AN4635 datasheet(16/34 Pages) STMICROELECTRONICS

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Power consumption comparison

AN4635

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DocID027290 Rev 1

6.2

Short periods of Sleep mode and Low power run

While under most circumstances it is worthwhile using WFE to put the core to suspended

mode while waiting for a flag or event, it may also work against the effort of optimizing the

power consumption in some cases. When the core clock speed is just enough to perform

the requested operation, adding WFE instruction may actually increase the power

consumption a bit. Consider using the low power run regulator mode instead. The low power

run effect is terminated with each wakeup from either Stop or Sleep mode, but until that

wakeup it saves a lot of energy.

In Figure 3 the red line represents configuration where the execution in low power mode is

terminated by the code that goes to Sleep mode after each byte transmission. The green

line is a code that runs with voltage regulator set to range 3 and makes no attempts to use

Sleep mode during transmission. Finally the black line is the power consumption of a code

that stays in LPRUN until it goes to Stop mode after finishing the transmission. All three

codes then use the regulator range 3 along with Sleep mode after each byte during

reception phase. It is worth noting that in case of such a short time in Stop mode it would be

more efficient to leave the code in LPRUN all the time.

Figure 3. Sleep mode vs. LPRUN

Table 5. Configurations - Sleep mode vs. LPRUN

Curve in Figure 3

Black

Green

Red

Idle mode between

transmissions

Stop

JTAG interface

OFF

Current average

over phases 2, 3 and 4

7.67 µA

8.78 µA

9.36 µA

Clock

130 kHz MSI

Voltage regulator

LPRUN, then range 3

Range 3

AHB/APB ratio

1/1

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