I read that 8-bit is faster than SPI, how much faster is 8-bit than SPI?

Say, if I wanted to run a video loop of a wheel spinning, would the speed difference between SPI and 8-bit mode be obvious?

  • 2
    There's no good answer to this question... "how much faster?" ... It depends on many factors. In general, it is faster to read and write a full byte at a time rather than place data in a shift register and output it one bit at a time synchronized with a clock signal. If the clock is the same frequency on both the serial interface and the parallel interface (8-bit), then the data rate on the parallel might be effectively 8 times the serial rate... However, implementation limitations need to be considered.... "Video of a spinning wheel" ... what is the desired frame rate, res? SPI clock OK? Nov 17 '17 at 0:52

I assume you are talking about connecting a display on the GPIO of the PI.

GPIO bandwidth

Here are some speed benchmarks for the GPIOS: github page, and one can toggle a GPIO on the PI 3 at 65.8Mhz.

This is the speed for toggling one bit. If you want to drive a 8-bit parallel bus, you will need to drive the 8-bits of data + the control bits (at least the write signal), so you will probably need to write one more register in every loop, reducing the speed by 33% -> ~43.8 MHz. So the maximum theoretical bandwith is ~43.8 MB/s.

SPI bandwidth

According to this answer, the SPI can run at 125 MHz, giving a maximum bandwidth of 15.625 MB/s (125/8 bits).

Bandwidth conclusion

The GPIO method using one full core at 100% can be almost 3 times faster than the SPI, but since the SPI is driven by DMA, it will use much less CPU.

On a Rapsberry PI 2, the gain will be slightly less, and on a Raspberry PI 1 it will even be slower than the SPI.

(Real) conclusion

If you really need a video output, all Raspberry PI except the zero have a DSI connector, which is really the better way of connecting a display to it (the software and the hardware already exist).

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