lgabiot:
Is there any penalties using flash memory to store constant instead of RAM?
Yes, CPU time. Loading data from SRAM is a two cycle machine instruction. Loading data from Flash is three cycles. But, Loading from Flash requires using the Z register which is at least another two cycles to initialize. And, having to dedicate the Z register can interfere with nearby code generation (less than optimal). A good rule of thumb: Flash data is about three times slower to access than SRAM data.
Unless you are developing very time critical code, the CPU time penalty is irrelevant.
(I could optimise all the constant of my sketch to use PROGMEM for instance, or just the biggest ones?).
Biggest and least frequently accessed are the low hanging fruit.
I guess it also an issue of time to write code, to keep it as simple as possible to be maintainable, and not over-optimise.
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