Buffered XTALOUT

In motherboard applications it may be desirable to have the

ICS2495 provide the bus clock for the rest of the system. This

eliminates the need for an additional 14.31818 MHz crystal

oscillator in the system, saving money as well as board space.

Depending on the load, it may be judicious to buffer XTA-

LOUT when using it to provide the system clock.

Output Circuit Considerations

As the dot clock is usually the highest frequency present in a

video graphics system, consideration should be given to EMI.

To minimize problems with meeting FCC EMI requirements,

the trace which connects VCLK or MCLK and other compo-

nents in the system should be kept as short as possible. The

ICS2495 outputs have been designed to minimize overshoot.

In addition, it may be helpful to place a ferrite bead in these

signal paths to limit the propagation of high-order harmonics

of this signal. A suitable device would be a Ferroxcube 56-590-

65/4B or equivalent. This device should be placed physically

close to the ICS2495. A 33 to 47 Ohm series resistor, some-

times called source termination, in this path may be necessary

to reduce ringing and reflection of the signal and may thereby

reduce phase jitter as well as EMI.

External Frequency Sources

EXTFREQ on versions so equipped by the programming, is

an input to a digital multiplexer. When this input is enabled by

the FS0-3 selection, the signal driving pin 2 will appear at

VCLK (15) instead of the PLL output. Internally, the PLL will

remain in lock at the frequency selected by the ROM code.

The programming option also exists to output the crystal oscil-

lator output on VCLK. In the case where XTAL1 is being

driven by an external oscillator, then this frequency would

appear on VCLK if so programmed.

Digital Inputs

FS0 (3), FS1 (4), FS2 (6), and FS3 (7), are the TTL compatible

frequency select inputs for the binary code corresponding to

the frequency desired. STROBE (5) when high, allows new

data into the frequency select latches; and when low, prevents

address changes per Figure 2. The internal power-on-clear

signal will force an initial frequency code corresponding to an

all zeros input state. MS0 (8) and MS1 (9) are the correspond-

ing memory select inputs and are not strobed.

ICS2495

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