Serial Peripheral Interface - SPI

Introduction:

SPI allows full-duplex, synchronous, serial communications with peripheral devices. Software can either poll the SPI status flags or SPI operation can be interrupt driven.

Features:

• Communication between SPI and external device is always synchronous.
• SPI is interfaced with external peripheral device for data transfer.
• SPI can transfer data to an external peripheral device.
• SPI can receive data from an external peripheral device.
• SPI can work in master or slave mode.
• In master mode, SPI clock drives the slave peripheral.
• In slave mode, the external clock drives SPI.

SPI Interfacing with a device:

MOSI: 

Master Out Slave In - Output from SPI, connected to Serial Data Input of the peripheral device.

MISO: 

Master In Slave Out - Input to the SPI, connected to Serial Data Output of the peripheral device.

SCLK: 

Clock output from the SPI to the clock input to slave.

SS: 

Chip select output from the SPI - master selects the chip with which it wants to communicate.

Single master, single slave SPI implementation: 

Single master, multiple slave SPI implementation:

Data Transfer mechanisim:

1. The software module begins an SPI transmission by writing a byte to the SPI transmit data register.
2. This byte is transferred to the shift register, when SPI transmit data register becomes empty the byte begins shifting out a bit at a time on the MOSI pin synchronized with the master clock signal.
3. The transfer will continue for 8 clock cycles, transferring all 8-bits.
4. The transmission ends when the whole byte is shifted out of the master SPI shift register and into the slave shift register.
5. If SPI is receiving, the incoming byte will come to the shift register from the MISO pin.
6. When one complete byte is received by the shift register, byte is copied to receive data register.

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Saravana