AN2591

Expanding the number of inputs

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2

Expanding the number of inputs

To expand the number input lines, for example to get the state of several bi-state sensors or

to scan switch states, two techniques can be used: using multiplexers or shift registers.

2.1

Using a multiplexer

2.1.1

Expanding the number of inputs to 8 lines

Figure 5 shows an example of how a multiplexer can be connected to the microcontroller. To

get a bit value from a multiplexer input, apply the address value of the input via address lines

(A, B and C) and read it in via the multiplexer OUT output (pin 5). To get all 8 bits of data

available on the inputs (D0 to D7), you must read all bit states sequentially from the MSB

(address value = 0x7) to LSB (address value = 0x0) by decrementing the bit address and

applying it on GPIO0.0 to GPIO0.2. For each bit address you read the bit value and shift it

into the microcontroller on GPIO0.3 by software to build a data byte. The multiplexer used is

the M74HC151. This allows you to save 8 - 4 = 4 lines.

Figure 5.

Expander for 8 inputs using a multiplexer: M74HC151

µC

GPIO0.3

Out

GPIO0.0

M74HC151

5

GPIO0.1

GPIO0.2

D0

D1

D2

D3

D4

D5

D6

D7

4

2

3

1

15

14

13

12

Vcc

16

ST

7

3.3V

GND

C

B

A

8 inputs

11

10

9

8