Energy-efficient processing

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Energy-efficient processing

The STM32L0 is built around the Cortex-M0+, an industry standard 32-bit core, which has

been designed, among other criteria, for low power applications.

The Cortex-M0+ offers a class-leading performance and code density. Although

performance is not naturally linked with low current consumption, it is a key benefit for most

of the low power applications which have to wake up periodically to execute software tasks.

In this case, the Cortex-M0+ spends less time in Run mode due to its processing

performance, thus maximizing the time in deep sleep mode. If we consider only the

processing consumption, expressed in mA/DMIPS (DMIPS standing for Dhrystone MIPS

measured using the public benchmark Rev 2.0), the performance of the Cortex M0+ is

significantly better than that of the other architectures, in particular 16-bit microcontrollers.

The performance in DMIPS/MHz being given by the core and its memory interface, the

processing consumption in mA/DMIPS can be maximized using voltage scaling. This

method, also called undervolting, consists of adapting dynamically the supply voltage of the

internal logic with the operating frequency. The STM32L0xx offers three dynamically

selectable voltage ranges, as summarized in the following figure, from 1.8 V (range 1) down

to 1.2 V (range 3), which offers a gain of more than 25% in terms of consumption.

Figure 1. STM32L0xx performance versus VDD and VCORE range

A typical example is portable healthcare equipment with USB device capability.

As long as it works in standalone mode, 4 MHz are sufficient to acquire and process the

data from the analog front-end. In this case, the internal logic can be supplied with 1.2 V

only.

However, executing a USB software stack when the system is connected to the USB

interface of a PC requires more processing power: in this case, the device can be placed in

"high-performance mode", where the internal voltage is 1.8 V. It can then execute code at 32

MHz while the USB peripheral is supplied by a 48-MHz clock. Voltage scaling is used to deal

with the contradictory requirements of these two operating modes without having to

compromise on the dynamic current consumption.