STM32L Cortex-M3 microcontroller for usage in low

STM32L Cortex-M3 microcontroller for usage in low
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Technical article
STM32L Cortex-M3 microcontroller for usage
in low-power healthcare applications
Changes in the structure of the populations of the Western civilization and changing habits
in Asian countries lead to increasing demands for portable diagnostic devices. On top of
that, the Net Generation is more willing to communicate with their physician through
electronic devices than previous generations where the only possibility was a personal visit
to the doctor. Fueled by the cost pressure in the public health sector, local diagnostic and
remote surveillance via internet have become a central point for cost reduction.
Beyond that, healthcare and medical applications have very specific needs in terms of
energy consumption, analog to digital converter (ADC) and digital to analog converter
(DAC), and require state-of-the art communication interfaces such as USB. Many devices
are battery-operated, but even if mains-operated they often feature a battery-powered
backup mode.
The STM32L ultra low power microcontroller family is the most recent member of
STMicroelectronics’ Cortex-M3 based family of STM32 microcontrollers. With an important
number of peripheral blocks it offers a high level of integration, leading to low system costs.
It pairs high processing power with low energy consumption and thus ideally matches the
requirements of this kind of equipment.
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STM32L brings integration to a new level
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STM32L brings integration to a new level
A typical portable medical application is usually having at least two subsystems:
●
an analog part for control and measurement of sensors
●
a digital part, covering analysis algorithms and display as the main functions, as well as
calibration and result storage, communication, time keeping and other functions (e.g.
power management)
Figure 1.
Glucose meter block diagram as a typical example for healthcare
applications
The proprietary mixed signal EEPROM technology used in the STM32L permits to integrate
many of the components on a single chip leading to a reduction of the bill of material, and
simplifying the power consumption and battery management.
The STM32L features a 24 channel 12-bit ADC and two 12-bit DACs. An external standard
voltage reference can be used, keeping a very good isolation between the noisier digital part
and the analog blocks.
The AD-converter with its very fast conversion time can handle all the measurements, from
the electrochemical or optical sensor to the temperature and power monitoring. With over
sampling techniques up to 14 bit ADC accuracy can reached thus meeting the most
demanding requirements in this domain.
The 12-bit DACs can be used to finely control the sensor bias and an internal data EEPROM
memory simplifies data logging. The integrated real time clock offers the same
performances as a high end stand-alone RTC.
A complete choice of communication ports is available, allowing digital communication with
the sensors via I²C and SPI as well as data transfer to a host through RS232, IrDA or USB.
The LCD glass controller features can directly drive up to 8x40 segments and significantly
reduces the need of external components due to an integrated step-up converter.
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STM32L – ultra low power architecture
STM32L – ultra low power architecture
Low power microcontroller applications are usually characterized by a continuous
alternation of
●
idle mode – with minimal power consumption and very low power continuous functions
(e.g. time keeping, low power LCD display) and
●
run mode – microcontroller is active, executes application tasks.
The STM32L allows reducing the overall energy consumption inside such an application by
1.
reducing the power in run mode
2.
reducing the power in sleep mode
3.
minimizing the time spent in run mode
Usually 1. and 3. are opposed. In order to reduce the time spent in run mode, a highly
performing CPU is needed with usually a higher consumption. The STM32L microcontroller
takes advantage of the following elements in order to overcome this contradiction.
STM32L CPU
The ARM Cortex-M3 CPU displays a very good energy to performance ratio to be
expressed in µA/DMIPS (Dhrystone Million Instructions Per Second). This value is more
representative for the energy needed to complete a software task than the µA/MHz which is
not accounting for the number of cycles per instruction or the efficiency of the instruction
itself. Here the 32-bit architecture outperforms 8- and 16-bit architectures that are eventually
better in µA/Mhz, but worse in µA/DMIPS.
Further time savings in run mode result from Cortex M3-specific low interrupt latency,
automatic context save and restore and tail-chaining (consecutive interrupts are executed
without extra context save and restore).
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Advanced low voltage technology and programmable LDO regulator
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Advanced low voltage technology and programmable LDO
regulator
One of the characteristics of CMOS technology is that in active mode, most energy is used
to charge and discharge a huge number of very small value capacitors intrinsic to the
technology. The energy for charging these capacitors is:
2
E = V core × Q = ( V core ) × C
where Vcore is the supply voltage of the digital part of the microcontroller and C is the sum of
the capacitance of all the active nodes in the circuit.
The advanced 130 nm technology of the STM32L, allows minimizing the individual nodes’
capacitance, the compactness of the Cortex-M3 core being beneficial for the total number of
nodes. In addition, clock gating allows cutting the dynamic consumption of unused parts of
the circuit.
An internal low dropout (LDO) regulator and voltage scaling contribute to reducing the Vcore.
The internal LDO ensures that the digital part is always supplied with the minimum required
voltage to ensure a certain performance. It can be programmed at three discrete voltage
levels. As it can be seen in Figure 2, the best current-to-performance ratio is obtained at
Vcore=1.2 V.
Figure 2.
Current consumption/processing performance and maximum CPU clock
frequency vs. digital core supply voltage
The idea is to keep the STM32L running at Vcore=1.2 V and to switch to a higher
performance range only when the microcontroller needs to perform a specific task in a
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Sleep mode power consumption
limited time. As both Vcore and clock speed are easily programmable, this allows the user to
dynamically adjust the energy vs. performance during runtime.
