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BOR Level fluctuation

Question asked by xol.xol on Jan 1, 2016
We are using STM32F205. Circuit is working at 3.3V Most of the critical components are 3.3V. I need to select proper BOR level.

Datasheet states following:
00: BOR Level 3 (VBOR3). reset threshold level from 2.70 to 3.60 V
01: BOR Level 2 (VBOR2). reset threshold level from 2.40 to 2.70 V
10: BOR Level 1 (VBOR1). reset threshold level from 2.10 to 2.40 V
11: BOR off (VBOR0), reset threshold level from 1.8 to 2.10 V

Ideally for us is BOR at 3.0V and we can leave with 2.7V. But the high BOR limit of 3.6V makes it impossible to use BOR Level 3 in 3.3V system. The lower levels are not good for us again because the components are working at nominal voltage of 3.3V and can not go down below 2.7V

Although experimentally we came to conclusion that BOR Level 3 works at about 3.0 - 3.1V. My question is:  Where such a wide fluctuation of 2.7-3.3V comes from? Is it something constant per specific CPU IC and can be sorted? Or does BOR Level is drifting depending on ambient temperature or any other parameter? If BOR works at 3.0V in laboratory can we trast that it will stay at the same 3.0V level in field?