Raspberry PI - Adding more outputs using the 74HC595 shift output chip

Connecting the Raspberry PI to a shift output board uses less GPIO pins and giving you eight digital outputs. To connect this you need to enable the SPI (Serial Peripheral Interface) chip interface. The chip commands eight parallel outputs from a serial data stream. It is possible to chain more than one chip to increase the amount of outputs or use the another chip for inputs (74HC165) or analogue inputs (MCP3008).

Install and enable the SPI interface

1. First make sure the system is up to date
   sudo apt-get update
   sudo apt-get upgrade
   sudo reboot
2. To install the development software that includes spi_dev type the following command. Note:Raspberry PI needs internet connection for this to work.
   sudo apt-get install python-dev python3-dev
   The command will indicate the package size before installing. Then install the spidev for Pyhon with the below commands.
   cd ~
   git clone https://github.com/doceme/py-spidev.git
   cd py-spidev
   make
   sudo make install
3. 
   
4. Enable the SPI interface 
5. sudo raspi-config select Advanced the enable SPI and set to load by default.

6. Reboot the Raspberry PI
7. sudo reboot
8. To check that the interface is enabled type lsmod and check that spi_bcm2835 is listed.

74HC595 connections

Pin	Signal	Description
1	Q1	Parallel data output 1
2	Q2	Parallel data output 2
3	Q3	Parallel data output 3
4	Q4	Parallel data output 4
5	Q5	Parallel data output 5
6	Q6	Parallel data output 6
7	Q7	Parallel data output 7
8	GND	Ground
9	Q7S	Serial data output
10	MR	Master reset
11	SHCP	Shift clock input
12	STCP	Storage clock input
13	OE	Output enable
14	DS	Data serial input
15	Q0	Parallel data output 0
16	VCC	Positive supply voltage

How the SPI interface works

SHCP is the constant clock used for processes. MR will reset all shift registers to low (This can be held High to be disabled). Serial data is sent to DS. When OE is enabled (This can be fixed low) and STCP is high shift registers are transferred to outputs. STCP storage clock can be connected to the Raspberry PI chip select of the SPI interface.

Wiring Connections

Diodes are connected to Q0 to Q8 with 500Ω resistor to limit the voltage. The other connections are to the Raspberry PI.

Pin	Signal	Description	GPIO	RPI Pin
8	GND	Ground	GND	6
10	MR	Master reset	3.3 Volts	1
11	SHCP	Shift clock	SCLK	23
12	STCP	Storage clock	CS1	26
13	OE	Output enable	GND	6
14	DS	Serial data input	MOSI	19
16	VCC	Positive supply voltage	3.3 Volts	1

The SPI interface sequence and timming is taken care of buy the Raspberry PI SPI interface. The code required is as below.

import spidev
import time

spi = spidev.SpiDev()
A=0

def spiWrite(channel,OutValue):

    spi.open(0,channel) # Port 0 , Chip Select 1
    spiValue = spi.xfer2([1,OutValue])
    time.sleep(2)
    spi.close()
    return spiValue

while True:
    print ("Output Value = {}".format(A))
    resp = spiWrite(1,A)
    A=A+1
    if (A>254):

        A=0

This program will write values of 0 to 255 to chip. The output LED's will show the binary values of the integer. So for example when A =11 lights 1,2 and 4 will be on 1+2+8 as below.
Output No      1  2  3  4  5  6  7  8
Decimal value  1  2  4  8  16 32 64 128