Duration: 2:19

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It's very complicated, more complicated than one can imagine if faster upload speed is the goal. What is the cause of this error? It's not from a bug in the code, but it's the fact that SCK is not stable at that high speed vs the target MCU, so decreasing it solves the problem, but it's a dilemma between fast and safe. Assuming both MCUs are operating at exact 8MHz, the programmer can output a 4MHz max SCK, but with SPI slave being in 8MHz, it requires to be clocked in with lower than 8/4=2MHz to be stable. Using 2MHz is ideal, but it's not stable, thus the errors, so to be 100% sure 1MHz should be used, but it's twice slower than 2MHz, so not great when fast upload speed is the goal.Here I am hacking the extended option in avrdude to let the user select between Fast and Safe SCK mode; Fast will work with MCU at slightly slower than 8MHz or for example 7.999Mhz at with 2MHz sck.