AN3404
Application note
Low power considerations for STM8TL53xx devices
Introduction
This document is intended for touch sensing application designers who require an overview
of low power modes in the STM8TL53xx devices. It describes how to use the general
features of these devices in low power modes by explaining the differences between the
various modes. It focuses on how to reduce consumption when using the ProxSense
peripheral and demonstrates how this is managed by the STM8TL5x Touch Sensing Library
in addition to giving some code examples.
This application note is not intended to replace the STM8TL53xx datasheet. All values given
in this document are for guidance only. For guaranteed values, please refer to the
STM8TL53xx datasheet.
November 2011 Doc ID 018847 Rev 2 1/31
www.st.com
Contents AN3404
Contents
1 Power consumption factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 Internal supply structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3 Clock management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Clock system overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Default clock source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3 Clock configuration and power management . . . . . . . . . . . . . . . . . . . . . . . 8
3.4 Clock selection versus power consumption . . . . . . . . . . . . . . . . . . . . . . . . 8
4 Low power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1 Flash memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2 Overview of low power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3 Slowing down the clock frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4 Peripheral clock gating (PCG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.5 Execution from RAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.6 Wait mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.6.1 Entering Wait mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.6.2 Exiting Wait for interrupt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.6.3 Exiting Wait for event mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.7 Halt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.7.1 Entering Halt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.7.2 Exiting Halt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.8 Active-halt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.8.1 Entering Active-halt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.8.2 Exiting Active-halt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.9 Activation level control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5 General power management tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1 Choosing the optimal low power mode for your application . . . . . . . . . . . 16
5.2 GPIO initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.3 Dynamic control of pull-up resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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AN3404 Contents
5.4 Waiting loops/delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.5 Minimizing power consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6 ProxSense and low power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1 Possible CPU low power modes combined with ProxSense . . . . . . . . . . 19
6.2 Main factor of the ProxSense acquisition consumption . . . . . . . . . . . . . . 19
6.3 Low power features in the ProxSense peripheral . . . . . . . . . . . . . . . . . . . 20
6.3.1 LOW_POWER bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.3.2 Stabilization time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.3.3 Bias parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.3.4 Inactive state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.3.5 Receiver disabling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7 Low power mode management by the STM8TL5x Touch Sensing
Firmware Library 22
7.1 Configuration available in the stm8_tsl_conf.h file . . . . . . . . . . . . . . . . . . 22
7.1.1 Acquisition time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1.2 LOW_POWER bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1.3 Stabilization time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1.4 Bias parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1.5 Receiver configuration when disabled . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.1.6 Transmitter configuration when disabled . . . . . . . . . . . . . . . . . . . . . . . . 24
7.2 Practical code example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.2.1 Low power management with all acquisition banks . . . . . . . . . . . . . . . . 24
7.2.2 Very low power management with proximity detection . . . . . . . . . . . . . 25
8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Doc ID 018847 Rev 2 3/30
Power consumption factors AN3404
Dynamic power C V2f××=
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1 Power consumption factors
The STM8TL53xx microcontrollers are digital logic devices using the complementary metal
oxide semiconductor (CMOS) technology. In these type of devices, power consumption is a
sum of:
● Static power (mainly caused by transistor polarization and leakage)
● Dynamic power which depends on the supply voltage and the clock frequency
Dynamic power is calculated using Equation 1.
Equation 1
where:
● C is the CMOS load capacitance
● V is the supply voltage
● f is the clock frequency
Static consumption is negligible compared to dynamic consumption when the clock is
running. In some low power modes, when no clock is running, static consumption is the
main consumption source.
Total consumption is a sum of static and dynamic consumption as given by Equation 2.
Equation 2
where:
● I
● I
● I
is the supply current
DD
DynamicRun
Static
is the current consumption dependent on the CPU frequency
is the current consumption independent on the CPU frequency
4/30 Doc ID 018847 Rev 2
AN3404 Power consumption factors
Consequently, power consumption depends on:
● The microcontroller unit (MCU) chip size
This includes the technology used, the number of transistors, and the analog
features/peripherals embedded and used in the application.
