AN2591

Expanding the number of outputs

5/19

The shift register receives data serially from the microcontroller via the Sin input (pin 14)

using synchronous transmission. The M74HC595 includes two registers. The first one is a

shift register to shift in the data, the second one is a latch register to enable the data on the

outputs. This register prevents the output values being disturbed while data is being shifted

into the shift register.

You need to perform two steps to make the data available on the M74HC595 outputs:

1.

Shift the data from the microcontroller to the shift register via the Sin line:

–

Shift each bit of data via the microcontroller (GPIO0.0) and

–

Apply a clock pulse (via SCK line) to shift this bit into the register. You need 8 clock

times to load all bits of the data byte into the shift register. At this stage the data is

not yet available on the outputs (Q0 to Q7).

2.

To make it available on the outputs (Q0 to Q7), you must apply a latch pulse via the

RCK line to load the data into the second register.

The number of lines saved is 8 - 3 = 5. This is same amount of saving as with the

M74HC138 decoder but the difference that you can activate or deactivate any of the outputs

independently, unlike with the decoder when only one output can be activated at a time.

Figure 3.

Expander for 8 outputs using a shift register: M74HC595

1.2.2

Expanding the number of outputs to 16 lines

In this section uses the same technique as described in Section 1.2.1, but the number of

outputs is expanded to 16 lines. This is done by cascading two shift registers (the same IC

reference used in Section 1.2.1).

The M74HC595 SDO output is used to connect the two shift registers to allow the data flow

to pass from the first register into the second one. This creates a 16-bit shift register.

µC

GPIO0.0

M74HC595

GPIO0.1

GPIO0.2

Q0

Q1

Q2

Q3

Q4

Q5

Q6

Q7

15

2

1

3

4

5

6

7

3.3v

GND

Sin

RCK

SCK

Vcc

G

SCLR

16

10

14

12

11

13

8

SDO

9

8 outputs