SPI (Serial Peripheral Interface)

 What is SPI ?

SPI (Serial Peripheral Interface) is a synchronous, full-duplex communication protocol used to transfer data between a master (e.g., microcontroller) and one or more slaves (e.g., sensors, memory chips).

SPI uses 4 main lines:

Line Full Name                 Direction Purpose
MOSI      Master Out Slave In          Master → Slave          Master sends data to slave
MISO      Master In Slave Out         Slave  → Master           Slave sends data to master
SCLK      Serial Clock         Master → Slave          Clock signal generated by master
SS      Slave Select         Master → Slave          Selects which slave to talk to


SPI Communication Characteristics :
  • Synchronous – Clock signal is shared.
  • Full Duplex – Transmit and receive at the same time.
  • Master/Slave Architecture
  • Faster than UART or I2C, but only supports short-distance, board-level communication.

 Embedded C (for Arduino-like systems)

#include <SPI.h>

void setup() {
  Serial.begin(9600);
  SPI.begin(); // Initialize SPI
  digitalWrite(SS, HIGH); // Deselect slave
}

void loop() {
  digitalWrite(SS, LOW); // Select slave
  byte response = SPI.transfer(0xA5); // Send 0xA5, receive data
  digitalWrite(SS, HIGH); // Deselect slave

  Serial.print("Received: ");
  Serial.println(response, HEX);

  delay(1000);
}

Python (with spidev on Raspberry Pi)

import spidev
import time

spi = spidev.SpiDev()
spi.open(0, 0)  # Open bus 0, device 0
spi.max_speed_hz = 50000

while True:
    response = spi.xfer([0xA5])  # Send one byte, receive one byte
    print("Received:", response[0])
    time.sleep(1)





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