Friday, 4 October 2013

serial communication protocol I²C vs SPI: is there a winner?

                      Serial Communication Protocol                                

 I²C vs SPI: is there a winner?

Let’s compare I²C and SPI on several key protocol aspects:

- Bus topology / routing / resources:
I²C needs 2 lines and that’s it, while SPI formally defines at least 4 signals and more, if you add slaves. Some unofficial SPI variants only need 3 wires, that is a SCLK, SS and a bi-directional MISO/MOSI line. Still, this implementation would require one SS line per slave. SPI requires additional work, logic and/or pins if a multi-master architecture has to be built on SPI. The only problem I²C when building a system is a limited device address space on 7 bits, overcome with the 10-bits extension.
From this point of view, I²C is a clear winner over SPI in sparing pins, board routing and how easy it is to build an I²C network.

- Throughput / Speed:
If data must be transferred at ‘high speed’, SPI is clearly the protocol of choice, over I²C. SPI is full-duplex; I²C is not. SPI does not define any speed limit; implementations often go over 10 Mbps. I²C is limited to 1Mbps in Fast Mode+ and to 3.4 Mbps in High Speed Mode – this last one requiring specific I/O buffers, not always easily available.

- Elegance:
It is often said that I²C is much more elegant than SPI, and that this last one is a very ‘rough’ (if not ‘dumb’) protocol. Actually, we tend to think the two protocols are equally elegant and comparable on robustness.
I²C is elegant because it offers very advanced features – such as automatic multi-master conflicts handling and built-in addressing management – on a very light infrastructure. It can be very complex, however and somewhat lacks performance.
SPI, on the other hand, is very easy to understand and to implement and offers a great deal of flexibility for extensions and variations. Simplicity is where the elegance of SPI lies. SPI should be considered as a good platform for building custom protocol stacks for communication between ICs. So, according to the engineer’s need, using SPI may need more work but offers increased data transfer performance and almost total freedom.
Both SPI and I2C offer good support for communication with low-speed devices, but SPI is better suited to applications in which devices transfer data streams, while I²C is better at multi master ‘register access’ application.
Used properly, the two protocols offer the same level of robustness and have been equally successful among vendors. EEPROM (Electrically-Erasable Programmable Read-Only Memory), ADC (Analog-to-Digital Converter), DAC (Digital-to-Analog Converter), RTC (Real-time clocks), microcontrollers, sensors, LCD (Liquid Crystal Display) controllers are largely available with I²C, SPI or the 2 interfaces.


        In the world of communication protocols, I²C and SPI are often considered as ‘little’ communication protocols compared to Ethernet, USB, SATA, PCI-Express and others, that present throughput in the x100 megabit per second range if not gigabit per second. Though, one must not forget what each protocol is meant for. Ethernet, USB, SATA are meant for ‘outside the box communications’ and data exchanges between whole systems. When there is a need to implement a communication between integrated circuit such as a micro-controller and a set of relatively slow peripheral, there is no point at using any excessively complex protocols. There, I²C and SPI perfectly fit the bill and have become so popular that it is very likely that any embedded system engineer will use them during his/her career.

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