Datasheet

141
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
The following code examples show how to initialize the SPI as a Slave and how to perform a simple reception.
Note: 1. The example code assumes that the part specific header file is included.
16.1 SS Pin Functionality
16.1.1 Slave Mode
When the SPI is configured as a slave, the slave select (SS) pin is always input. When SS is held low, the SPI is activated,
and MISO becomes an output if configured so by the user. All other pins are inputs. When SS
is driven high, all pins are
inputs, and the SPI is passive, which means that it will not receive incoming data. Note that the SPI logic will be reset once
the SS
pin is driven high.
The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous with the master clock
generator. When the SS
pin is driven high, the SPI slave will immediately reset the send and receive logic, and drop any
partially received data in the shift register.
Assembly Code Example
(1)
SPI_SlaveInit:
; Set MISO output, all others input
ldi r17,(1<<DD_MISO)
out DDR_SPI,r17
; Enable SPI
ldi r17,(1<<SPE)
out SPCR,r17
ret
SPI_SlaveReceive:
; Wait for reception complete
sbis SPSR,SPIF
rjmp SPI_SlaveReceive
; Read received data and return
in r16,SPDR
ret
C Code Example
(1)
void SPI_SlaveInit(void)
{
/* Set MISO output, all others input */
DDR_SPI = (1<<DD_MISO);
/* Enable SPI */
SPCR = (1<<SPE);
}
char SPI_SlaveReceive(void)
{
/* Wait for reception complete */
while(!(SPSR & (1<<SPIF)))
;
/* Return Data Register */
return SPDR;
}