i.MX RT1064 Clarifications about VDD_SNVS_IN

[axelrietschin]

Hi,

I'm studying the EVK and the documentation around VDD_SNVS_IN and my first guess is that input is meant to be connected to a 3V Lithium coin cell (e.g. CR2032 etc)

Si far so good but then the processor references manual says at 13.4.1.1.4 that "if VDDHIGH_IN is present, then the SNVS_IN supply is internally shorted to the VDDHIGH_IN supply to allow coin cell recharging if necessary."

VDD_HIGH_IN is 3.3V so that would send that tension to the coin cell? But my understanding is those 3V cells aren't rechargeable?

The manual also says at 13.4.1.4.1 that the "SNVS/RTC low power domain [...] should be supplied from an external single cell LiION battery." but then the nominal tension of a Li-Ion cell is 3.7V, which exceeds the 3.6V max input tension allowed on VDD_SNVS_IN, and that 3.7V cell probably won't charge well at 3.3V.

What is a good battery to use there, and what circuit is necessary to charge it from 3.3V coming out of VDD_SNVS_IN when the system is up and also match the 3V input requirement for VDD_SNVS_IN when the system is down?

In my project, VDD_HIGH_IN will be powered most of the time so the external battery charging must be controlled.

Thanks in advance,

Axel

[gusarambula · Accepted solution]

Hello Axel,

You are correct in that the CR2032 is not rechargeable. You would need to use a battery that fits the specific voltage, which are not as commonplace as the CR2032.

I would consider the wording LiION an oversight, the SNVS battery it’s usually described as a coin cell battery in the SNVS power chapter and in other documents. The problem being that LiION batteries usually have slightly higher voltages than the required specs.

There are some rechargeable coin batteries on the market that you can use. You may use MS614 or ML414 batteries, for example. These are not LiION, but rather Manganese Lithium batteries, but these are rechargeable and have a 3.0V nominal value.

I hope that this information helps. Please let me know if there’s anything else that I can do for you.

Regards,
Gustavo