● The MCU supply voltage
The amount of current used in CMOS logic is directly proportional to the square of the
power supply voltage (V²). Thus, power consumption may be reduced by lowering the
MCU supply voltage. This is less critical for STM8TL53xx devices than for other
microcontrollers, as an internal voltage regulator is used. However, the MCU supply
voltage could have an impact on the remaining components on the board.
● The clock frequency
Power consumption may be reduced by decreasing the clock frequency when fast
processing is not required by the application.
● The number of active peripherals or MCU features used (such as timers,
communication peripherals, watchdogs, ProxSense, etc.)
The greater the number of active peripherals or features, the greater the amount of
power consumed.
● The operating mode
Power consumption depends on which mode a particular application is running
(example: central processing unit (CPU) on/off, oscillator on/off). For an application
powered by a battery, the consumption is very important. Usually, the average
consumption should be below a certain target value to ensure an optimum battery
lifetime. This means that an application can consume more for short periods of time
and keep its average current consumption below the target value.
Doc ID 018847 Rev 2 5/30
Power supply AN3404
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The STM8TL53xx family embeds two regulators which provide a supply voltage (V
the core and internal peripherals.
Figure 1. Power supply overview
CORE
) for
1. If V
is lower than 1.8 V, the 1.8 V domain is supplied by the voltage on the VDD input. For low power
DD
modes where the low power voltage regulator (LPVR) is used, this domain is supplied by a 1.55 V voltage.
The main voltage regulator (MVR) provides a 1.8 V supply voltage. It has a high current
capability, as it can deliver up to 25 mA. However, the consumption of this regulator is higher
than the consumption of the LPVR. Consequently, the MVR is used during a standard
operation only.
The consumption of the LPVR is very low as required for low power modes. The LPVR can
deliver up to 200 µA, providing 1.55 V to the digital part of the MCU.
After reset, the MVR provides a supply voltage (V
microcontroller. Depending on the functional mode, the MVR can be switched off. In this
case, the LPVR continues to provide the V
the power-on reset/power-down reset (POR/PDR). This system ensures a proper startup
and reset of the MCU, while V
V
falls below the PDR threshold.
DD
6/30 Doc ID 018847 Rev 2
DD
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rises above the POR threshold. It resets the MCU when
) to the internal digital parts of the
CORE
voltage. The power supply is monitored by
AN3404 Power supply
2.1 Internal supply structure
STM8TL53xx devices operate from 1.65 V up to 3.6 V, when connected to one pair of supply
pins. There are no dedicated supply pins for the analog voltage domain. It is recommended
to use a decoupling ceramic capacitor placed close to the supply pins.
● In Run and Wait modes, both MVR and LPVR provide the V
●
In Halt/Active-halt modes, the LPVR is automatically used while the MVR is switched
off by the system in order to reduce current consumption.
CORE
Doc ID 018847 Rev 2 7/30
Clock management AN3404
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f 150×μA()MHz⁄[]215 μA[]+=
3 Clock management
3.1 Clock system overview
The 16 MHz high-speed internal RC oscillator (HSI) is the only clock source that can be
used to drive the system clock. The 38 kHz low-speed internal RC oscillator (LSI) is only
used to supply the auto-wakeup unit (AWU) and watchdog.
Each peripheral clock can be switched on or off independently, in order to optimize power
consumption when the peripheral is not used. This is done by using the peripheral clock
gating (PCG) feature. See the “Clock control (CLK)” section of the STM8TL53xx
microcontroller family reference manual (RM0312) for more details.
3.2 Default clock source
The default clock source after reset is HSI/8. The user can then switch the clock to different
frequencies by choosing the prescaler (/1, /2, /4 or /8) for the internal RC 16 MHz (HSI)
clock through the HSIDIV[1:0] bits in the Clock divider (CLK_CKDIVR) register.
3.3 Clock configuration and power management
In addition to the flexibility of the clock sources, different complementary clock
configurations and features are available to optimize the power consumption of the device:
● Each peripheral clock can be switched on/off through the Peripheral clock gating
registers 1 and 2 (CLK_PCKENRx).
