Problem with working on 2.5V
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‎2018-03-01 2:10 AM
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?
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‎2018-03-01 2:21 AM
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).
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‎2018-03-01 2:23 AM
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.
A complex system designed from scratch never works and cannot be patched up to make it work.
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‎2018-03-01 3:32 AM
But why STM works incorrectly if it is capable to work from 1.8V?
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‎2018-03-01 3:39 AM
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 ... ?
A complex system designed from scratch never works and cannot be patched up to make it work.
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‎2018-03-01 6:09 AM
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.
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‎2018-03-01 6:19 AM
Bottom line: your power supply is bad - you need to fix it!
A complex system designed from scratch never works and cannot be patched up to make it work.
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‎2018-03-02 12:04 AM
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.
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‎2018-03-02 2:29 AM
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..
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‎2018-03-02 2:56 AM
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!
A complex system designed from scratch never works and cannot be patched up to make it work.
