DCO Functional Description

The DCO feature allows users to steer (pull) output

frequency by up to ±3200 ppm with 5 to 94 ppt resolution

through the I2C or SPI digital interface.

There are several advantages of DCO relative to analog

voltage control (VCXO)

Frequency Control Resolution as low as 5 ppt. This

high resolution minimizes accumulated time error in

synchronization applications.

Lower system cost – A VCXO may need a Digital to

Analog Converter (DAC) to drive the control voltage

input. In a DCO, the frequency control is achieved

digitally by register writes to the control registers via

I2C, thereby eliminating the need for a DAC.

Better Noise Immunity – The analog signal used to

drive the voltage control pin of a VCXO can be

sensitive to noise and the trace over which the signal

is routed can be susceptible to noise coupling from

the system. The DCO does not suffer from analog

noise coupling since the frequency control is

performed digitally through I2C.

No Frequency Pull non-linearity. The frequency

pulling is achieved via fractional feedback divider of

the PLL, eliminating any pull non-linearity concern

which is typical of quartz based VCXOs. This

improves dynamic performance in closed loop

operations.

Programmable Wide Pull Range – The DCO pulling

mechanism is via the fractional feedback divider and

is therefore not constrained by resonator pullability

as in quartz based solutions. The SiT3521 offers 16

frequency pull range options from ±6.25ppm to

±3200ppm, thereby giving system designers great

flexibility.

In the DCO mode, the device powers up at the nominal

operating frequency and pull range specified by the

ordering code. After power-up both the pull range and

output frequency can be controlled via I2C/SPI writes to

the respective control registers. The maximum output

frequency change is constrained by the pull range limits.

The pull range is specified by the value loaded in the

digital pull range control register. The 16 pull range

choices are specified in the control register and range

from ±6.25ppm to ±3200ppm.

Table 22 below shows the frequency resolution vs. pull

range programmed value.

Table 22. Frequency Resolution vs. Pull Range

Programmed Pull Range

Frequency Precision

±25ppm

5x10-12

±50ppm

5x10-12

±80ppm

5x10-12

±100ppm

5x10-12

±125ppm

5x10-12

±150ppm

5x10-12

±200ppm

5x10-12

±400ppm

1x10-11

±600ppm

1.4x10-11

±800ppm

2.1x10-11

±1200ppm

3.2x10-11

±1600ppm

4.7x10-11

±3200ppm

9.4x10-11

The ppm frequency offset is specified by the 26-bit DCO

Frequency control register in two’s complement format as

described in the I2C/SPI Register Descriptions. The power

up default value is

00000000000000000000000000b

which sets the output frequency at its nominal value

(0ppm). To change the output frequency, a frequency

control word is written to 0x00[15:0] (Least Significant

Word) and 0x01[9:0] (Most Significant Word). The LSW

value should be written first followed by the MSW value;

the frequency change is initiated after the MSW value is

written.

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