● System clock dividers from 1 to 8 (HSIDIV[1:0] bits in the CLK_CKDIVR register) are
available.
Note: System clocks are used to supply both CPU and peripherals.
The STM8TL53xx is focused on low consumption. This is why all peripheral clocks are
gated by default. Before accessing any peripheral register, it is mandatory to enable the
clock for the given peripheral.
3.4 Clock selection versus power consumption
The selected clock type and speed is one of the major factors influencing power
consumption of the MCU (see Section 1: Power consumption factors). Total consumption for
the STM8TL53xx devices is given by Equation 3.
Equation 3
Note: The values given in Equation 3 are measured with all peripherals disabled.
8/30 Doc ID 018847 Rev 2
AN3404 Clock management
Slowing down the clock decreases the immediate consumption but, often this is not the ideal
solution. By slowing down the clock, the CPU performance is also reduced and a longer
time is required to perform an action or computation. If we consider the average
consumption, it might be better to use the highest available clock speed to perform the
required operation, and then force the MCU into one of the low power modes (like Activehalt mode) for the remaining time frame. This should be taken into account during the
design of application flowcharts.
Doc ID 018847 Rev 2 9/30
Low power modes AN3404
4 Low power modes
By default, after a reset, the microcontroller is in Run mode. The default CPU clock is 2 MHz
(HSI/8). Several low power modes are available to save power when the CPU is not used for
a standard operation (for instance: waiting for an external event). It is up to the user to select
the mode that gives the best compromise between low power consumption, short startup
time, good peripheral functionality, and availability of wakeup sources. Those power modes
are listed in
Power consumption in Run and Wait modes can be reduced by one of the following means:
● Slowing down the system clocks
● Executing code from RAM
● Gating the clocks to the peripherals when they are not used
4.1 Flash memory
On STM8TL53xx devices, the Flash is automatically switched off when Halt or Active-halt
mode is entered.
Section 4.2: Overview of low power modes.
4.2 Overview of low power modes
The STM8TL53xx devices feature the following main low power modes:
● Wait mode: CPU stopped and peripherals kept running
● Active-halt mode: CPU stopped; ProxSense (PXS), AWU, and independent watchdog
(IWDG) kept running if they are already enabled.
● Halt mode: CPU and peripheral clocks stopped
In all low power modes, the general purpose I/Os continue to actively drive outputs.
The MCU can be woken up from these low power modes by specific interrupts, including
through the ProxSense and incoming communications on the serial peripheral interface
(SPI), inter-integrated circuit (I2C), and/or universal synchronous/asynchronous receiver
transmitter (USART). Refer to the interrupt mapping table in the STM8TL53xx datasheet.
Ta bl e 1 lists the STM8TL53xx low power modes, shows the basic behavior of STM8TL53xx
devices, and demonstrates the influence of different low power modes on the CPU,
peripherals and consumption.
10/30 Doc ID 018847 Rev 2
AN3404 Low power modes
Table 1. Low power mode summary
Functions
Low power
modes
Wait
Active-halt
with
ProxSense
Active-halt
with AWU
Halt Off Off On 0.4 µA
1. Value during an acquisition (see the STM8TL53xx datasheet) with 1 transmitter and 1 receiver.
Entry
instruction
CPU Peripherals HSI LSI Flash RAM
WFE or
WFI
Halt Off
Off
Can be
enabled
On On On On 500 µA
4.3 Slowing down the clock frequency
In Run mode, choosing the clock frequency is very important to ensure the best compromise
between performance and consumption. The selection is done by programming the
prescaler registers. These registers can also be used to slow down the peripherals before
entering a low power mode.
Low power mode
consumptions
STM8TL53xx
typical values
@ 3 V/25°C
On Off On 600 µA
On Off On 1.0 µA
(1)
4.4 Peripheral clock gating (PCG)
Peripheral clock gating (PCG) can be used for additional power saving. This can be done at
any time by selectively enabling or disabling the clock connection to individual peripherals
through the CLK_PCKENRx registers. These settings are effective both in Run and Wait
modes.
