The interconnection between master and slave CPUs with SPI is shown in Figure 15-2. 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.
Figure 15-2. SPI Master-slave Interconnection
MSB MASTER
LSB
MISO
MOSI
MISO
MOSI
MSB
SLAVE
LSB
8-bit Shift Register
8-bit Shift Register
Shift
Enable
SCK
SS
SCK
SS
SPI
Clock Generator
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 frequency of the SPI clock should never exceed fclkio/4.
When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden according to Table 15-1. For
more details on automatic port overrides, refer to Section 9.3 “Alternate Port Functions” on page 55.
Table 15-1. SPI Pin Overrides(1)
Pin
MOSI
MISO
SCK
SS
Direction, Master SPI
User defined
Input
Direction, Slave SPI
Input
User defined
Input
User defined
User defined
Input
Note:
1. See Section 9.3.2 “Alternate Functions of Port B” on page 58 for a detailed description of how to define the
direction of the user defined SPI pins.
134
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
