SPI Communication Protocol in Embedded

Communication plays an essential role in the embedded system designing. Without going to the protocols, the peripheral expansion is highly complex and high-power consuming. The embedded system basically uses serial communication to communicate with the peripherals.
There are many serial communication protocols, such as UART, CAN, USB, I2C and SPI communication. The serial communication protocol’s characteristics include high speed and low data loss. It makes system-level designing easier, and ensures reliable data transfer.

Serial Data Communication

Electrically-coded information is called a serial data, which is transmitted bit by bit from one device to another through a set of protocols. In the embedded system, control sensors and actuators data is received or transmitted to the controller devices such as microcontrollers so that the data is further analyzed and processed. As the microcontrollers work with the digital data, the information from the analog sensors, actuators and other peripherals is converted into one byte (8-bit) binary word prior to being transmitted to the microcontroller.

Serial Data Communication

This serial data is transmitted with respect to certain clock pulse. The data transmission rate is referred to as the baud rate. The number of data bits that can be transmitted per second is called as baud rate. Suppose the data is of 12 bytes, then each byte is converted into 8bits so that the total size of the data transmission is about 96bits/sec of the data (12bytes*8 bits per byte). If the data can be transmitted once every second, the baud rates are around 96bits/sec or 96 baud. The display screen refreshes the data value once every second.

Serial Peripheral Interface Basics

The SPI communication stands for serial peripheral interface communication protocol, which was developed by the Motorola in 1972. SPI interface is available on popular communication controllers such as PIC, AVR, and ARM controller, etc. It has synchronous serial communication data link that operates in full duplex, which means the data signals carry on both the directions simultaneously.

SPI protocol consists of four wires such as MISO, MOSI, CLK, SS used for master/slave communication. The master is a microcontroller, and the slaves are other peripherals like sensors, GSM modem and GPS modem, etc. The multiple slaves are interfaced to the master through a SPI serial bus. The SPI protocol does not support the Multi-master communication and it is used for a short distance within a circuit board.

SPI Lines

MISO (Master in Slave out): The MISO line is configured as an input in a master device and as an output in a slave device.

MOSI (Master out Slave in): The MOSI is a line configured as an output in a master device and as an input in a slave device wherein it is used to synchronize the data movement.

SCK (serial clock): This signal is always driven by the master for synchronous data transfer between the master and the slave. It is used to synchronize the data movement both in and out through the MOSI and MISO lines.

SS (Slave Select) and CS (Chip Select): This signal is driven by the master to select 
individual slaves/Peripheral devices. It is an input line used to select the slave devices.

Master Slave Communication with SPI Serial Bus

Single Master and Single Slave SPI Implementation

Here, the communication is always initiated by the master. The master device first configures the clock frequency which is less than or equal to the maximum frequency that the slave device supports. The master then selects the desired slave for communication by dragging the chip select line (SS) of that particular slave device to go low state and active. The master generates the information on to the MOSI line that carries the data from master to slave.

Master Slave Communication

Single Master and Multiple Slave Implementations

This is a multiple slave configuration with one master and multiple slaves through the SPI serial bus. The multiple slaves are connected in parallel to the master device with the SPI serial bus. Here, all the clock lines and data lines are connected together, but the chip select pin from each slave device must be connected to a separate slave select pin on the maser device.

Single Master and multiple slaves

In this process, the control of each slave device is performed by a chip select line (SS). The chip select pin goes low to activate the slave device and goes high to disable the slave device.The data transfer is organized by using the shift registers at both master and slave devices with a given word size of about 8-bit and 16-bit, respectively. Both the devices are connected in a ring form so that the maser shift register value is transmitted through the MOSI line, and then the slave shifts data in its shift register. The data is usually shifted out with the MSB first and shifting new LSB into the same register.

Data Transfer between Master and Slave

Significance of Clock Polarity and Phase

Generally the transmission and reception of data is performed with respect to the clock pulses at rising edges and falling edges. The Advanced microcontrollers have two frequencies: internal frequency and external frequency. SPI peripherals could be added by sharing the MISO, MOSI and SCLK lines. The peripherals are of different types or speeds like ADC, DAC, etc. So we need to change the SPCR settings between the transfers to different peripherals.

SPCR register

The SPI bus operates in one of the 4 different transfer modes with a clock polarity (CPOL) and clock phase (CPHA) which defines a clock format to be used. The clock polarity and the phase clock rates depend on which peripheral device you are trying to communicate with the master.
CPHA=0, CPOL=0: The first bit starts as a lower signal – the data is sampled at rising edge and the data changes on falling edge.

CPHA=0, CPOL=1: The first bit starts with a lower clock – the data is sampled at falling edge and the data changes on rising edge.

CPHA=1, CPOL=0: The first bit starts with a higher clock – the data is sampled at falling edge and the data changes on rising edge.

CPHA=1, CPOL=1: The first bit starts with a higher clock – the data is sampled at rising edge, and the data changes on falling edge.

SPI Bus timings

SPI Communication Protocol

Many microcontrollers have inbuilt SPI protocols that handle all of the sending and receiving data. Any of the data mode operations (R/W) is controlled by a control and status registers of the SPI Protocol. Here, you can observe the EEPROM interface to the PIC16f877a microcontroller through the SPI protocol.

Here, 25LC104 EEROM is a 131072 bytes memory wherein the microcontroller transfers two bytes of data to the EEROM memorythrough a SPI serial bus. The program for this interfacing is given below.

Master to Slave communication through SPI serial bus

#include<pic16f877x.h>
Sbit SS=RC^2;
Sbit SCK=RC^3;
Sbit SDI=RC^4;
Sbit SDO=RC^5;
Void initialize EEROM();
Void main()
{
SSPSPAT=0x00;
SSPCON=0x31;
SMP=0;
SCK=0;
SDO=0;
SS=1;
EE_adress=0x00;
SPI_write( 0x80);
SPI_write(1234);
SS=0;
}

Advantages of SPI Protocol

• It is a full duplex communication.
• It is high-speed data bus 10MHzs.
• It is not limited to 8bits while transferring
• Hardware interfacing is simple through SPI.
• Slave uses a master clock and doesn’t need precious oscillators.