[sdiy] LFSR using lookup tables

sleepy_dog at gmx.de sleepy_dog at gmx.de
Wed Feb 19 19:48:45 CET 2020


John Speth via Synth-diy:
> LUTs instead of "code" (hard logic or procedural) illustrates the
> classic speed vs memory tradeoff. If you have more of one than the
> other, you can craft your algorithm towards one method or the other.
> I'm skeptical that there is a need for LUTs in the very simple design
> domain of shift then xor. My gut tells me that shift then xor is
> always more efficient than using LUTs, both in terms of memory and
> speed resource usage. I'm eager to be shown the opposite.
>
> JJS

not only speed vs mem.
Cortex M* class MCUs have program memory (flash) wait states which are
compensated by a small cache.
Depending on its size, your table size, and the access pattern, you
might severely degrade the performance with the table'd version.
You could load the table from flash into SRAM, the old tradeoff again,
but this seems especially "wasteful", unless you can easily spend the
RAM I guess.

* (at least those which are not the slowest, cheapest, from ST that I know)

- Steve

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