Dynamic Voltage-Frequency Scaling table for a microcontroller

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acolin
 
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Dynamic Voltage-Frequency Scaling table for a microcontroller

Post by acolin »

Not AdaFruit specific, just a general EE question. Why would a datasheet [1] for a microcontroller specify a range for supply voltage (1.8-3.6v) and a set of frequencies (2-138MHz) but never mention what is the minimum required voltage for each frequency? Is it up to the customer to find this out somehow? The lack of this information doesn't imply the chip can run at max freq at min voltage, does it?

The datasheet even provides current measurements for different frequencies but all at the same voltage (3.6v).

DVFS tables for some SoCs can be found in Linux kernel code (released with a corresponding consumer device), but in the case of retailed microcontrollers, why would this information not be part of the datasheet?

[1] http://www.st.com/st-web-ui/static/acti ... 037051.pdf

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brucef
 
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Re: Dynamic Voltage-Frequency Scaling table for a microcontroller

Post by brucef »

Looks to me like the answer is "it's complicated."

Table 15 on page 79 is probably where you want to start.

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adafruit_support_rick
 
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Re: Dynamic Voltage-Frequency Scaling table for a microcontroller

Post by adafruit_support_rick »

Have a look at section 2.2.16 Voltage Regulator:
Regulator ON:
There are three power modes configured by software when regulator is ON:
• MR is used in the nominal regulation mode (With different voltage scaling in Run)
In Main regulator mode (MR mode), different voltage scaling are provided to reach the
best compromise between maximum frequency and dynamic power consumption.
Refer to Table 14: General operating conditions.
...
and
Regulator OFF

Since the internal voltage scaling is not manage internally, the external voltage value must
be aligned with the targetted maximum frequency. Refer to Table 14: General operating
conditions.
I read this to say that, with the VR on, the frequency/voltage matching happens automatically. With the VR off, it's up to you. Table 14 is on page 77.

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acolin
 
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Re: Dynamic Voltage-Frequency Scaling table for a microcontroller

Post by acolin »

Thank you. Yes, I've been trying to understand all of the above places in the datasheet. Turned out that the reference manual [1] sheds some light on this. Looks like there is indeed automatic voltage scaling within one of two scales selectable by the user. From page 67:
4.1.3 Voltage regulator
An embedded linear voltage regulator supplies all the digital circuitries except for the backup
domain and the Standby circuitry. The regulator output voltage is around 1.2 V.
...
In Run mode, the regulator supplies full power to the 1.2 V domain (core, memories
and digital peripherals). In this mode, the regulator output voltage (around 1.2 V) can
be scaled by software to different voltage values (scale 1 or scale 2 configured through
the VOS bit of the PWR_CR register). The voltage scaling allows optimizing the power
consumption when the device is clocked below the maximum system frequency (see
Section 4.4.1: PWR power control register (PWR_CR).
....
Bit 14 VOS: Regulator voltage scaling output selection
This bit controls the main internal voltage regulator output voltage to achieve a tradeoff
between performance and power consumption when the device does not operate at the
maximum frequency (refer to the datasheets for more details).
0: Scale 2 mode
1: Scale 1 mode (default value at reset)
and from Table 14 (page 78) of datasheet:
Regulator ON: 1.2 V internal voltage on VCAP_1/VCAP_2 pins
Max frequency 144MHz, VOS bit in PWR_CR register = 0 : 1.08-1.20v
Max frequency 168MHz, VOS bit in PWR_CR register= 1 : 1.20-1.32v
Regulator OFF: 1.2 V external voltage must be supplied from external regulator
on VCAP_1/VCAP_2 pins
Max frequency 144MHz : 1.10-1.20v
Max frequency 168MHz : 1.20-1.30v
While the mapping table is hidden from us, it seems measurable by setting clock rates and looking at the voltage at Vcap1/Vcap2 pins, where external capacitors for the built-in LDO are supposed to be connected, and should lead to the output of the regulator. I'll try if I get a hold of the device. Others have suggested that the mapping is hidden because it is a trade secret and that a linear interpolation of the voltage range could be a rough guess.

[1] http://www.st.com/st-web-ui/static/acti ... 031020.pdf

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