23.2 Overview

The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the ATmega8A and peripheral devices or between several AVR devices.

Note: 1. Refer to Pin Configurations, table Port B Pins Alternate Functions in Alternate Functions of Port B for SPI pin placement.

The interconnection between Master and Slave CPUs with SPI is shown in the figure below. The system consists of two shift registers, and a Master Clock generator. The SPI Master initiates the communication cycle when pulling low the Slave Select SS pin of the desired Slave. Master and Slave prepare the data to be sent in their respective Shift Registers, and the Master generates the required clock pulses on the SCK line to interchange data. Data is always shifted from Master to Slave on the Master Out – Slave In, MOSI, line, and from Slave to Master on the Master In – Slave Out, MISO, line. After each data packet, the Master will synchronize the Slave by pulling high the Slave Select, SS, line.

When configured as a Master, the SPI interface has no automatic control of the SS line. This must be handled by user software before communication can start. When this is done, writing a byte to the SPI Data Register starts the SPI clock generator, and the hardware shifts the eight bits into the Slave. After shifting one byte, the SPI clock generator stops, setting the end of Transmission Flag (SPIF). If the SPI interrupt enable bit (SPIE) in the SPCR Register is set, an interrupt is requested. The Master may continue to shift the next byte by writing it into SPDR, or signal the end of packet by pulling high the Slave Select, SS line. The last incoming byte will be kept in the Buffer Register for later use.

When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long as the SS pin is driven high. In this state, software may update the contents of the SPI Data Register, SPDR, but the data will not be shifted out by incoming clock pulses on the SCK pin until the SS pin is driven low. As one byte has been completely shifted, the end of Transmission Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE, in the SPCR Register is set, an interrupt is requested. The Slave may continue to place new data to be sent into SPDR before reading the incoming data. The last incoming byte will be kept in the Buffer Register for later use.

The system is single buffered in the transmit direction and double buffered in the receive direction. This means that bytes to be transmitted cannot be written to the SPI Data Register before the entire shift cycle is completed. When receiving data, however, a received character must be read from the SPI Data Register before the next character has been completely shifted in. Otherwise, the first byte is lost.

In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure correct sampling of the clock signal, the minimum low and high periods should be:

Low period: longer than 2 CPU clock cycles.

High period: longer than 2 CPU clock cycles.

When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden according to the table below. For more details on automatic port overrides, refer to Alternate Port Functions.

The following code examples show how to initialize the SPI as a Master and how to perform a simple transmission. DDR_SPI in the examples must be replaced by the actual Data Direction Register controlling the SPI pins. DD_MOSI, DD_MISO and DD_SCK must be replaced by the actual data direction bits for these pins. E.g. if MOSI is placed on pin PB5, replace DD_MOSI with DDB5 and DDR_SPI with DDRB.

SPI_MasterInit:
   ; Set MOSI and SCK output, all others input
   ldi    r17,(1<<DD_MOSI)|(1<<DD_SCK)
   out    DDR_SPI,r17
   ; Enable SPI, Master, set clock rate fck/16
   ldi    r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)
   out    SPCR,r17
   ret
SPI_MasterTransmit:
   ; Start transmission of data (r16)
   out    SPDR,r16
Wait_Transmit:
   ; Wait for transmission complete
   sbis   SPSR,SPIF
   rjmp   Wait_Transmit
   ret

void SPI_MasterInit(void)
{
   /* Set MOSI and SCK output, all others input */
   DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK);
   /* Enable SPI, Master, set clock rate fck/16 */
   SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);
}

void SPI_MasterTransmit(char cData)
{
   /* Start transmission */
   SPDR = cData;
   /* Wait for transmission complete */
   while(!(SPSR & (1<<SPIF)))
      ;
}

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

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;
}