Note: As all peripheral clocks are gated by default after reset on STM8TL53xx devices, it is
mandatory to enable the clock for a given peripheral before accessing any peripheral
register.
4.5 Execution from RAM
The code can be executed from RAM in order to save power consumption. However, due to
the difference between the size of the data and the instruction bus from Flash (32-bit) and
RAM (8-bit), performance is degraded when executed from RAM. It is important to take into
account the performance and the ratio between Run/Halt operations.
Figure 2 shows the different bus widths for Flash and RAM memories.
Doc ID 018847 Rev 2 11/30
Low power modes AN3404
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4.6 Wait mode
The STM8TL53xx devices support two different Wait modes: Wait for interrupt (WFI) and
Wait for event (WFE). Both modes are designed to reduce STM8TL53xx device power
consumption by switching off the core when it is not used. Wait mode is mainly used when
the STM8TL53xx is waiting for an external or internal event so that the program execution
can continue.
The polling loop on an associated peripheral flag can be efficiently replaced by a wait
instruction (WFI() or WFE()) after having correctly configured the interrupt related to this flag
or the event in the WFE register.
Wait mode can be combined with peripheral clock gating and a low-speed clock source to
further reduce the power consumption of the device.
Wait mode also offers the lowest wakeup time which is interesting for applications
requesting a fast response time.
4.6.1 Entering Wait mode
Wait mode is entered by executing the WFI or WFE assembly instruction. This stops the
CPU, but other peripherals and the interrupt controller can continue to run. When entering
Wait mode, the global interrupts are automatically enabled.
● Before entering WFI mode, at least one interrupt must be enabled.
● Before entering WFE mode, at least one event source must be enabled.
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AN3404 Low power modes
4.6.2 Exiting Wait for interrupt mode
When an internal or external interrupt request occurs, the CPU wakes up from Wait mode,
serves the interrupt routine and resumes processing.
The following list gives examples of peripherals or features with interrupts having exit-fromwait capability:
● ProxSense
● I2C
● USART
● SPI
● AWU
● External interrupt
● Timers
Refer to the STM8TL53xx reference manual (RM0312) for more details.
4.6.3 Exiting Wait for event mode
When an internal or external event request occurs, the CPU wakes up from Wait mode and
resumes processing.
The following list gives examples of peripherals or features with events having exit-from-wait
capability:
● ProxSense
● I2C
● USART
● SPI
● External interrupt
● Timers
If an interrupt occurs during Wait for event mode, the related interrupt service routine is
executed. After this routine, the MCU goes back to Wait for event mode.
Refer to the STM8TL53xx reference manual (RM0312) for more details.
4.7 Halt mode
In this mode the system clock is stopped. This means that the CPU and all the peripherals
requiring clocks are disabled, except for the following cases:
● The HSI clock is not stopped if used by the SWIM
● The system clock source is not stopped if a Flash/Data EEPROM write operation is in
progress.
● The LSI clock is not stopped if used by the SWIM, by the IWDG or if the “IWDG_HALT”
option bit is disabled.
In Halt mode, none of the peripherals is clocked and the digital part of the MCU consumes
almost no power.
Doc ID 018847 Rev 2 13/30
Low power modes AN3404
4.7.1 Entering Halt mode
The MCU enters Halt mode when a HALT instruction is executed. Before executing a HALT
instruction, the application must clear any pending peripheral interrupt by clearing the
interrupt pending bit in the corresponding peripheral configuration register. Otherwise, the
HALT instruction is executed but the MCU wakes up immediately and the program execution
continues.
4.7.2 Exiting Halt mode
Wakeup from Halt mode is triggered by an external interrupt. This wakeup is sourced by a
general purpose I/O port configured as an interrupt input or by an alternate function pin
capable of triggering a peripheral interrupt.
The system clock is then restarted at the last selected clock source before the system
enters Halt mode.
In an interrupt-based application, where most of the processing is done through the interrupt
routines, the main program may be suspended by setting the activation level bit (AL) in the
Global configuration (CFG_GCR) register. Refer to
page 15.
