SPI communication between PICs

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Contents

Overview

SPI or Serial Peripheral Interface is a communication method that was once used to connect devices such as printers, cameras, scanners, etc. to a desktop computer. This function has largely been taken over by USB, however SPI can still be a useful communication tool for some applications. SPI runs using a master/slave set-up and can run in full duplex mode, meaning that signals can be transmitted between the master and the slave simultaneously.

SPI is still used to control some peripheral devices and has some advantages over IC2 (another type of serial data communication). SPI can send much higher rates of data than I2C and requires no addressing to separate slave devices. Compared to parallel buses, SPI has the benefit of requiring only simple wiring.

Below are some peripheral devices that still use SPI.

• Converters (ADC and DAC)

• Memories (EEPROM and FLASH)

• Real Time Clocks (RTC)

• Sensors (temperature, pressure, etc.)

• Others (signal mixer, potentiometer, LCD controller, UART, CAN controller, USB controller, amplifier)


Basic Operation<b> SPI requires four lines, and is therefore often termed the “four wire” serial bus. These four lines are described in the table below.

Spi-diagram.png
Line Name Description
SCLK Serial Clock Output from master
MOSI/SIMO Master Output, Slave Input Output from master
MISO/SOMI Master Input, Slave Output Output from slave
SS Slave Select Output from master (active low)


The master, as its name suggests, controls all communication. By controlling the clock, the master decides when data is sent and received.

Using the “Slave Select” line, the master also chooses which slave with which to communicate. Note that multiple slaves may be selected, simply by applying a logic low bias to the desired SS lines, as illustrated in the diagram to the right. If a slave is not selected (SS is high) it disregards signals sent by the master. Although possible, multi-master systems are rarely implemented.

Within each clock cycle a full duplex communication is carried out. Each bit is shuffled into the slave line and a bit is shuffled from the slave line back to the master line. The master first sends logic low to select the slave. There is no general protocol for the transmission. The slave can then either just receive or receive and reply to the Master. The Master can also either just send data or send and receive data.


<b>References<b>

SPI Background.www.totalphase.com

Serial Peripheral Interface. Www.wikipedia.org

mct.net

Circuit

Spi-circuit.jpg

see www.totalphase.com

Above shows the Master connected to three slaves. Each slave must be enabled through the slave select pin in order to communicate with the Master.

Spi.jpg

The two PICs can be wired directly by connecting these input and output pins of the diagram above.

Code (Code mentioned on this page does NOT work when tried with the PIC18F4520)

/*MASTER CODE*/
/* This code is valid only for devices that support hardware spi such as the PIC18F4520. For all other devices use
software spi. This code does not work on PIC18F4520 and will require more experimentation. This is the code that
looks most promising to start with from our review of codes on the internet for hardware SPI*/

/* For hardware spi, library functions such as spi_setup(), spi_read() and spi_write() can be used */
/* For software spi, spi_xfer() can be used alongwith the #use spi directive*/
/* We are not entirely sure if an output pin of the master should be connected to the SS pin (A5 for PIC18f4520) 
of the slave and so we have commented out code that should be included if an output pin (PIN_D0 in this case) of 
the master is connected to the SS pin of the slave/ 

#include <18f4520.h>
#fuses HS,XT,NOLVP,NOWDT,NOPROTECT
#use delay(clock=20000000)

void main(){

int value; 	//for 8 bit transfer which is default
int value_read; // to be read from spi

setup_spi(SPI_MASTER|SPI_H_TO_L|SPI_CLK_DIV_16); // sets the PIC as a master which generates the clock signal, a slower clock can be  generated by changing 16 to 64

 while (TRUE) {
        
      value = 4;     // value to be sent to spi

    //output_low(PIN_D0); //Turns on slave if slave select is used           
    
      spi_write(value);
    
      value_read = spi_read();
     
      delay_us(100);   
  
   // output_high(PIN_D0); //Turns off slave if slave select is used   

   }

}

/*SLAVE CODE*/

/* This code is valid only for devices that support hardware spi such as the PIC18F4520. For all other devices use
software spi. This code does not work on PIC18F4520 and will require more experimentation. This is the code that
looks most promising to start with from our review of codes on the internet for hardware SPI*/

/* For hardware spi, library functions such as spi_setup(), spi_read() and spi_write() can be used */
/* For software spi, spi_xfer() can be used alongwith the #use spi directive*/
/* We are not entirely sure if an output pin of the master should be connected to the SS pin (A5 for PIC18f4520) 
of the slave */
/* In most cases, simply grounding the SS pin of the slave should work */
/* slave code is really hard to find on the internet */
 

#include <18f4520.h>
#fuses HS,XT,NOLVP,NOWDT,NOPROTECT
#use delay(clock=20000000)

void main(){

 	
int value_read; // to be read from spi

setup_spi(SPI_SLAVE|SPI_H_TO_L); // sets the PIC as a slave which recieves a clock signal and data is transferred during H to L

 while (TRUE) {
        
          
      value_read = spi_read(); // according to the reference manual this should work but also see the code below

     // if(spi_data_is_in())   // Checks to see if data is ready to be read
	//	value_read = spi_read(); //reads the data from spi
     
      delay_us(100);   

      output_d(value_read);  //display the number that has been read
      spi_write(0);        // This is probably unnecessary
  
   }

}


/* WATCH OUT FOR UPDATES */

Most of the sample code was obtained from the following sources

The CCS user manual

CCS forum

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