SPI Protocol

SPI(Serial Peripheral Interface) is a single master multiple slave serial transmission protocol that works in full duplex mode.

• There can be only one master but there can be multiple slaves.
• MOSI- Master out Slave In
• MISO- Master In Slave Out
• SCLK- Clock Signal
• SS or CS- Slave Select or Chip Selecthe 
• The data is transmitted using shift register.
• With the number of clocks equal to the word size(generally 8) the master and slave would have exchanged the value in their shift register.
• They can now process the data they have received or/and can load new data in the shift register.

Lets Talk Business and Discuss The actual Protocol

• To start off , the the master configures the clock rate of transmission(frequency should be less than the maximum supportable frequency).
• The master then sends a chip select signal 0 on the chip select line which it wants to choose.
• Since the lines are active high so by default they are high(initially).
• Sometimes a waiting time is involved, so the master must wait for that duration before it starts the clock cycle.
• During each SPI clock cycle a full transmission happens with this operation-

1. The master sends one bit on MOSI line and the slave reads on the same line.
2. The slave sends one bit on the MISO line and the master reads on the same line.

• We may not need all these transmission for our purpose but as per the protocol this has to happen.