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ICS2495 Dataheets PDF



Part Number ICS2495
Manufacturers Integrated Circuit Systems
Logo Integrated Circuit Systems
Description Dual Video/Memory Clock Generator
Datasheet ICS2495 DatasheetICS2495 Datasheet (PDF)

Integrated Circuit Systems, Inc. ICS2495 Dual Video/Memory Clock Generator Features • • • • • • • Low cost - eliminates need for multiple crystal clock oscillators in video display subsystems Mask-programmable frequencies Pre-programmed versions for Industry Standard VGA chips Glitch-free frequency transitions Internal clock remains locked when the external frequency input is selected Low power CMOS device technology Small footprint - 16-pin DIP or SOIC • • • • • • • Buffered Xtal Out Integral.

  ICS2495   ICS2495


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Integrated Circuit Systems, Inc. ICS2495 Dual Video/Memory Clock Generator Features • • • • • • • Low cost - eliminates need for multiple crystal clock oscillators in video display subsystems Mask-programmable frequencies Pre-programmed versions for Industry Standard VGA chips Glitch-free frequency transitions Internal clock remains locked when the external frequency input is selected Low power CMOS device technology Small footprint - 16-pin DIP or SOIC • • • • • • • Buffered Xtal Out Integral Loop Filter components Fast acquisition of selected frequencies, strobed or nonstrobed Guaranteed performance up to 135 MHz Excellent power supply rejection Advanced PLL for low phase-jitter Frequency change detection circuitry enhances new frequency acquisition and eliminates problems caused by programs that rewrite frequency information Description The ICS2495 Clock Generator is an integrated circuit dual phase-locked loop frequency synthesizer capable of generating 16 video frequencies and 4 memory clock frequencies for use with high performance video display systems. Utilizing CMOS technology to implement all linear, digital and memory functions, the ICS2495 provides a low-power, small-footprint, low-cost solution to the generation of video dot clocks. Outputs are compatible with XGA, VGA, EGA, MCGA, CGA, MDA, as well as the higher frequencies needed for advanced applications in desktop publishing and workstation graphics. Provision is made via a single level custom mask to implement customer specific frequency sets. Phase-locked loop circuitry permits rapid glitch-free transitions between clock frequencies. In addition to providing 16 clock rates, the ICS2495 has provisions to multiplex an externally-generated signal source into the VCLK signal path. Internal phase-locked frequencies continue to remain locked at their preset values when this mode is selected. This feature permits instantaneous transition from an external frequency to an internally-generated frequency. Printed circuit board testing is simplified by the use of these modes as an external clock generated by the ATE tester can be fed through, permitting synchronous testing of the entire system. Pin Configuration XTAL2 EXTFREQ FS0 FS1 STROBE FS2 FS3 MS0 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 XTAL1 VCLK XTALOUT VSS VDD N/C MCLK MS1 16-Pin DIP or SOIC Notes: 1. ICS2495M(SOIC) pinout is identical to ICS2495N(DIP). ICS2595RevA090694 ICS2495 Reference Oscillator & Crystal Selection In cases where the on-chip crystal oscillator is used to generate the reference frequency, the accuracy of the crystal oscillation frequency will have a very small effect on output accuracy. The external crystal and the on-chip circuit implement a Pierce oscillator. In a Pierce oscillator, the crystal is operated in its parallel-resonant (also called anti-resonant mode). This means that its actual frequency of oscillation depends on the effective capacitance that appears across the terminals of the quartz crystal. Use of a crystal that is characterized for use in a series-resonant circuit is fine, although the actual oscillation frequency will be slightly higher than the value stamped on the crystal can (typically 0.025%-0.05% or so). Normally, this error is not significant in video graphics applications, which is why the ICS2495 will typically derive its frequency reference from a series resonant crystal connected between pins 1 and 16. As the entire operation of the phase-locked loop depends on having a stable reference frequency, the crystal should be mounted as close as possible to the package. Avoid routing digital signals or the ICS2495 outputs underneath or near these traces. It is also desirable to ground the crystal can to the ground plane, if possible. Layout Considerations Utilizing the ICS2495 in video graphics adapter cards or on PS2 motherboards is simple, but does require precautions in board layout if satisfactory jitter-free performance is to be realized. Care should be exercised to ensure that components not related to the ICS2495 do not share its ground. In applications utilizing a multi-layer board, V SS should be directly connected to the ground plane. Frequency Reference The internal reference oscillator contains all of the passive components required. An appropriate crystal should be connected between XTAL1 (16) and XTAL2 (1). In IBM compatible applications this will typically be a 14.31818 MHz crystal, but fundamental mode crystals between 10 MHz and 25 MHz have been tested. Maintain short lead lengths between the crystal and the ICS2495. In some applications, it may be desirable to utilize the bus clock. If the signal amplitude is equal to or greater than 3.5 volts, it may be connected directly to XTAL1 (16). If the signal amplitude is less than 3.5 volts, connect the clock through a .047 microfarad capacitor to XTAL1 (16), and keep the lead length of the capacitor to XTAL1 (16) to a minimum to reduce noise susceptibility . This input is int.


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