fs6377 AMI Semiconductor, Inc., fs6377 Datasheet - Page 3

• Manufacturer Part Number: fs6377
• Description: Fs6377-01g Programmable 3-pll Clock Generator Ic
• Manufacturer: AMI Semiconductor, Inc.

FS6377-01x Programmable 3-PLL Clock Generator IC
3.0 Functional Block Description
3.1 Phase Locked Loops (PLLs)
Each of the three on-chip PLLs is a standard phase- and frequency-locked loop architecture that multiplies a reference frequency to a
desired frequency by a ratio of integers. This frequency multiplication is exact.
As shown in Figure 3, each PLL consists of a reference divider, a phase-frequency detector (PFD), a charge pump, an internal loop
filter, a voltage-controlled oscillator (VCO), and a feedback divider.
During operation, the reference frequency (f
The divider value is called the "modulus," and is denoted as N
The PFD controls the frequency of the VCO (f
continuously variable frequency clock source for the PLL. The output of the VCO is fed back to the PFD through the feedback divider
(the modulus is denoted by N
The PFD will drive the VCO up or down in frequency until the divided reference frequency and the divided VCO frequency appearing at
the inputs of the PFD are equal. The input/output relationship between the reference frequency and the VCO frequency is:
3.1.1. Reference Divider
The reference divider is designed for low phase jitter. The divider accepts the output of the reference oscillator and provides a divided-
down frequency to the PFD. The reference divider is an 8-bit divider, and can be programmed for any modulus from 1 to 255 by
programming the equivalent binary value. A divide-by-256 can also be achieved by programming the eight bits to 00h.
3.1.2. Feedback Divider
The feedback divider is based on a dual-modulus prescaler technique. The technique allows the same granularity as a fully
programmable feedback divider, while still allowing the programmable portion to operate at low speed. A high-speed pre-divider (also
called a prescaler) is placed between the VCO and the programmable feedback divider because of the high speeds at which the VCO
can operate. The dual-modulus technique insures reliable operation at any speed that the VCO can achieve and reduces the overall
power consumption of the divider.
AMI Semiconductor
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Specifications subject to change without notice
– Oct., 2007 – Rev. 3.0
F
) to close the loop.
REF
VCO
), generated by the on-board crystal oscillator, is first reduced by the reference divider.
) through the charge pump and loop filter. The VCO provides a high speed, low noise,
Figure 3: PLL Diagram
R
for the reference divider. The divided reference is then fed into the PFD.
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Data Sheet