14

ICS94209

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Fig. 1

Shared Pin Operation -

Input/Output Pins

The I/O pins designated by (input/output) on the ICS94209

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.

Via to

VDD

Clock trace to load

Series Term. Res.

Programming

Header

Via to Gnd

Device

Pad

2K

W

8.2K

W

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.