the brown out (BOR) must still be enabled. according to the data sheet it will
reset the chip somwhere between 3.7 and 4.3 volts.
which seems to match your problem

> i have a circuit that must be operated about 3.6 v. the first version for
> simplicity was done at 5v and its works ok, but when i down the supply to
> 3.6 v, things go wrong.
>
> at 4.3 v it works fine, and the device is windowed. the BOR is disabled. the
> xtal is 3.579 Mhz, coupled with 22 pf capacitor. I assume that the device is
> working, because is showing alternating the first two digits, but it is
> perhaps reseting and starting again and again.

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