14

Integrated

Circuit

Systems, Inc.

ICS952621

0756A—09/10/04

Fig. 1

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.

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.