Sleep mode power consumption
The sleep mode, a low power idle mode is usually the longest period in a portable
application’s life. Even though the consumption in this mode is several orders of magnitude
lower than in run mode, it still can make a difference in the total energy consumption.
On the STM32L, a number of low power modes are implemented, allowing the application
designer to optimize his implementation according to the energy budget (i.e. battery size)
and application needs.
Table 1.
Mode
Typ.
consumption
at 25 °C
The low power modes of the STM32L
Active (run) Active (run)
from FLASH
from RAM
230 µA/MHz
186 µA/MHz
Low
Low
Power
Power run
sleep + 1
@ 32 kHz
timer
10,4 µA
6,1 µA
Stop
Standby
with/without
RTC
with/without
RTC
1,3 / 0,43 µA
1,0 / 0,27 µA
The ultra low power option of the used 130 nm technology together with low power modes of
the internal LDO lead finally to 430 nA typical consumption with the entire system
configuration retained, featuring a very fast start-up of few µs and no context loss.
Specific care for low power consumption has also been taken when designing the LCD
controller of the STM32L which is an important feature in portable healthcare applications. It
integrates a very low power step-up converter that allows keeping the display active for long
periods of time on a wide range of VDD values.
A/D converter optimized for low power
The resolution of the successive approximation converter is 12 bits, at a conversion speed
of 1 MSPS. The 24 single-ended inputs have a range of 0 to VREF+. The resolution can be
reduced to 10, 8 and 6 bits, allowing speeding up the conversion.
Apart from advanced features as conversion injection, multiple trigger sources, DMA,
selectable sampling time for each channel and auto calibration, it specifically supports the
low power operation of the application. The ADC is supplied from the high speed internal RC
oscillator and can thus run independently from the rest of the MCU which may be in power
down while the ADC is still converting. On the other hand, by entering power down
automatically after the programmed conversions, the ADC reduces the consumption to the
minimum required. At 32 kHz CPU clock for instance, an instruction may take 31 µs.
Assuming 5 ADC conversions at 1 µs, the ADC can go in power down after 5 µs before the
CPU command is even executed and thus minimize the power consumption.
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STM32L and USB in Medical Applications
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Applications, which require better resolution of acquired data, may use a technique, called
oversampling. The method is described in the STMicroelectronics’ application note AN2668.
The oversampling uses the inherent noise or artificially added signal (by e.g. triangular or
white-like noise modulation available from the integrated DACs) to the one being measured,
sampling of such signal with higher sampling rate and for instance fast averaging of the
data. This allows, to some extent, overcoming the limits of the quantization. The number of
samples required to extend the ADC resolution differs for the dithering signals: the linear
triangular voltage sweep across 1 LSB requires 2P+1-times higher sampling speed, while
white noise modulation requires 4P-times higher sampling speed, P being no. of additional
bits. Reasonable accuracy enhancement with the STM32L is possible up to 14 bits, still
keeping a high effective conversion rate and short active time of the ADC.
STM32L and USB in Medical Applications
The USB communication protocol is no longer reserved to classic PC peripherals and is
nowadays the most common wired channel for the next-generation UART in embedded
systems that are connected to a host PC and in some cases via this host to the internet.
The success of USB in medical applications has called for standardization and as a result
the dedicated device class called Personal Healthcare Device Class (PHDC) has been
defined through the USB- implementers’ forum. This new class enables health related
devices such as exercise watches, blood pressure monitors, thermometers, weighing
scales, glucose meters etc. to connect to a host and ease the communication between
individual and fitness coach or patient and doctor. The USB PHDC was the first wired
communication transport layer approved and adopted by the Continua Health alliance, an
open industry consortium with more than 200 member companies around the world with the
goal to harmonize the developments in the healthcare sector.
STMicroelectronics is member of Continua and provides the PHDC USB package with the
medical applications USB Stack on for the STM32L MCUs along with a complete set of USB
classes such as mass storage, human interface devices, audio and DFU for firmware
updates on the field. This stack (Figure 3) is based on USB PHDC and IEEE-11073
standards and enables communication between the device and the host according to
Continua standards.
Two example applications featuring, thermometer (IEEE-11073:10408) and glucose meter
(IEEE-11073:10417) are included to run out-of the box on STM32L152-EVAL evaluation
board connected to the Continua reference software (Continua Enabling Software Library)
emulated on a personal computer.
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Summary
Figure 3.
STM32L USB stack for healthcare devices. Mass storage and firmware
upgrade are optional.
This package from STMicroelectronics allows medical devices manufacturers to concentrate
on developing medical applications instead of implementing USB stacks and low-layer
firmware while gaining valuable time when going to certify their end-products with
recognized standards.
Summary
Due to a high level of integration, the STM32L allows a reduction of the bill of material in
healthcare applications, leading to a significant reduction of the system cost. Its consequent
low power architecture paired with high processing performance ideally matches the
requirements of this kind of equipment. STMicroelectronics supports the cost efficient and
rapid development of healthcare and medical devices with a complete set of development
tool and free of charge software libraries.
The usage of future-proof technologies and industry standards ensures a long term
availability of this microcontroller that is matching the development cycles and product life
times of this market segment.
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Revision history
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Revision history
Table 2.
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Document revision history
Date
Revision
01-Feb-2011
1
Changes
Initial release.
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