ST AN3193 Application note
Introduction
AN3193
Application note
STM32L15x
ultralow power features overview
The STM32L15x product line belongs to the STMicroelectronics ultralow power
EnergyLite
performance Cortex
packages.
Both microcontroller families are based on the ST's proprietary 130 nm ultralow leakage
process and have many analog and digital peripherals in common, which eases the
transition from one architecture to the other and offers users the opportunity to capitalize on
the knowledge acquired on one platform.
This application note aims at describing the key low power features of the STM32L15x
family and explains their benefits for applications where energy consumption is a major
concern.
Important note: This document is not intended to replace STM32L datasheets. All values
given in this document are for guidance only. Please refer to the related datasheet to get
guaranteed values and up-to-date characterization data.
TM
platform and complements the 8-bit STM8L1xx family with 32-bit high-
TM
-M3 based microcontrollers offering an extended memory and bigger
April 2010 Doc ID 17369 Rev 1 1/14
www.st.com
Table of contents AN3193
Table of contents
1 STM32L main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Energy-efficient processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Numerous low power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4 A set of peripherals tailored for low power . . . . . . . . . . . . . . . . . . . . . . . 8
5 A versatile clock management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6 Ultra-safe supply monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2/14 Doc ID 17369 Rev 1
AN3193 STM32L main features
1 STM32L main features
Based on the solid foundations of the award-winning STM32F10x family, the STM32L
embeds various innovations which minimize the consumption in different configurations,
while maintaining most of the existing peripherals and a quasi pin-to-pin compatibility.
For a given manufacturing process and die area, the consumption of a microcontroller
largely depends on two factors which can be controlled dynamically: voltage and frequency.
In the STM32L devices, an internal low drop regulator supplies most of the logic circuitry
with a fixed voltage: this guarantees that consumption is kept minimal whatever the supply
voltage, along the lifetime of portable battery-supplied products, down to 1.65 V.
If we consider the clock sources, several cascaded clock prescalers, gating techniques and
peripheral-by-peripheral clock management allow only the necessary logic gates to be
activated, and at the adequate frequency. These are now design practices commonly used
for reducing the consumption in Run mode. For the STM32L, additional efforts have been
done in this direction with the implementation of voltage scaling to reach an even higher
processing efficiency.
However, all ultralow power requirements cannot be met by simply focusing on run time: for
most applications, the challenge is to spend the minimum time and energy in this mode and
find the adequate low power mode.
The improvements do not come only from the deep sleep modes optimized to eliminate
every ten nA of leakage. The system has also been complemented with seven low power
modes and a set of peripherals tuned for low power (such as the calendar real-time clock
and glass LCD controller). These items are described in more details herafter.
Doc ID 17369 Rev 1 3/14
Energy-efficient processing AN3193
2 Energy-efficient processing
The STM32L is built around the Cortex-M3, an industry standard 32-bit core, which has
been designed, among other criteria, for low power applications. The Cortex-M3 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-M3 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 M3 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 STM32L 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.
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AN3193 Energy-efficient processing
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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 "highperformance 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.
Doc ID 17369 Rev 1 5/14
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