AN2591

Expanding the number of inputs

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2.2

Using shift registers

2.2.1

Expanding the number of inputs to 8 lines

A PISO (Parallel In Serial Out) shift register can also be used to increase the number of

inputs. Figure 7 shows how to connect a shift register M74HC165 to a microcontroller using

only 3 GPIOs.

The shift register sends data serially to the microcontroller via its QH output (pin 9) using

synchronous communication.

First, you load the 8 bit data available on the inputs (A to H) into the register by applying a

load pulse (set it low and then high again), the 8 bits of data are loaded in parallel into the

register. The logic level that was present at the H input appears immediately on the QH

output when the latch pulse is applied. Then, on each subsequent rising edge of the clock

(pin 2), the logic level of the next bit appears on the QH output and can be read via the

GPIO0.2 input and shifted by software to build a byte of data. 8 bits of data take 7 clock

pulses (generated by the microcontroller) to shift out one byte and not 8 clocks because

initially the higher bit was present on QH.

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

Figure 7.

Expander for 8 inputs using a shift register: M74HC165

µC

GPIO0.0

M74HC165

GPIO0.1

GPIO0.2

A

B

C

D

E

F

G

H

11

13

12

14

3

4

5

6

3.3v

GND

S/L

CK

QH

Vcc

SI

16

1

2

9

10

8

8 inputs

CK INH

15

QH