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AN3193

A set of peripherals tailored for low power

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A set of peripherals tailored for low power

Several peripherals require special attention, either because of their intrinsic high

consumption or because they are always powered up.

The STM32L1xx embeds a 12-bit / 1 MSps ADC. This very fast but accurate converter can

jeopardize the battery lifetime if left powered-up continuously, with a 1.45 mA typical

consumption. With its calibration-free architecture, fast power-up time of 3.5 µs and a

conversion time of 1 µs, 6 conversions can be achieved within less than 10 µs, and the ADC

can be shut down immediately afterwards. If this is done at a 1-kHz rate, it represents not

more than 10.5 µA of the ADC related average consumption.

The time required to switch on and off a peripheral is negligible at high speed, but can

become significant for a low operating frequency: at 1 MHz, each instruction lasts as long as

the ADC conversion itself. When the ADC operates at such a low frequency, the conversion

time is also significantly increased without benefits for the end application. To reduce the

consumption and allow the most efficient use of this fast converter, the ADC digital interface

has been designed to operate in a completely independent manner, at its maximum speed,

whatever the CPU operating frequency (which can range from “sub-kHz” up to 32 MHz),

using an internal 16 MHz clock source. It also includes auto shut-down modes to reduce the

software overhead and to cope with a very low CPU frequency. At 32 kHz, each instruction

lasts 30 µs: a burst of conversion can be launched between two cycles, with the ADC

powered-off automatically, so that the ADC extra consumption can be limited to the

necessary time only (for single conversions, during 4.5 µs out of the 30 µs CPU period).

Three peripherals have been developed to operate continuously even in Stop mode, where

the system clock is stopped, with the main oscillator and memory powered down.



A pair of ultralow comparators is available to monitor analog voltages with a current

down to 3 µA. These comparators can wake up the MCU as soon as the external

voltage reaches the selected threshold and they can be combined together to provide a

window comparator. One of these comparators has a rail-to-rail input capability and its

output can be redirected to a timer for a general purpose use.



An RTC peripheral provides a clock/calendar with two alarms, includes a periodic

wake-up unit and several application specific functions (timestamp, tamper detection

…). It can remain enabled in the lowest power mode (standby) where most of the chip

is powered down, and wake up the full MCU circuitry in case of an alarm or tamper

detection for instance. It also contains 80 bytes of backup registers to store contextual

information when exiting from standby mode. This peripheral has been designed using

asynchronous design techniques to minimize its consumption (below 1 µA).



The glass LCD is one of the most common displays in low power applications, because

of its inherently low current consumption, low price and customization easiness. The

STM32L1xx includes a versatile LCD controller, which can drive displays with up to 8

common lines and 40 segments, with the capability of selecting individually the I/O

ports assigned to the LCD for an optimal use of the chip alternate functions. It also

controls an optional internal step-up converter to maintain the LCD contrast on a wide

included).