Enter low power run mode directly at boot

[Kemsa]

Hello community,

I have this difficult task of powering an STM32G0B1 from a pin that was really not tasked for that (because the connector I have to plug onto provides no supply pin). In short, I can't draw more than 500µA current at any time or I risk lowering the voltage too much for MCU.

I have tried so far to reduce clock and use low power run mode. I get a power consumption around 300µA, which is good enough for me. The problem is that the current drawn at boot before entering low power mode is around 1mA for about 30ms, as you can see on oscilloscope below (1mV=1µA)

 
Is it possible to start program directly in low power mode without ever entering "normal" mode?

[TDK · Accepted solution]

> Is it possible to start program directly in low power mode without ever entering "normal" mode?

No. Chip always boots to normal mode.

You can see the entry conditions for low power mode here. All of them involve doing something after the chip is in run mode.

You can enter a low power mode immediately (< 1 ms) upon start if you do it right at ResetHandler, though this will involve breaking the normal startup routine that CubeMX uses.

[Michal Dudka]

BTW: What about to use capacitor to supply MCU during startup ? For example 220uF capacitor drops from 3.3V to 3.0V after 50ms if source current 1mA. And can be charged from 0V to 3.3V in less then two seconds.

[Kemsa]

That's the solution we are trying right now. But because it's a pin we are highjacking that is used for another purpose, we must first delay input, set MCU behind a delayed mosfet and so far, the large capa doesn't play nice with mosfet (smaller ones are fine, but not enough capa...)

[Kemsa]

Thanks for your reply @TDK . It is as I feared...Although 1ms instead of 30ms might be enough for capas to fill the necessary current.

Where would you change the reset handler? I have no experience with that so far...
Restoring the changes after each re-generation by cubeMX would be an acceptable compromise. 

[TDK]

Looks like SystemInit is called immediately after startup, so put the clock initialization there or put it in another function and jump to it (bl) before SystemInit.

Here's the first part of the startup code.

Reset_Handler:  
  ldr   sp, =_estack    		 /* set stack pointer */

/* Call the clock system initialization function.*/
  bl  SystemInit   

/* Copy the data segment initializers from flash to SRAM */  
  ldr r0, =_sdata
  ldr r1, =_edata
  ldr r2, =_sidata
  movs r3, #0
  b LoopCopyDataInit

CopyDataInit:
  ldr r4, [r2, r3]
  str r4, [r0, r3]
  adds r3, r3, #4
...