Cypress FS784 Clock User Manual

Low EMI Spectrum Spread Clock
Cypress Semiconductor Corporation 198 Champion Court San Jose, CA 95134-1709 408-943-2600
Document #: 38-07029 Rev. *F Revised January 2, 2005
Spread Spectrum clock generator (SSCG) with 1×, 2×,
and 4× outputs
6- to 82-MHz operating frequency range
Modulates external clocks including crystals, crystal
oscillators, or ceramic resonators
Programmable modulation with simple R-C external
loop filter (LF)
Center spread modulation
3V-5V power supply
TTL-/CMOS-compatible outputs
Low short-term jitter
Low-power Dissipation
3.3 VDC = 37 mW – typical
5.0 VDC = 115 mW – typical
Available in 8-pin SOIC and TSSOP packages
Desktop/notebook computers
Printers, copiers, and MFP
Scanners and fax
LCD displays and monitors
Automotive and embedded systems
Networking, LAN/WAN
Digital cameras and camcorders
Programmable EMI reduction
Fast time to market
Lower cost of compliance
No degradation in rise/fall times
Lower component and PCB layer count
Functional Description
The Cypress FS781/82/84 are Spread Spectrum clock
generator ICs (SSCG) designed for the purpose of reducing
electromagnetic interference (EMI) found in today’s
high-speed digital systems.
The FS781/82/84 SSCG clocks use a Cypress-proprietary
technology to modulate the input clock frequency, XIN, by
modulating the frequency of the digital clock. By modulating
the reference clock the measured EMI at the fundamental and
harmonic frequencies of FSOUT is greatly reduced. This
reduction in radiated energy can significantly reduce the cost
of complying with regulatory requirements without degrading
digital waveforms.
The Cypress FS781/82/84 clocks are very simple and
versatile devices to use. By programming the two range select
lines, S0 and S1, any frequency from 6- to 82-MHz operating
range can be selected. The FS781/2/4 are designed to
operate over a very wide range of input frequencies and
provides 1×, 2×, and 4× modulated clock outputs.
The FS78x devices have a simple frequency selection table
that allows operation from 6 MHz to 82 MHz in four separate
ranges. The bandwidth of the frequency spread at FSOUT is
determined by the values of the loop filter components. The
modulation rate is determined internally by the input frequency
and the selected input frequency range.
The Bandwidth of these products can be programmed from as
little as 1.0% up to as much as 4.0% by selecting the proper
loop filter value. Refer to the Loop Filter Selection chart in
Table 2 and Table 3 for the recommended values. Due to a
wide range of application requirements, an external loop filter
(LF) is used on the FS78x products. The user can select the
exact amount of frequency modulation suitable for the appli-
cation. Using a fixed internal loop filter would severely limit
the use of a wide range of modulation bandwidths (Spread %)
to a few discrete values. Refer to FS791/2/4 products for appli-
cations requiring 80- to 140-MHz frequency range.
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