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Introduction

Serial Peripheral Interface (SPI) is a bus interface connection protocol initiated by Motorola.

• The SPI interface uses four wires for communication. Therefore it is also called the four-wire serial communication protocol.
• SPI is a full-duplex master-slave communication protocol. This means that only one master station and one slave station can communicate on the interface bus at the same time.
• SPI-enabled devices work in two basic modes of SPI operation, namely SPI master mode and SPI slave mode.
• The master device is responsible for initiating communication. The master device generates the serial clock for synchronous data transfer. The master device can process multiple slave devices on the bus by selecting the bus one by one.

The ESP8266 based on NodeMCU has hardware SPI, with 4 pins that can be used for SPI communication. Through this SPI interface, we can connect any device that supports SPI to NodeMCU and communicate with it.

The SPI pins (SD1, CMD, SD0, CLK) of ESP8266 are specifically used for Quad-SPI communication with ESP-12E flash, so they cannot be used for SPI applications. We can use the hardware SPI interface for user-side applications.

The figure below shows the four SPI interface pins used internally for flash. It includes four I/O (4-bit data bus) with synchronous clock (SDIO_CLK) and chip select pin (SDIO_CMD), that is, four (SDIO_DATA0-SDIO_DATA3) bidirectional (I/P and O/P) data signals. It is mainly used to obtain more bandwidth/throughput than dual I/O (2-bit data bus) interfaces.

NodeMCU SPI pins

NodeMCU SPI pins

 

MISO（Master In Slave Out）

The master receives data, and the slave transmits data through this pin.

MOSI（Master Out Slave In）

The master sends data, and the slave receives data through this pin.

SCLK (serial clock)

The master generates this clock for the communication used by the slave.

Only the master can start the serial clock.

CS (Chip Select)

The host can select the slave through this pin and start communicating with it.

 

example

Let's write an Arduino sketch of SPI communication for NodeMCU. Here NodeMCU is the master device, and we use Arduino as the slave device.

In this example, we send the "Hello Slave" string as the end "\n" of the string from the NodeMCU master. The slave device receives the string and prints it on the serial monitor.

NodeMCU Arduino SPI interface diagram

 

Arduino program for NodeMCU main SPI

#include<SPI.h>

char buff[]="Hello Slave\n";

void setup() {
 Serial.begin(9600); /* begin serial with 9600 baud */
 SPI.begin();  /* begin SPI */
}

void loop() {
 for(inti=0; i<sizeof buff; i++)  /* transfer buff data per second */
  SPI.transfer(buff[i]);
 delay(1000);  
}

 

Arduino Uno slave SPI Arduino program

#include <SPI.h>

char buff [100];
volatile byte index;
volatile bool receivedone;  /* use reception complete flag */

void setup (void)
{
  Serial.begin (9600);
  SPCR |= bit(SPE);         /* Enable SPI */
  pinMode(MISO, OUTPUT);    /* Make MISO pin as OUTPUT */
  index = 0;
  receivedone = false;
  SPI.attachInterrupt();    /* Attach SPI interrupt */
}

void loop (void)
{
  if (receivedone)          /* Check and print received buffer if any */
  {
    buff[index] = 0;
    Serial.println(buff);
    index = 0;
    receivedone = false;
  }
}

// SPI interrupt routine
ISR (SPI_STC_vect)
{
  uint8_t oldsrg = SREG;
  cli();
  char c = SPDR;
  if (index <sizeof buff)
  {
    buff [index++] = c;
    if (c == '\n'){     /* Check for newline character as end of msg */
     receivedone = true;
    }
  }
  SREG = oldsrg;
}

 

Slave output window

The output is received on the slave device sent from the master device.

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