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Sample code of Interrupt based SPI Slave?

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Sample code of Interrupt based SPI Slave?
PostPosted: Sun Apr 30, 2006 1:39 am     Reply with quote

Hi everyone,
I am trying to setup a communication link between the two PICs using SPI. So far, the slave receives data from the master correctly for about 70% of the time.

The Slave is using SS signal, with interrrupt on SSPIF.

Does anyone have a sample code of a WORKIING Interrupt based SPI Slave?

Thank you and best regards,
PCM programmer

Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Mon May 01, 2006 12:54 am     Reply with quote

Does anyone have a sample code of a WORKING Interrupt based SPI Slave?

Here is a demo program. This program takes characters that
are typed in on the PC's keyboard and sends them out via
software SPI to the interrupt-driven SPI slave, where they are
stored in a buffer. The characters are then read from the
buffer and sent back to the PC, where they are displayed in
a terminal window.

In a real implementation, there would be two separate boards
and you would likely use hardware SPI in the Master board.
I had to use a software Master for this demo because it's
all done in one PIC, and the PIC only has one hardware SSP

// This program demonstrates an SPI slave with the PicDem2-Plus
// board. Both Master and Slave are contained in one 16F877
// chip. The Slave uses the hardware SPI pins on Port C and
// Port A.  The Master uses software SPI on Port B.
// See the following website for an explanation of SPI modes.

#include <16F877.H>
#device *=16
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)

// Software SPI pins used by Master.
#define MASTER_SS  PIN_B2  // Connect to Slave \SS (Pin A5)
#define MASTER_SCK PIN_B3  // Connect to Slave SCK (Pin C3)
#define MASTER_SDI PIN_B4  // Connect to Slave SDO (Pin C5)
#define MASTER_SDO PIN_B5  // Connect to Slave SDI (Pin C4)

// Use Charlie U's SPI mode definitions.
#define SPI_MODE_0_0 0x4000
#define SPI_MODE_0_1 0x0000
#define SPI_MODE_1_0 0x0010
#define SPI_MODE_1_1 0x4010

// Variables used for the SPI slave receive buffer.
#define BUFFER_SIZE  80
int8 buffer[BUFFER_SIZE];
int8 next_in = 0;
int8 next_out = 0;

#define spi_kbhit (next_in != next_out)

// Create an interrupt-driven SPI receive
// buffer, similar to the way it's done for
// RS-232 in the CCS example file, EX_SISR.C.

void ssp_isr(void)
buffer[next_in] = spi_read();


if(next_in >= BUFFER_SIZE)
   next_in = 0;


// Wait for a character to become available
// in the spi slave's receive buffer.  Then
// get it from the buffer and return it.

int8 spi_bgetc(void)
int8 c;


c = buffer[next_out];

if(next_out >= BUFFER_SIZE)
   next_out = 0;

// Transmit a byte via the software SPI port.
// Use SPI mode 0,0.

void master_spi_write(char data)
int8 i;



for(i = 0; i < 8; i++)
    if(bit_test(data, 7))       

    data <<= 1;     



void main()
char c;

// Initialize the hardware SSP for SPI Slave mode.
setup_spi(SPI_SLAVE | SPI_MODE_0_0);

// Initialize the software Master SPI pins.

// Enable interrupts for the SPI slave.

// If the user presses a key on the PC, get the
// character and send it out the software SPI
// master port.   Then when the character is
// received by the hardware SPI slave and placed
// in its buffer, get it and send it back to the PC.

   if(kbhit())  // Char available from PC ?
      c = getc();  // If so, get it
      master_spi_write(c);  // Send it via SPI

   if(spi_kbhit)  // Char available from SPI slave ?
      c = spi_bgetc();  // If so, get it
      putc(c);  // Send it to the PC


Joined: 20 Feb 2008
Posts: 27
Location: mexico

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PostPosted: Sat Sep 06, 2008 11:50 am     Reply with quote

Could you help us with a similar example for Master and one for Slave? different files

I am using the pic18f2520 and CCS 4.065

My idea is to have one Master which will send to Slave everything it gets from rs232 via SPI and a Slave which will send everything it receives from rs232 to Master via SPI.

I am having problems to make this a seamless communications bridge.

I am using 1 master and 8 slaves all 18f2520.

Have somebody use a similar protocol? this is not for a commercial use.
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