SPITransfer tx, rx

Command Availability:

Available on Microchip PIC microcontrollers with Synchronous Serial Port (SSP) module.


This command simultaneously sends and receives a byte of data using the SPI protocol. It behaves differently depending on whether the microcontroller has been set to act as a master or a slave. When operating as a master, SPITransfer will initiate a transfer. The data in tx will be sent to the slave, whilst the byte that is buffered in the slave will be read into rx. In slave mode, the SPITransfer command will pause the program until a transfer is initiated by the master. At this point, it will send the data in tx whilst reading the transmission from the master into the rx variable.


There are two example programs for this command - one to run on the slave microcontroller , and one on the master. A reading is taken from a sensor on the slave, and sent across to the master which shows the data on its LCD screen.

Slave Program:

	'Select chip model and configuration
	#chip 16F88, 20
	#config MCLR_OFF

	'Set directions of SPI pins
	dir PORTB.2 out
	dir PORTB.1 in
	dir PORTB.4 in
	'Set direction of analogue pin
	dir PORTA.0 in

	'Set SPI mode to slave
	SPIMode Slave

	'Allow other microcontroller to initialise LCD
	Wait 1 sec

	'Main loop - takes a reading, and then waits to send it across.
	'Note that rx is 0 - this is because no data is to be received.
	SPITransfer ReadAD(AN0), 0

Master Program:

	'General hardware configuration
	#chip 16F877A, 20

	'LCD connection settings
	#define LCD_IO 8
	#define LCD_RS PORTD.0
	#define LCD_RW PORTD.1
	#define LCD_Enable PORTD.2

	'Set SPI pin directions
	dir PORTC.5 out
	dir PORTC.4 in
	dir PORTC.3 out

	'Set SPI Mode to master, with fast clock
	SPIMode MasterFast

	'Main Loop
	'Read a byte from the slave
	'No data to send, so tx is 0
	SPITransfer 0, Temp

	'Display data
	if Temp > 0 then
	  Print "Light: "
	  LCDInt Temp
	  Temp = 0
	end if

	'Wait to allow time for the LCD to show the given value
	wait 100 ms