Raspberry PI - Adding more inputs using the 74HC165 shift input chip

Connecting the Raspberry PI to a shift input board uses less GPIO pins and giving you eight inputs. To connect this you need to enable the SPI (Serial Peripheral Interface) chip interface. The chip reads eight parallel inputs and converts it to a serial data stream. It is possible to chain more than one chip to increase the amount of inputs or use the another chip for outputs (74HC595) 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.

74HC165 connections

Pin	Signal	Description
1	PL	Asynchronous parallel load input (active LOW)
2	CP	Clock input
3	D4	Parallel input 5
4	D5	Parallel input 6
5	D6	Parallel input 7
6	D7	Parallel input 8
7	Q7	Complementary output from the last stage
8	GND	Ground
9	Q7	Serial output from the last read
10	DS	Serial data input
11	D0	Parallel input 1
12	D1	Parallel input 2
13	D2	Parallel input 3
14	D3	Parallel input 4
15	CE	Clock enable input (active LOW)
16	VCC	Positive supply voltage

How the SPI interface data is read

When CE is high the chip will not output values. When PL is low inputs D0 to D7 are read, then when PL is high D0 to D7 and the serial value is sent to Q7 (Pin 9). Q7 (Pin 7) sends the inverse of Q7 (Pin 9). CP is a clock pulse required for the operations to work. DS is linked to the output of another chip and is used to send that chips data when the CE is high.

Wiring Connections

A tac switches are connected to D0 to D8 inputs with 10KΩ pull down resistor to keep the values to low when switch is not connected. Any input connections (D0 to D8) not being used must be connected to GND or the value could change randomly. The other connections are to the Raspberry PI. 
The CE is pulled high as there is only one chip in this system.

Pin	Signal	Description	GPIO	RPI Pin
1	PL	Parallel load inputs	CS1	26
2	CP	Clock input	SCLK	23
8	GND	Ground	GND	6
9	Q7	Serial output from the last read	MISO	21
15	CE	Clock enable	GND	6
16	VCC	Positive supply voltage	3.3 Volts	1

The SPI interface sequence and timing is taken care of buy the Raspberry PI SPI interface. The code required is as below. 
The 74HC165 does not really support the SPI interface properly because PL must be cycled to read the parallel inputs this is done by reading (or writing) to the other channel. Read channel 1 then 0 and the data will be read when on channel 0 but the chip has connected to channel 1. The chip works well when used with the 74HC595 chip in series.

import spidev
import time

spi = spidev.SpiDev()

def spiRead(channel):

    spi.open(0,channel)
    spiValue = spi.xfer2([0])
    time.sleep(0.5)
    spi.close()
    return spiValue

while True:
    resp = spiRead(1)
    resp = spiRead(0)
    print ("Input Responce = {}".format(resp))


The program load the SPI and time modules. The spiRead function opens the channel and read from address zero waits half a second then returns the value. The main loop reads from channel 1 then channel 0 to make the chip read the inputs, then value and printed to the screen.