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Serial Peripheral Interface (SPI) in AVR Microcontroller

AVR microcontroller support wired communication protocols. Most of the microcontroller comes with internal modules that support communication protocols.

The commonly supported protocols in AVR microcontroller are:

• SPI (Serial Peripheral Interface)
• TWI (Two Wire Interface)
• UART (Universal Asynchronous Receiver Transmitter) communication
• USI (Universal Serial Interface)
• USB (Universal Serial Bus)
• Ethernet etc.

SPI is the simplest protocols among all the communication protocols. 8 Bit data registers inside a device is connected by wires and these data registers works as shift registers for controlling the data exchange inside the SPI network. The device or devices that will controls the operation inside the network is known as the master.

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Register Description:

The table for status register of SPI is shown below:

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SPI (Serial Peripheral Interface) Status Register
Bit	7	6	5	4	3	2	1	0
BitName	SPIF	WCOL	–	–	–	–	–	SPI2X
Read/Write	Read	Read	Read	Read	Read	Read	Read	Read
Initialvalue	0	0	0	0	0	0	0	0

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Bit 0 – SPI2X (Double SPI Speed Bit):

When this bit is high then the SPI speed (system clock frequency) will be doubled when the SPI is in master mode. This means the minimum SCK period is 2 CPU clock periods. When the SPI is configured as slave then the SPI is work at lower then clock period frequency.

Bit 1…5-Reserved Bits:

In SPI status register bit 1 to bit 5 is reserved bits used for any further connection operation.

Bit 6-WCOL (Write Collision Flag):

The WCOL bit is set high if the SPI Data Register is used during a data transfer. The WCOL bit is cleared by first reading the SPI register with WCOL set, and then used for accessing the SPI Data Register.

Bit 7-SPIF (SPI Interrupt Flag):

When a serial transfer of data is complete then the SPIF flag is set. SPIF flag is cleared by hardware when executing the corresponding interrupt handling vector.

The table for control register of SPI is shown below:

SPI Control Register- SPCR
Bit	7	6	5	4	3	2	1	0
BitName	SPIE	SPE	DORD	MSTR	CPOL	CPHA	SPR1	SPR0
Read/Write	RW	RW	RW	RW	RW	RW	RW	RW
Initial value	0	0	0	0	0	0	0	0

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Enabling the SPI operation in AVR Microcontroller:

• For enabling SPI operations the steps given below are sufficient:

1. SET or RESET the MSTR bit inside SPCR register.
2. SET the SPI Enable bit inside SPCR register.
3. Configure the value of DDR (Data Direction Register) of digital I/O port to which the SPI belongs.
4. It is not compulsory to configure the Prescaler bits.

SPI exchange the contents of SPDR register of master and slave. Therefore precautions must be taken so the garbage contents doesn’t get used or transmitted.