Problem with working on 2.5V

[Artem Dmitriev]

Posted on March 01, 2018 at 11:10

Hello!

On the PCB from LDO(

MCP1700T-3302E

) 3.3V are supplied 2 chips:

GPS

:

SIM

68

M,

STM32L475RET6.

GPS power supply is controlled by STM.

When STM turn on GPS module voltage temporarily drops to 2,5V.

At this time MCU works incorrectly if frequency is 80MHz (hang or HardFault handler are called). If frequency 40Mhz all fine.

What could be the problem?

Note: this post was migrated and contained many threaded conversations, some content may be missing.

[AvaTar]

Posted on March 01, 2018 at 11:21

Just guesswork, but:

When STM turn on GPS module voltage temporarily drops to 2,5V.

At this time MCU works incorrectly if frequency is 80MHz (hang or HardFault handler are called). If frequency 40Mhz all fine.

This suggests you have a power supply issue.

The current consumption of the MCU rises proportional to the core clock frequency.

I guess either your supply itself is too weak, and falters temporarily under the load, or you need better buffering (larger caps).

[Andrew Neil]

Posted on March 01, 2018 at 11:23

When STM turn on GPS module voltage temporarily drops to 2,5V.

It is very common that things like GPS draw a considerably higher 'starting' current than 'running' current.

Your power supply design needs to ensure that your 3.3V rail can cope with this - clearly, it can't!

You need to fix the 

power supply design.

[Artem Dmitriev]

Posted on March 01, 2018 at 11:32

But why STM works incorrectly if it is capable to work from 1.8V?

[Andrew Neil]

Posted on March 01, 2018 at 11:39

You need to study the datasheet carefully - there are limitations when running at lower voltages...

Have you tried just running the STM32 at a steady 2.5V ... ?

[AvaTar]

Posted on March 01, 2018 at 14:09

Most probably because the current surge from the GPS activation pulls Vcc down below 1.8V, or to the limit.

Dropouts in the microsecond range usually suffice to derail the core.

You would need a scope (and doing the probing near the supply pins of the STM32) to see what is going on there.

[Andrew Neil]

Posted on March 01, 2018 at 14:19

Bottom line: your power supply is bad - you need to fix it!

[Artem Dmitriev]

Posted on March 02, 2018 at 08:04

You need to study the datasheet carefully - there are limitations when running at lower voltages...

I tried to study datasheet but i can't find info regarding CPU maximum speed vs VCC voltage.

[aa bb]

Posted on March 02, 2018 at 11:29

How do you enable the power to your GPS module? At startup, GPS drow a lot of current. you could add a slow inrush current with mosfet..

[Andrew Neil]

Posted on March 02, 2018 at 10:56

So, again, haveyou tried just running the STM32 at a steady 2.5V ... ?

As

meyer.frank

‌ says, your voltage may well be dropping below 1.8V.

Again, the bottom line is: your power supply is bad - you need to fix it!