How can I provide a 1Mhz HSE Byp clock for low power... without wasting more power than I save??

[BarryWhit]

Hi all,

I've been working on reducing the power consumption of a project I'm working on using the STM32G431.

Stop modes are not an option on this chip since I need to keep some peripherals active, but I've been

able to reduce the needed system clock to below 1Mhz, and use the LP-Run/Sleep modes exclusively.

I've gone as far as I can in terms of reducing overall frequency. According to the CubeMx power estimator pane, I can  reduce the power consumption further by about 75% by avoiding the use of a crystal and supplying an external clock

signal instead. This so-called "HSE Bypass" mode powers down the oscillator circuit as well as the PLL. 

But I've run into a wall in implementing this. If I can't use a crystal, what do I put on the board to generate my (say, 1Mhz) clock? I don't have an FPGA or a big-brother CPU on the board to provide the clock. And to add one would negate the power savings several times over (to say nothing of the BOM cost). 

I've searched for dedicated clock generators, but what I've found so far seems to aim at much higher frequencies and still requires 10x the power used by the G431 to run.

Are there any jellybean chips/solutions for this? basically, I need an IC that generates a (say) 1Mhz clock while consuming only 10-100uA. 

[Edited for clarity]

[BarryWhit]

Yes, I now see that digikey also carries the SiT1579 MEMS Oscillator and its siblings, as Andrew.Neil suggested earlier.

[AScha.3]

If you can handle the case ... :)

maybe SIT8021AI-J4-XXS-1.162000  (digikey) might get you some uA less, about 100uA at 1,1MHz :

But its case : 1.5 mm x 0.8 mm , 0,41 mm pitch - only for reflow + pick'n place.

[Uwe Bonnes]

The difference between HSE external and HSI is whopping and much more than the expected 190 uA for HSI. Maybe someone at ST can explain.

[BarryWhit]

ST has replied to questions I posted following this discussion. That thread is 

Question: Power consumption and clock sources for low power modes in G4 family 

The gist is:

1. The power consumption associated with HSI is not 190us, because associated circuitry needed for using it raises that figure considerably.

2. The datasheet does provide information about the power consumption when using external crystals, but in a separate table, not as part of the run mode current consumption tables. 

3. the Power Consumption Calculator tool in CubeIDE is an extended version of the datasheet tables

4. HSE. Bypass (i.e external clock) does provides the lowest power consumption overall in Low-Power modes, assuming the external clock source doesn't use up much power (and others have identified suitable solutions for achieving this in this thread).

My thanks to everyone who offered advice and suggestions.

Update:

Not strictly about my question, but a recommended read for anyone working on low-power:

ST Post: Tips for using stm32 low power modes