Expanding the number of outputs

AN2591

4/19

This example uses a 4-to-16 line decoder (M74HC4514 or M74HC4515) to expand the

number of outputs to 16. The number of lines saved is 16 - 5 = 11 lines.

Contrary to the solution described in Section 1.1.1, the microcontroller GPIOs connected to

the decoder address lines can also be used for other application purposes. This is made

possible by the decoder strobe input which stores the address of the decoder output. The

decoder output values remain unchanged while ST = 0, so in this state you can change the

GPIO values and you are free to use the lines (GPIO0.0 to GPIO0.3) for other application

purposes. In this case, the number of lines saved is 16 - 1 (for strobe line) = 15 lines.

Figure 2.

Expander for 16 outputs using a M74HC4514 or M74HC4515 decoder

Note:

M74HC4514 and M74HC4515 are similar and pin to pin compatible except that the latter

has inverted outputs.

1.2

Using a shift register

1.2.1

Expanding the number of outputs to 8 lines

This is another technique you can use to free up some microcontroller lines. This example

uses a 74HC595 SIPO (Serial In / Parallel Out) shift register that requires only 3

microcontroller GPIOs to generate one byte of data on its outputs (Q0 to Q7) (see

M74HC595 datasheet).

µC

GPIO0.0

M74HC4514 /

GPIO0.1

GPIO0.2

Y0

Y1

Y2

Y3

Y4

Y14

Y15

11

10

9

8

7

16

15

3.3v

GND

A

B

C

Vcc

INH

24

2

3

21

23

12

ST

D

GPIO0.3

M74HC4515

22

GPIO0.4

Strobe

Can also be used for

other application purposes

1

16 outputs