During periods of low VCC, the Flash program can be
corrupted because the supply voltage is too low for the CPU and the Flash to operate
properly. These issues are the same as for board level systems using the Flash, and the
same design solutions should be applied.
A Flash program corruption can be caused
by two situations when the voltage is too low. First, a regular write sequence to the
Flash requires a minimum voltage to operate correctly. Secondly, the CPU itself can
execute instructions incorrectly, if the supply voltage for executing instructions is
too low.
Flash corruption can easily be avoided by
following these design recommendations (one is sufficient):
- 1.If there is no need for a Boot
Loader update in the system, program the Boot Loader Lock bits to prevent any Boot
Loader software updates.
- 2.Keep the AVR RESET active (low)
during periods of insufficient power supply voltage. This can be done by enabling
the internal Brown-out Detector (BOD) if the operating voltage matches the detection
level. If not, an external low VCC reset protection circuit can be used. If a reset occurs while a write
operation is in progress, the write operation will be completed provided that the
power supply voltage is sufficient.
- 3.Keep the AVR core in Power-down
sleep mode during periods of low VCC. This will prevent the CPU from attempting to
decode and execute instructions, effectively protecting the SPMCR Register and thus
the Flash from unintentional writes.