Serial
Peripheral
Interface – SPI
The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the
ATmega8 and peripheral devices or between several AVR devices. The ATmega8 SPI includes
the following features:
• Full-duplex, Three-wire Synchronous Data Transfer
• Master or Slave Operation
• LSB First or MSB First Data Transfer
• Seven Programmable Bit Rates
• End of Transmission Interrupt Flag
• Write Collision Flag Protection
• Wake-up from Idle Mode
• Double Speed (CK/2) Master SPI Mode
Figure 57. SPI Block Diagram(1)
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Note:
1. Refer to “Pin Configurations” on page 2, and Table 22 on page 58 for SPI pin placement.
The interconnection between Master and Slave CPUs with SPI is shown in Figure 58. The sys-
tem 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 Mas-
ter 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.
124
ATmega8(L)
2486T–AVR–05/08
ATMEL [ ATMEL ]
