13

ICS94241

0453C—10/26/04

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.