vbat mode

[yang hong]

I am using Nucleo144 32f429zi board. I have removed SB156 to connect VDD to VBAT. There are 2 questions.

1) when I power off controller, I have battery to connect to vbat to keep RTC work, normally current should be around uA level according to stm32f4xx manual, I am using fluke 123 to measure battery current with 10mV/mA, but I got 70mA after controller is powered off. I am not sure my measurement is correct or not. If it is correct, how can I reduce current after I power off controller.

2) my understanding stm32f4 has internal circuit to detect POR and PDR, a internal switch will switch controller between RUN MODE AND BAT MODE. PVD interrupt can't not work on POWER DOWN. How can I turn controller to standby/vbat mode when main control power is off.

[waclawek.jan]

You don't need to do anything to use the VBAT mode, the switchover is automatic upon powerdown.

Did you do any other modification of the Nucleo board? Make sure that VDDA is still connected to VDD.

JW

[yang hong]

hello,

Thank you for your advice. I just remove sb156 and I didn't do anything. I did some verification. I believe my fluke 123 1mV/mA current measurement is not good for this case. I think I need other meter, something like fluke 177 to have 400 mA current measurement.

it looks working now.

[waclawek.jan]

Put an appropriately sized resistor in between the battery and VBAT, and measure the voltage drop across it, to measure the current.

JW

[TDK]

> I am not sure my measurement is correct or not.

70mA sure seems like a lot to me for a processor in showdown mode. I'd recheck your measurement.

[waclawek.jan]

> 70mA sure seems like a lot to me for a processor in shutdown mode. I'd recheck your measurement.

It certainly is. I assumed "it looks working now" means it's already in the uA realm.

I've seen the processor actually running from VBAT after having removed primary power source, depleting a CR2032 battery in a minute or so, when I by mistake disconnected VDDA from VDD. There may have been other random circumstances involved, I did not investigate deeper nor did I attempt to reproduce. That's why I asked above about VDDA. I'm not sure about the exact mechanism there, it certainly violates the operating envelope (DS requires VDDA being within 50mV of VDD, probably not without a reason).

JW