How quickly I can change pin states on Arduino?

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hi,

i'm thinking of implementing project require quite lot of data transferred shift registers , i'm thinking limitations in performance expect arduino plan project properly. instead of using digitalwrite() sets state of pins individually understanding can write state directly porta/b/c/d global variables, sets state of 8 pins in each port @ once. if understood correctly, memory write on atmega takes 2 cycles, @ 16mhz mean can change state of 8 pins @ 8mhz, right?

thanks, jarkko

well can write @ rate have else fetch data , slow down.
for transferring data shift register @ using spi interface, think run @ 1mhz


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