328P vs 328PB brownout problems

Other Hardware 3rd Party Boards
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So I've designed my own project board using a 328pb surface mount as my processor. I'm experiencing a strange issue where the processor doesn't seem to start when its powered off and on within a certain time period. I have automotive 12v with a 1000uf capacitor feeding my 7805 voltage regulator that powers my board. When the board is powered down, I can see the voltage slowly decaying to 0v over a few seconds. If the device is powered back up when the voltage is falling, then the processor doesn't restart. I have another arduino running a relay turning the device on and off at a faster and faster rate to test this.

If I use the 328P chip instead of the 328PB then it works perfectly fine. I'm using the exact same fuse settings for the two processors and the exact same program. I've also tried it with or without the bootloader. Anybody have some insight as to why the 328pb would act this way?

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The following application note from Microchip/Atmel states:

ATmega328PB is not a drop-in replacement for ATmega328 variants, but a new device. ... For differences in errata, typical, and electrical characteristics between ATmega328 variants and ATmega328PB, refer to the specific device datasheets.

Atmel 42559: Differences between ATmega328P and ATmega328PB

Also check the datasheet for the ATmega328PB section 33.6 System and Reset Characteristics

There are conditions of how fast or slow power is allowed to rise or fall. See Note 2:

  1. The Power-on Reset will not work unless the supply voltage has been below VPOT (falling)