Each segment is 4096 bits. Erasing turns all the bits to ones. Writing can only set bits to zero.

If you store the erase timestamp in the first 64 bits, that leaves 4032 bits. If you set a bit to zero every second without erasing the page, each page can last for 4033 seconds. So that multiplies your endurance calculation by 4033.

This will only work if the flash doesn't have ECC. If it does, then rewrites will make the error detection codes invalid, and it will most likely crash.

Note that power to the micro must absolutely never be shut down during a flash write. If that happens, the behavior is usually undefined, which means the controller may write garbage into random pages unrelated to the one you're writing to, or other undesirable things that will brick it.

Is there any micro these days that has 1.5M of Flash that doesn't have a special RTC power domain that can keep some data around for years on a coin battery?

Each segment is 4096 bits. Erasing turns all the bits to ones. Writing can only set bits to zero.

If you store the erase timestamp in the first 64 bits, that leaves 4032 bits. If you set a bit to zero every second without erasing the page, each page can last for 4033 seconds. So that multiplies your endurance calculation by 4033.

This will only work if the flash doesn't have ECC. If it does, then rewrites will make the error detection codes invalid, and it will most likely crash.

Note that power to the micro must absolutely never be shut down during a flash write. If that happens, the behavior is usually undefined, which means the controller may write garbage into random pages unrelated to the one you're writing to, or other undesirable things that will brick it.

I have never tried this, and honestly I would recommend using battery backed up RAM instead. If your micro has 1.5M of Flash it most likely already has a special RTC power domain that can keep some data around for years on a coin battery.