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May 31, 2015

Introduction to Communication Standards and Interfaces - Part 3

by 4hathacker  |  in Robotronics at  8:55 AM

Welcome to our next discussion in this series. As stated in the previous post, we are going to deal here with the working aspects of SPI protocol. For introductory details about these protocols and any other protocol, visit our previous posts.

Serial Peripheral Interface is a clocked serial link in which transfer of data is faster according to clock. SPI uses 4 signal lines for data transfer. As such, there are no 'specific official' details of SPI are discussed here. (This article can be referred to understand the basic working of SPI and then for your project work you can easily cope up with your datasheet of microntroller involved in the project)
The four signal buses are named as:
SCLK (Serial Clock Signal) is sent from master to all slaves to maintain the synchronisation.
MOSI (Master Out Slave In) is a data line from master to slave.
MISO (Master In Slave Out) is a data line from slave to master.
SS (Slave Select Signal) used to select the slave that communicates with the master.

SPI is a single master communication protocol while it may have a single slave or multiple slaves as shown above. When the master wants to communicate with its slaves, it selects that slave by pulling the corresponding SS line low. Then activates the clock signal.

The master generates signal information on MOSI line and samples at MISO line. Data bits on MOSI and MISO toggles on SCLK falling edge and sampled on SCLK rising edge. So, for toggling of data and samples SCLK is used. But how to define which toggling edge for data and which for samples. To make this thing easier, we have SPI modes. There are four SPI modes as shown in figure below.

Points to remember about SPI Protocol:

  • It is less well specified as compared to I2C.
  • No data reception acknowledgement is there.
  • Maximum data rate is undefined.
  • Actually the SPI master is unaware of the existence of slave unless something additional is done outside the SPI interface.
  • Nowadays, there are many variants of SPI that uses continuous clock signal and arbitrary transfer length.

This is all about SPI. If you need to know more about SPI feel free to ask in the comment box. In the next post we are going to discuss about I2C protocol.

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