GPS module fails to cold-boot / re-acquire satellite lock after STM32 wakes from STOP mode

[muxtech]

Hello,
We are developing a battery-operated GPS asset tracker. To conserve power, the STM32 enters STOP mode every 10 minutes and wakes up via RTC alarm to fetch the current location.
Problem:
After waking up from STOP mode and toggling the power/enable pin of the GPS module, the GNSS chip takes over 60–90 seconds to re-acquire a 3D fix (Cold Start), which drains the battery significantly. Even when backup power (VBACKUP pin) is maintained to save ephemeris data, warm start is not triggering reliably.
Setup:
MCU: STM32L4 Low-Power series
Power Management: WFI / STOP 2 mode
Is there a best practice for managing GPIO states and UART lines before entering STOP mode to prevent current leakage into the GPS module's RX/TX pins?
Best regards.

[Saket_Om]

Hello ​@muxtech 

 

Is there a best practice for managing GPIO states and UART lines before entering STOP mode to prevent current leakage into the GPS module's RX/TX pins?

 

Typical practice is to configure pins as analog to avoid unnecessary current.

[Andrew Neil]

Typical practice is to configure pins as analog to avoid unnecessary current.

Indeed; eg, see Application Note AN4899, Guidelines for GPIO hardware settings and low-power consumption on STM32 MCUs:

https://www.st.com/resource/en/application_note/an4899-guidelines-for-gpio-hardware-settings-and-lowpower-consumption-on-stm32-mcus-stmicroelectronics.pdf

https://www.st.com/resource/en/application_note/an4899-stm32-microcontroller-gpio-hardware-settings-and-lowpower-consumption-stmicroelectronics.pdf

 

​@muxtech However, this effectively leaves the connections to the GNSS “floating” - which it may not like; eg,

 

So, again, this becomes a question about the GNSS device.

[Andrew Neil]

This sounds like more of a question about the GNSS module than the STM32.

You haven’t said what GNSS module you’re using.

 

Even when backup power (VBACKUP pin) is maintained to save ephemeris data, warm start is not triggering reliably.

You need to give more details:

 

PS:

You’ve tagged this as STM32F4, but the post text says STM32L4 - so which is it?

Please give the full part number.