FS714x

4.1.1. Reference Divider

The reference divider is designed for low phase jitter. The divider accepts the output of either the crystal oscillator circuit or an external

reference frequency. The reference divider is a 12 bit divider, and can be programmed for any modulus from 1 to 4095 (divide by 1 not

available on date codes prior to 0108).

4.1.2. Feedback Divider

The feedback divider is based on a dual-modulus divider (also called dual-modulus prescaler) technique. It permits division by any

integer value between 12 and 16383. Simply program the FBKDIV register with the binary equivalent of the desired modulus. Selected

moduli below 12 are also permitted. Moduli of: 4, 5, 8, 9, and 10 are also allowed (4 and 5 are not available on date codes prior to

0108).

4.1.3. Post Divider

The post divider consists of three individually programmable dividers, as shown in Figure 3.

Figure 3: Post Divider

The post divider performs several useful functions. First, it allows the VCO to be operated in a narrower range of speeds compared to

the variety of output clock speeds that the device is required to generate. Second, the extra integer in the denominator permits more

flexibility in the programming of the loop for many applications where frequencies must be achieved exactly.

Note that a nominal 50/50 duty factor is always preserved (even for selections which have an odd modulus).

See Table 8 for additional information.

4.1.4. Crystal Oscillator

The FS7140 is equipped with a Pierce-type crystal oscillator. The crystal is operated in parallel resonant mode. Internal load

capacitance is provided for the crystal. While a recommended load capacitance for the crystal is specified, crystals for other standard

load capacitances may be used if great precision of the reference frequency (100ppm or less) is not required.

4.1.5. Reference Divider Source MUX

The source of frequency for the reference divider can be chosen to be the device crystal oscillator or the REF pin by the REFDSRC bit.

When not using the crystal oscillator, it is preferred to connect XIN to VSS. Do not connect to XOUT.

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