As I know that LM3S5951 internal flash memory just can program around 100 times, but my design must allow user on site flash upgrade through USB port, so any suggestion?
may I disable internal flash memory and then use external Nor flash memory through EPI (LM3S5791) to access? that means all of firmware is put into external nor flash and execute. is it possible?
The 100 programming cycles is a minimum number across temperature and life time. Many customer applications that require in the field updates will only be updated a few times over the product life. Depending on your number of updates expected the system may work as it is.
If you need more than 100 updates in the field then you still have a few options.
1) You cannot totally disable internal flash. You can create a permanent bootloader of sorts that checks for the update request and burns update to external flash. If the update request is not present then you simple run from existing code in external flash or wait for an update if for some reason there is no code externally. I believe we have a boot_eth_ext example that does something like this over ethernet for the DK-LM3S9B96 The bootloader itself in internal flash would not get updated and thus you don't have to worry about programming cycles.
2) The 100 programming cycles only impacts certain parts from our product lineup. Depending on the peripherals and features you require perhaps an alternate stellaris part can perform the same functions.
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