SN74CBT3251

SCDS019M – MAY 1995 – REVISED DECEMBER 2015

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9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

The SN74CBT351 device can be used for a wide variety of applications, including expanding MCU GPIOs.

9.2 Typical Application

One application of the SN74CBT3251 device is to use in conjunction with a microcontroller to poll a keypad.

Figure 4 shows the basic schematic for such a polling system. The microcontroller uses the channel-select pins

to cycle through the different channels while reading the input to see if a user is pressing any of the keys. This is

a very robust setup that allows for simultaneous key presses with very little power consumption. It also uses very

few pins on the microcontroller. The down side of polling is that the microcontroller must frequently scan the keys

for a press.

Figure 4. Keypad Polling Application

9.2.1 Design Requirements

These devices use CMOS technology and have balanced output drive. Take care to avoid bus contention

because it can drive currents that would exceed maximum limits.

9.2.2 Detailed Design Procedure

1. Recommended input conditions:

–

For switch time specifications, see propagation delay times in Recommended Operating Conditions.

–

Inputs must not be pulled below ground.

–

Conditions.

8

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