The Smart Timing Choice



The Smart Timing Choice



SiT3921

Digitally Controlled Differential Oscillator (DCXO)

Rev. 1.1

Page 4 of 12

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Figure 3. Mode 1 Frame Timing

Figure 4. Mode 2 Frame Timing

Frame Timing Parameters

Calculating Pull Range PPM offset

The frequency control value must be encoded as a 2's complement number (16-bit in mode 1 and 23-bit in mode 2), represent-

ing the full scale range of the device. For example, for a ±1600ppm device in mode 2, the 23-bit number represents the full

±1600ppm range.

The upper 16 bits of the value are written to address 0x06. If the high-resolution register (address 0x07) is used, the other 7 bits

are written to the lowest seven bits of address 0x07.

Here are the steps to calculate the frequency control value:

1. Find the scale factor (calculated for half of the pull range) from the tables below where PR is the Pull Range:

2. Enter the desired_PPM in equation below:

Frequency control (decimal value) = round (desired_PPM * K).

3. For any frequency shifts (positive or negative PPM), convert the frequency control value to a 2’s complement binary number.

Parameter

Symbol

Min.

Max.

Unit

Frame Length

40

—

Frame to Frame Delay

2—

Frequency Settling Time

—30

Frame to Frequency Delay

—8

K (scale)Factor

Mode

K = Scale Factor

1

(2^15-1) / (PR*1.00135625)

2

(2^22-1) / (PR*1.00135625)

C ontro l pin

O u tput

freque nc y

0x F A 0A

0 x 06

0xF A D A

0x06

C o ntro l pin

Ou tp u t

frequenc y

(L S W )

(M S W )

0x F A 0 A

0 x 0 7

0x F A D A

0x0 6