In Shutdown, how is RTC powered?

[waclawek.jan]

While I was experimenting with the 'L476 Disco, the RM wording goes across multiple families, so it may be relevant to most/all of them.

In particular, for Shutdown mode (which is supposedly the lowest-VDD-consumption mode), the RM says:

What does "switch to Backup domain is not supported" mean, exactly (assuming VDD and VBAT have separate sources)? 

1. as soon as Shutdown mode is entered, RTC (backup domain) is switched to VBAT even if VDD is present, potentially draining the VBAT source (battery, capacitor)
2. in Shutdown, while VDD is present, RTC is powered from VDD; but when VDD is removed, RTC is *not* switched to VBAT, resulting in backup-domain reset and/or RTC corruption
3. in Shutdown, while VDD is present, RTC is powered from VDD; but when VDD is removed, most of the time the switch flips to VBAT as expected, but this is not reliable and sometimes RTC corruption occurs
4. in Shutdown, it's unpredictible, what is the backup-domain switch state, even while VDD is present

These are very different scenarios and it's important to know, which one of them is valid, to be able to plan the lowest power consumption mode properly.

Can ST please comment.

Thanks,

JW

[Viktor POHORELY · Accepted solution]

After entering shut down mode, operation is guaranteed as long as VDD is above minimum required voltage.

No current is drain from the VBAT circuitry.

Once VDD drops below 1.6 V, there is no guarantee anymore, only more or less probable behaviors.

As per datasheet, once VDD allows device to enter operating range (specified as 1.71 V for all devices, might be between 1.62 to 1.70 V where BOR0 release reset for given device), functionality is guaranteed down to VBOR0 specified as 1.60 to 1.69 V. As there is no BOR enabled, 1.6 V will apply.

It is valid for devices you have listed.

L4 RM will not be changed for this point as it is considered as fully mature device. But I will propose to update wording for further RM's.

[waclawek.jan]

Hi@Viktor POHORELY ,

Thanks for the clarification. This has been settled, then.

Jan

[MM..1]

Yes , but technicaly i mean shutdown is here for reduce power with stay Vdd, as internal Pswitch.

Then if is planned or predicted Vdd lost , use Standby is safe. Or?

[waclawek.jan]

Whatever the actual behaviour is, if there are doubts about it, it's hard to plan the proper strategy.

In applications like those based on energy harvesting there may be wildly varying power supply relationships, yet you can predict e.g. that a fully charged capacitor will sustain VDD in Shutdown for a certain time (plus allow a short run to decide further action, eg. would the capacitor be not charged enough in meantime, switch to Standby, which as you say is in this regards safe). But you still may want to minimize drain at VBAT at all cost to preserve life of e.g. a tiny primary cell connected to it for as long as possible, i.e. you don't want it to be drained would in Shutdown the switch behaviour unpredictiby even in presence of VDD (which now we know is not the case, but I did have my doubts based on the observations).

JW