Section 4.9: Activation level control on
4.8 Active-halt
In Active-halt mode, the main oscillator, the CPU, and almost all peripherals are stopped.
Only the LSI RC oscillator runs to drive the SWIM and IWDG, if enabled.
4.8.1 Entering Active-halt mode
The user can enter this mode by a HALT instruction, once auto-wakeup (AWU) is enabled or
a ProxSense acquisition is ongoing
ProxSense acquisition must have started or the AWU must have been correctly configured
and enabled, before executing the HALT instruction.
4.8.2 Exiting Active-halt mode
Wakeup from Active-halt mode is triggered by an external interrupt, a ProxSense interrupt or
an AWU interrupt.
The system clock is then restarted at the last selected clock source before the system
enters Halt mode.
In an interrupt-based application, where most of the processing is done through the interrupt
routines, the main program may be suspended by setting the activation level bit (AL) in the
Global configuration (CFG_GCR) register. Refer to
.
Section 4.9: Activation level control.
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AN3404 Low power modes
4.9 Activation level control
STM8TL53xx devices support an automatic activation level control feature. Consequently,
the user can configure the MCU so that it directly returns to a low power mode after it has
been woken up from such a mode through interrupts.
In an interrupt-based application, where most of the processing is done through the interrupt
routines, the main program may be suspended by setting the activation level bit (AL) in the
CFG_GCR register. Setting the AL bit in the CFG_GCR register (see
level control on page 15) causes the CPU to return to Low power mode after exiting an
interrupt service routine without restoring the main execution context when the IRET
instruction is executed. This saves power by removing both the save/restore context activity
and the need for a main software loop execution for power management (in order to return
to low power mode).
To return to the main loop, the AL bit has to be cleared by software. This must be done at
least two clock cycles before
the IRET instruction is executed.
Section 4.9: Activation
Doc ID 018847 Rev 2 15/30
General power management tips AN3404
5 General power management tips
5.1 Choosing the optimal low power mode for your application
Different low power modes can be selected depending on your application:
● Applications powered by a battery where the MCU is in sleep mode most of the time:
– If the MCU is woken up due to external events, no time tracking is necessary and
power consumption has to be as low as possible. Halt mode is advised to extend
battery life as much as possible.
– Active-halt mode with AWU clocked by the LSI is advised if the application does
not only depend on external events, but needs a nonaccurate periodic wakeup.
● Applications powered by a battery where the MCU is awake most of the time:
– Active-halt mode is advised if the MCU has to perform a few periodic actions using
no peripheral except the ProxSense.
– Wait mode is advised if at least one peripheral should be enabled all the time and
an interrupt or event can wake up the MCU.
● Applications supplied by mains but where consumption is critical:
– Run mode, with a clock prescaler adapted to the application requirement, is
advised if the MCU has to run all the time and low power modes cannot be used.
5.2 GPIO initialization
By default, the I/Os are configured as floating input.
It is important to change the configuration of all I/Os that are not connected to defined logic
signals so as to obtain one of the following configurations:
● Input configuration with pull-up
● Output configuration with a low (or high) logic level
Otherwise, an increased consumption is generated by noise, as the internal Schmitt triggers
detecting this noise are toggling.
Floating I/Os could generate additional consumption in the range of a few 10 µA.
In 28-pin packages, the unbonded I/Os are connected to a defined level by factory option
configuration.
16/30 Doc ID 018847 Rev 2
AN3404 General power management tips
5.3 Dynamic control of pull-up resistor
Many applications have buttons which are used as a user interface. These buttons are
connected to I/Os that are connected to V
used in order to have a defined logic level on these I/Os when the button is off (not pressed).
Once the button is pressed, the I/O is grounded and it generates an additional current of ~
40 – 70 µA, drawn from the power supply. This current is negligible if the application is in
Run mode, but becomes very important for applications which run mainly in low power
modes.
For such low-power applications, the pull-up can be dynamically controlled. Once a button is
pressed, the related service routine is executed. This routine disables the pull-up and the
interrupt, in order to save consumption while the button is being pressed.
