SPI: The serial peripheral interface [video] (opens in new tab)

The parts when he troubleshoots are the most educational by far. He explains the theory, he puts it in practice, and debugs the result. Also, his editing is perfect IMO, he really knows how to teach.

A nice advantage of SPI is that it can not lock up the master.

This is not true of I2C, where the slave could stretch the clock forever.

This is also not true of UART, where the slave just might not respond. But in fact both SPI and I2C can be simpler than UART since they are strictly master/slave. With a UART, the slave could send you unrequested data, which means your software has to handle asynchronous events. Even if it doesn't you probably have to implement timeouts and input flushing.

SPI is simpler than I2C in that it is a single-master bus, so accesses are atomic (at least at the bus level, within your RTOS is another story...).

You can use simple logic gates as SPI peripherals: for example, 74HC595 works as a SPI accessible 8-bit output port. This is important because such chips are the cheapest available (vs. I2C output port or UART to parallel..)

Despite all this simplicity, SPI is very fast.

Where UART has the advantage is software: picocom on Linux or Teraterm on Windows. Basically you don't have to develop any host-side software if you use a UART. I like to use the old ymodem protocol for embedded system firmware updates.. since again no host-side software has to be developed.

Simplicity of what exactly? From the perspective of the device/hardware, SPI is much simpler. A UART based solution tends to be relatively much more complex on the hardware/device side. You can implement a basic compliant SPI slave in a tiny bit of silicon - like a handful of flip flops simple. Moreover, you realize many of the simplest SPI devices don't have an oscillator, so you're still going to need a clock line - and if its truly a UART/asynchronous it may not be convenient to get a high enough speed external clock.

The point of SPI is that it can be implemented with a simple shift register at either end. It's not about bandwidth so much as implementation simplicity.

For SAR[1] ADC's for example, the output can be generated while the ADC conversion is happening. This reduces complexity and latency, and allows you to easily control the sampling time by how fast you clock the SPI bus[2].

If the ADC had to output as text via UART it would need to complete the conversion and then convert the output to characters before stuffing it over an UART with start and stop bits and whatnot. And you'd need some other way to control sampling and conversion time. An internal clock? External, but separate clock? Either way, added complexity.

[1]: https://www.maximintegrated.com/en/design/technical-document...

[2]: https://www.microchip.com/en-us/product/MCP3202 (semi-random example, datasheet chapter 5 and figure 5-2)

Do you mean implicit clock to save the third line? Other than that SPI is about as simple as it gets. Plus there's a protocol for practically every requirement: i2c for simple slowish shared bus with clock, 1-wire shared bus without clock, SPI fast and simple, serial single ended (RS232) or differential (RS485) for stuff outside of your PCB...

Given that I2C and SPI are addressable replacing them with a bare TTL serial interface would be horrendous because now everyone needs to invent their own addressing scheme which they already had with the other two.

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