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stefanHri
stefanHri
Visitor II
March 28, 2019
Question

understanding the current consumption in run mode

  • March 28, 2019
  • 2 replies
  • 905 views

I am trying to understand the current consumption in run mode of STM32L063x8 reading the data sheet (DocID025660 Rev 7).

According to the cover page the current consumption of the MCU is 88uA/MHz in run mode and the maximal Frequency is 32MHz,

therefore the current consumption at the maximal frequency is I = 32MHz * 88uA/MHz = 2.816mA. 

What is the voltage for which the value of 88uA/MHz is given? Is the code corresponding to this value executed from FLASH? 

On page 58 in Table 28 the current consumption for code execution from FLASH at 32MHz is 6.3mA. This is more then twice of the calculated above 2.816mA.

How is this possible? According to section 6.3.4 (Supply current characteristics) all peripherals are off.

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2 replies

waclawek.jan
waclawek.jan
Super User
March 28, 2019

The frontpage is marketing, so it uses the most optimistic values, with no discussion whether they are applicable to real-world usage or not. This one may come from Table 29, performing while(1) at 4MHz at Vcore=1.2V. It's typ, so it's at VDD=VDDA=3.0V, "under ambient temperature" (presumably meaning room temperature, i.e. 25deg.C). Note, that would they use the same while(1) value from Tab.31 i.e. running from RAM, they would achieve an even slightly better value.

The pressure to get "nice numbers" onto the fronpage is huge, and the penalty for the "minor marketing lie" is only the outrage of the engineer which is usually constrained to his cubicle, so there's even an evolution in these:

0690X0000089BxYQAU.png

(Strangely, the lowest consumption at 32MHz does not come from while(1) but from Fibonacci!)

JW

waclawek.jan
waclawek.jan
Super User
March 28, 2019

Let's just have more fun with the "evolution" of frontpage matter... ;)

0690X0000089CDqQAM.png

Oh, so they added 2 SPI interfaces meantime? That's nice! Except that....

0690X0000089CMJQA2.png

"SPI mode of USART"... wait, what? Oh, you mean the *synchronous* mode of USART, that's where the S in USART comes from!

So this is a clear indication how marketing tries to wrap reality as far as it goes. It's nice of them they put up the "Up to" as a warning that lie may follow.

To make it clear, this is not all ST's fault, they are under pressure of the competition which does exactly the same. It's a big shame, but again, engineers are in these cases more and more often delegated to the role of the grumbling droid who has to hammer in the nails and can be neglected otherwise.

JW

PS. Not all changes are wrong there - the minimum working voltage of the comparators got corrected, typo corrected in "8x peripheral communication interfaces" and one remains in "Step-up converted on board" ;)

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