As the application has to check the button status regulary, the pull-up is enabled again
before any checks are made. If the button is continuously pressed, the process is repeated.
Once the button release is detected, the pull-up remains on.
For this task, an I/O can be left floating for a short time frame, given by a check period,
generating an extra consumption in the range of ~10 µA. Therefore, the total current
consumption for a button-press operation is lower than without such dynamic control.
once they are pressed. An internal pull-up is
SS
5.4 Waiting loops/delays
In some cases, the application has to wait for an input from the user, communication
peripheral, or another external unit.
If there is no useful code to execute while waiting for an input, it is recommended to avoid
using either a typical delay loop or a polling loop.
● Typical delay loop:
(delay = 0; delay < 0xFFFF; delay++)
_asm ("nop");
● Polling loop:
SPI_DR = data; // start communication
while (!( SPI_SR & SPI_SR_TXE)); // wait for TXE bit set
A Wait mode woken up by an interrupt on the SPI transmitter empty (SPI_SR_TXE flag) is
advised by the following sequence:
● Enable TXE interrupt:
SPI_ICR |= SPI_TXIE; // enable interrupt for Tx empty
An event is then generated after each byte is sent. The next data can be loaded in the Tx
buffer as follows:
SPI_DR = data; // start communication
_asm ("wfi"); //or wfi() if you include STM8TL53.h file in your
project
The sequence described above uses WFI mode to wait for Tx buffer empty. A delay loop can
be replaced by a Wait mode woken up by a programmed timer interrupt or better an Activehalt with AWU if no other peripheral is running. This can also be used for other
communication lines. For external sources (like interrupt from an I/O port), Halt mode can
also be considered.
Doc ID 018847 Rev 2 17/30
General power management tips AN3404
5.5 Minimizing power consumption
The following recommendations can help the user choose the right configurations to
minimize power consumption of the device in the application:
● Switch off all unused peripherals (peripherals are switched off by default) and use the
peripheral clock gating (PCG) feature (through the CLK_PCKENRx registers) to disable
the clock provided to the peripherals when not in use (these clocks are disabled by
default).
● All unused pins must be connected to a defined logic level. One option is to configure
them as output low level. Make sure there are no unused I/O pins configured as a
floating input. This needlessly leads to high consumption.
● Use Wait mode if external wakeup capability is needed in low power mode and if some
peripherals have to remain active.
● Use the appropriate V
power is consumed. Thanks to the internal regulator, which supplies internal structures
of the MCU, there is no major difference in the MCU consumption but, the difference
could be visible at the application level, for example, due to the current flowing through
the pull-up resistors out of the MCU.
● Use Active-halt and Halt modes as much as possible. To achieve this, shorten the time
spent in Run mode, for example, by using the highest clock speed in Run mode.
● When a low power mode cannot be used, use the minimum possible frequency for the
application. Select the frequency value that meets requirements.
● Use the dynamic control pull-up configuration if possible (see Section 5.3: Dynamic
control of pull-up resistor on page 17 ).
value for the application. The higher the VDD value, the more
DD
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AN3404 ProxSense and low power modes
6 ProxSense and low power modes
6.1 Possible CPU low power modes combined with ProxSense
Once a ProxSense acquisition is launched, the CPU and its clock are not needed to perform
the acquisition. Wait modes, entered by a WFE or a WFI instruction, can be used as well as
Active-halt mode by executing a HALT instruction. If WFE is used, either a PXS interrupt or
the PXS event must be enabled but, if WFI or HALT is used then a PXS interrupt must be
enabled. In most cases, the PXS end of completion interrupt is used.
In Active-halt, it is possible to combine the auto-wakeup with the ProxSense. The acquisition
can be time limited by configuring the maximum counter enable register (PXS_MAXENR)
and maximum counter value register (PXS_MAXR).
Table 2. ProxSense compatibility with low power modes
Low power mode Instruction Exit by PXS CPU HSI HSI_PXS SYNCHRO
(1)
Wait
Active-halt HALT Yes Off Off On No
Halt HALT No Off Off Off No
1. ‘SYNCHRO’ indicates compatibility with the synchronization feature.
