In this tutorial we’ll continue with the serial communication between 2 microcontrollers. You should always configure one PIC as master while all the others are to be configured as slaves. For more information check this SPI explanation.

SPI Master-Slave Configuration

Check the datasheet to see which pins are SDO (output), SDI (input) and SCK (SPI Clock). Once you have connected the PICs there are a couple of registers that need to be set.

This is for the master:

MOVLW	0x00	; Datasample middle, SCK act-idle.
MOVWF	SSPSTAT	; 
MOVLW	0x32	; SSP enabled, Idle at high, Fosc/64
MOVWF	SSPCON1

As you can see we use the internal timing components for the SPI clock, however you can opt to use TMR2 as well. Don’t forget to enable TMR2 and set its T2CON register if you do this.

This is for the slave:

MOVLW	0x00	; Datasample middle, SCK act-idle.
MOVWF	SSPSTAT	; 
MOVLW	0x35	; SSP enabled, Idle at high, SPI SLAVEMODE
MOVWF	SSPCON1

Make sure you don’t make a mistake here, if you have your CLK idle at high, see to it that the transition is configured correctly else you’ll miss the first pulse. For example, when the CLK is idle at high, its first pulse will be from high to low. When you configure the slave that it should do something when the CLK goes from low to high you will have missed that first pulse resulting in missed or incorrect data.

Now you have configured both master and slave you can send any data you wish. Let’s take a look on how to transmit data from master to slave:

Main
   goto	Main                    ; Infinite loop
 
SPIsend
   bcf		PIR1,ADIF	; Clear AD converter flag
   MOVFF	ADRESH,SSPBUF	; move ADRESH to send buffer
   RETURN
 
SPIread
   bcf		PIR1,SSPIF      ; Clear the flag
   MOVFF	SSPBUF,PORTB    ; Move received data to PORTB
   RETURN
 
inter				; using single interrupt priority
   BTFSC	PIR1,SSPIF	; if set-> transmission finished
   CALL	SPIread
   BTFSC	PIR1,ADIF	; wait until AD finished -> send
   CALL	SPIsend
   RETFIE
 
  END

In this example I’m sending values sampled by the AD converter to the slave. The code is pretty self explanatory. The ADC will generate an interrupt when it’s finished with converting and the SPI will generate one when it’s done transmitting the data that was in SSPBUF, which is the send buffer.

The slave has even simpler code:

main
   goto main
 
SPIread
   bcf 	  PIR1,SSPIF
   movff  SSPBUF,PORTA
   return
 
inter	
   BTFSC  PIR1,SSPIF
   CALL   SPIread			
   RETFIE
 
   END

The slave will generate an interrupt when the data byte is received, you just have to move the data in the buffer to another register such as PORTA if you’d like to display the data with LEDs for example.