FS6377

3.2 Post Divider Muxes

As shown in Figure 2, an input mux in front of each post divider stage can select from any one of the PLL frequencies or the reference

frequency. The frequency selection is done via the I

The input frequency on two of the four muxes (mux C and D in Figure 2) can be changed without reprogramming by a logic-level input

on the SEL_CD pin.

3.3 Post Dividers

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, it changes the basic PLL equation to

oscillator frequencies. The extra integer in the denominator permits more flexibility in the programming of the loop for many applications

where frequencies must be achieved exactly.

The modulus on two of the four post dividers muxes (post dividers C and D in Figure 2) can be altered without reprogramming by a

logic level on the SEL_CD pin.

4.0 Device Operation

The FS6377 powers up with all internal registers cleared to zero, delivering the crystal frequency to all outputs. For operation to occur,

the registers must be loaded in a most significant-bit (MSB) to least-significant-bit (LSB) order. The register mapping of the FS6377 is

shown in Table 3, and I

Control of the reference, feedback and post dividers is detailed in Table 5. Selection of these dividers directly controls how fast the VCO

will run. The maximum VCO speed is noted in

Table 13.

4.1 SEL_CD Input

The SEL_CD pin provides a way to alter the operation of PLL C, muxes C and D and post dividers C and D without having to reprogram

the device. A logic-low on the SEL_CD pin selects the control bits with a "C1" or "D1" notation, per Table 3. A logic-high on the

SEL_CD pin selects the control bits with "C2" or "D2" notation, per Table 3.

Note that changing between two running frequencies using the SEL_CD pin may produce glitches in the output, especially if the post-

divider(s) is/are altered.

4.2 Power-Down and Output Enable

A logic-high on the PD pin powers down only those portions of the FS6377 which have their respective power-down control bits

enabled. Note that the PD pin has an internal pull-up.

When a post divider is powered down, the associated output driver is forced low. When all PLLs and post dividers are powered down

the crystal oscillator is also powered down. The XIN pin is forced low, and the XOUT pin is pulled high.

A logic-low on the OE pin tristates all output clocks. Note that this pin has an internal pull-up.

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