Using SPI with shift registers

74HC165 and similar

// 74HC165 connection via SPI

#include <SPI.h>

//  UNO   SPI   '165
//  9*    n/a   storage clock (ST_CP, /PL, DIP #1)
//  10*   NSS   shift clock inhibit (CLK INH, /CE, DIP #15)
//  11    MOSI  n/c
//  12    MISO  serial data output (Q7, DIP #9)
//  13    SCK   shift clock (SH_CP, CP, DIP #2)

// Note that the storage clock and the NSS can be anywhere. The UNO pin 10
// must be OUTPUT for the SPI master to work (the "NSS" functionality of the
// pin is applicable for SPI *in slave mode* when it is an input), but it
// need not be the actual select pin.

const uint8_t IM_STO = 9;
const uint8_t IM_NSS = 10;

const unsigned long IM_MAX_SPEED = 4000000; // A guess; could probably do way faster
const uint8_t IM_BIT_ORDER = MSBFIRST;
const uint8_t IM_SPI_MODE = SPI_MODE0;

void setup() {
  pinMode(IM_STO, OUTPUT);
  digitalWrite(IM_STO, true);
  pinMode(IM_NSS, OUTPUT);
  digitalWrite(IM_NSS, true);

  Serial.begin(115200);
  Serial.println("start");

  SPI.begin();
}

void sendStorePulse() {
  digitalWrite(IM_STO, false);
  delayMicroseconds(5);
  digitalWrite(IM_STO, true);
  delayMicroseconds(5);
}

uint8_t loadBySpi() {
  // Select device and begin transaction
  digitalWrite(IM_NSS, false);
  SPI.beginTransaction(SPISettings(IM_MAX_SPEED, IM_BIT_ORDER, IM_SPI_MODE));

  // Perform transfer
  uint8_t receivedData = SPI.transfer(0x00);

  // End transaction and deselect device
  SPI.endTransaction();
  digitalWrite(IM_NSS, true);

  return receivedData;
}

void loop() {
  sendStorePulse();
  uint8_t reading = loadBySpi();

  Serial.print("SPI: ");
  Serial.print(reading, BIN);
  Serial.println();

  delay(1000);
}