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ICS2495M Datasheet(PDF) 2 Page - Integrated Circuit Systems |
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ICS2495M Datasheet(HTML) 2 Page - Integrated Circuit Systems |
2 / 7 page Figure 1 VCLK XTALOUT N/C MCLK MS1 EXTFREQ FS0 FS1 STROBE FS2 FS3 MS0 10 C3 C2 .1 22 ICS2495 5.0V NOTES: FS3-FS0, MS1-MS0, EXTFREQ, and STROBE inputs are all equipped with pull-ups and need not be tied high. Mount decoupling capacitors as close as possible to the device and connect device ground to the ground plane where available. Mount crystal and its circuit traces away from switching digital lines and the VCLK, MCLK and XTALOUT lines. 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. Power Supply Conditioning The ICS2495 is a member of the second generation of dot clock products. By incorporating the loop filter on chip and upgrad- ing the VCO, the ease of application has been substantially improved over earlier products. If a stable and noise-free power supply is available, no external components are required. How- ever, in most applications it is judicious to decouple the power supply as shown in Figure 1. 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 applica- tions 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 con- nected between XTAL1 (16) and XTAL2 (1). In IBM compat- ible 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 internally biased at V DD/ 2. Since TTL compatible clocks typically guarantee a V OH of only 2.8V, capacitively coupling the input restores noise immunity. The ICS2495 is not sensitive to the duty cycle of the bus clock; however, the quality of this signal varies considerably with different moth- erboard designs. As the quality of this signal is typically outside of the control of the graphics adapter card manufacturer, it is suggested that this signal be buffered on the graphics adapter board. XTAL2 (1) must be left open in this configuration. ICS2495 2 |
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