Bit-endianness is only relevant for serial transmission. You only have to worry about it in circumstances where you have hardware that assumes a different bit order than what you're communicating with. Otherwise, the internal layout of words is abstracted and you can ignore it.
It's relevant when dealing with the control registers of peripherals, even if that's over a parallel bus. It's at least important that you have the same bit order as the peripheral.
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Bit-endianness is only relevant for serial transmission. You only have to worry about it in circumstances where you have hardware that assumes a different bit order than what you're communicating with. Otherwise, the internal layout of words is abstracted and you can ignore it.
It's relevant when dealing with the control registers of peripherals, even if that's over a parallel bus. It's at least important that you have the same bit order as the peripheral.