Reset challenges nrf 51422

[Flemming Jensen]

Hi,

We are developing a product using the nrf 51422 where the power source is a 3 phase generator.

We rectify and charge on a supercap where the uProc is being supplied.

Now we see if the supercap is completely drained and then slowly charged to fx 2.5v we often see that the cpu does not start...

By hitting reset pin we can get the cpu to run again.

Shouldn't this device have a safe and reliable power on reset circuitry?

What am i doing wrong? :)

[Flemming Jensen]

Can I do something in software ?

Set a fuse regarding POR / BROWNOUT?

Or is this something that needs to be solved maybe using an external capacitor in hardware?

Looking forward to hearing from you :))

[Ole Bauck]

How slowly is the capacitor charged?

According to the Product Specification chapter 7, the supply rise time (0V to VDD) needs to be less than 100ms. As it says in the note:

"The on-chip power-on reset circuitry may not function properly for rise times outside the specified interval."

Your chip is probably going to an undefined state rather than a reset.

[Flemming Jensen]

Yes - this is probably what is going on. The cap could be charging very slowly.

So I guess we really need an external reset circuit :((