Communication Protocols – SPI

Dec 31, 2022

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Ever wondered how electronic devices communicate with each other? Just as humans need a ‘language’ to be able to talk, electronic devices or ICs need ‘communication protocols’ to exchange information (0s and 1s here). Factors like, how fast the data needs to be exchanged, how much data is needed to be transferred, or the availability of pin count, helps in deciding which protocol to use.

Before we discuss one such protocol – SPI (Serial Peripheral Interface), lets us understand few more terms. Communication protocols can be:

Serial

Transfer of 1 bit at a time

Parallel

Transfer of many bits at the same time

Synchronous

Data is transferred with reference to a ‘clock

Asynchronous

Data exchange doesn’t need a ‘clock’

Serial Peripheral Interface – SPI

Serial communication
Synchronous
Any amount of data can be transferred in a continuous stream (not in packets limited to certain number of bits)
Uses master-slave relationship (For example: a controller can act as ‘master’ and a memory device can act as ‘slave’
Only master can initiate a transaction. However, some slave devices may have an extra functionality (using a separate pin) to indicate the master that they need to send or receive data. But actual transaction is started by ‘master’ only.
Number of wires in most common configuration is 4

Master Slave Configuration:
MOSI: Master Out Slave In, MISO: Master In Slave Out, SCK: SPI Clock, CS: Chip-Select

The speed of data transmission can reach up to 10 Mbps
The configuration can have only 1 master and unlimited slaves (practically limited, based on other factors). There are following possible configurations:

Multiple Slave Configuration:
Master has 3 Chip-Select pins for 3 slaves

**Daisy Chain Configuration:**
**Master has only one Chip-Select connected to all slaves. The MISO and MOSI pins of consecutive flops are connected in serial as shown.**

Pros

No start/stop bits
Simple addressing
Faster than many other protocols
Full-Duplex (through MISO and MOSI) – Data can be sent and received at the same time
Clock is provided by master.

Cons

Minimum 4 wires (more than other protocols like UART, I2C which need only 2 wires)
No data acknowledgement by the slave
No error checking capability
Only single master is allowed

Working

Clock Phase and Clock Polarity

CPOL

Clock “Pol”arity

0 : clock idles at 0 (pulse of 1)

1 : clock idles at 1 (pulse of 0)

CPHA

Clock “Pha”se

0 : Data is captured on leading edge, and shifted on the trailing edge

1 : Data is captured on the trailing edge and shifted on the leading edge

The waveforms in all the possible clock phase and polarity combinations are as follows:

CPOL = 0 and CPHA = 0

Leading edge : Rising
Trailing Edge : Falling
Data captured on leading “Rising” edge
Data shifted/sampled on trailing “Falling” edge

CPOL = 0 and CPHA = 1

Leading edge : Rising
Trailing Edge : Falling
Data captured on trailing “Falling” edge
Data shifted/sampled on leading “Rising” edge

CPOL = 1 and CPHA = 0

Leading edge : Falling
Trailing Edge : Rising
Data captured on leading “Falling” edge
Data shifted/sampled on trailing “Rising” edge

CPOL = 1 and CPHA = 1

Leading edge : Falling
Trailing Edge : Rising
Data captured on trailing “Rising” edge
Data shifted/sampled on leading “Falling” edge

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Comments

One response to “Communication Protocols – SPI”

Anonymous

August 28, 2024

such an underrated blog this is.

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