Posted on August 30, 2011 at 12:55I don't care about the longer eras/write time of the MPU,
However when the MPU is stalled during that time I have an issue.
At any moment in time the MPU has to react on interrupts, no delay is allowed.
Is my conclusion right that the only option is to use external flash?
I enumerated three possible courses of action.
Your choices may be further constrained by information not presented.
You could add external RAM too, but it won't operate single cycle like the internal stuff. Still it would be nano-seconds, not micro/milli seconds.
The stall occurs on reads (instruction, data, literals, etc) against the active flash array/plane, if you don't read from it the processor does not stall. So it's imperative that you don't look at it from anywhere without checking if it is busy. Cache values you need for interrupts in RAM, for example.
The array also has a finite life, you should uses some lazy/deferred write strategy to cache the writes until it is safe for you to delay, like prior to shutdown/sleep or whatever. You can also minimize erase delays, and damage to the array by writing in a journalling fashion, where you write multiple instances of the data structure to the array before erasing it, and always pick the last (most recent) one when reading back the structure.
I believe the STM32F1 part with 1MB of flash (XL) has two flash planes, you should be able to write to one while executing from the second, but I have not personally tried that. Then again, that's probably quite an expensive part, so an external NV memory might still be the cheaper route.
