ICS9248-185 Datasheet, PDF (9/11 Pages) Integrated Circuit Systems – Frequency Generator & Integrated Buffers for PENTIUM/ProTM & K6

ICS9248 - 185
Shared Pin Operation -
Input/Output Pins
The I/O pins designated by (input/output) serve as dual
signal functions to the device. During initial power-up, they
act as input pins. The logic level (voltage) that is present on
these pins at this time is read and stored into a 5-bit internal
data latch. At the end of Power-On reset, (see AC
characteristics for timing values), the device changes the
mode of operations for these pins to an output function. In
this mode the pins produce the specified buffered clocks to
external loads.
To program (load) the internal configuration register for these
pins, a resistor is connected to either the VDD (logic 1) power
supply or the GND (logic 0) voltage potential. A 10 Kilohm
(10K) resistor is used to provide both the solid CMOS
programming voltage needed during the power-up
programming period and to provide an insignificant load on
the output clock during the subsequent operating period.
Figure 1 shows a means of implementing this function when
a switch or 2 pin header is used. With no jumper is installed
the pin will be pulled high. With the jumper in place the pin
will be pulled low. If programmability is not necessary, than
only a single resistor is necessary. The programming resistors
should be located close to the series termination resistor to
minimize the current loop area. It is more important to locate
the series termination resistor close to the driver than the
programming resistor.
Programming
Header
Via to Gnd
Via to
VDD
2K W
Device
Pad
8.2K W
Clock trace to load
Series Term. Res.
Fig. 1
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