AN2591

Expanding the number of outputs

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1

Expanding the number of outputs

This section describes how to increase the number of output lines using two techniques: by

using a decoder or by using shift registers.

1.1

Using a decoder

1.1.1

Expanding the number of outputs to 8 lines

In certain applications, you may need to use several microcontroller outputs but only one of

these outputs is activated at any given time. For example when you control a multiplexed

display (7-segment or LCD display), only one digit at a given time is selected. This is also

the case with matrix keypad management. In order to generate row signals to poll a keypad

to determine which key is pressed, you have to use several microcontroller outputs.

Figure 1 shows how to reduce the number of outputs used in this type of application.

This solution uses a 3-to-8 line decoder (M74HC138) to determine which one of the outputs

goes low (Y0 to Y7) by applying an address value on the three inputs A, B and C. The

number of lines saved is 8 - 3 = 5 lines.

Figure 1.

Expander for 8 outputs using a M74HC138 decoder

1.1.2

Expanding the number of outputs to 16 lines

Sometimes, more than 8 outputs are used although only one output needs to be active at a

time (for example for a multiplexed display with more than 8 digits, a matrix keypad with

more than 7 rows or a row of 16 LEDs for a bargraph level display, and so on).

µC

GPIO0.0

M74HC138

GPIO0.1

GPIO0.2

Y0

Y1

Y2

Y3

Y4

Y5

Y6

Y7

15

13

14

12

11

10

6

8

3.3v

GND

A

B

C

Vcc

G2B

16

1

2

3

5

8

G2A

G1

4

9