2. Once in Active-halt, the synchronization edge cannot be detected by the ProxSense peripheral but, Activehalt mode can be entered once the acquisition is started. To synchronize with an external signal, the
acquisition must be launched from an external interrupt routine.
WFE Yes Off Off On Yes
WFI Yes Off Off On Yes
Activation level (AL bit set in CFG_GCR register) can be used in the case of successive
acquisitions. The AL bit should be reset once the last acquisition has been performed in
order to return to the main processing and take the appropraite action according to the
acquisition results.
6.2 Main factor of the ProxSense acquisition consumption
The best way to reduce consumption of the ProxSense acquisition itself is to decrease the
acquisition time. This can be done by setting the ProxSense in order to get a lower count
when the key is not touched. In the STM8TL5x Touch Sensing Library, this is done by
reducing the KEY_TARGET_REFERENCE parameter in the touch sensing configuration
file. This must be done carefully as reducing this count also reduces the sensitivity of the
touchkey. A trade-off must be found between the consumption and the sensitivity.
(2)
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ProxSense and low power modes AN3404
6.3 Low power features in the ProxSense peripheral
Power consumption can be reduced in different ways between ProxSense acquisitions. The
ProxSense can be switched off and its clock gated by resetting the PCKENR17 bit in the
CLK_PCKENR1. Consequently, the ProxSense has the following special low-power
features.
6.3.1 LOW_POWER bit
The most efficient way to reduce power consumption between two acquisitions is to set the
LOW_POWER bit in the PXS control register 1 (PXS_CR1). In this case, the ProxSense
clock (HSI_PXS) and the ProxSense regulator are switched off as soon as an acquisition is
completed and the results are still available in the PXS counter register of receiver channel n
(PXS_RXnCNTR).
By switching off the ProxSense regulator, a stabilization delay is introduced at the beginning
of each new acquisition. This delay time depends on the number of enabled PXS_RX pins.
For a few receivers, a 1.7 ms delay must be selected.
For successive acquisitions, it is recommended to reset the LOW_POWER bit and to set the
ProxSense interrupt to a high priority. This saves the stabilization time on the one hand and
greatly reduces the time between the acquisitions while the ProxSense clock and regulator
are set. The LOW_POWER bit must be set again once the last acquisition has been
launched. In this way, the regulator and the clock are switched off as soon as the last
acquisition is completed.
6.3.2 Stabilization time
The stabilization time is needed to ensure the regulator is fully operational at start-up. This
delay can be set at different values in the PXS control register 3 (PXS_CR3). The values
are: 1.7 ms (default), 500 µs or 120 µs.
The more the regulator is loaded, the faster the stabilization. In other words, launching an
acquisition with many receivers enabled speeds up the regulator stabilization. But, it is the
impedance of the receivers that is more important than their number. The best way to check
that the stabilization time is long enough, is to perform two successive acquisitions without
switching off the regulator between them, in ensuring that the ProxSense is off before the
first acquisition. If the results of the two acquisitions are similar, this means the regulator is
correctly stabilized before the first acquisition.
In case of successive acquisitions, in order to select the lowest stabilization time, the
acquisition with the maximum number of receivers must be launched first.
6.3.3 Bias parameter
This is the nominal bias current for the OpAmp which maintains the PXS_RXn pins at the
final (discharged) voltage while the transmit lines are transitioning (back) low. This function
is valid only for projected capacitance measurements. A higher bias is needed when the
projected capacitance (the capacitance between each Rx line and each Tx line) is high.
This parameter is set in PXS_CR3. The lower its value, the less the consumption.
20/30 Doc ID 018847 Rev 2
AN3404 ProxSense and low power modes
6.3.4 Inactive state
It is recommended to set the inactive state of the receivers and transmitters to VSS in order
to avoid driving noise on these lines This also allows consumption to be reduced through the
PXS_RX and PXS_TX pins compared to if they were configured in floating state.
6.3.5 Receiver disabling
Between acquisitions, if the ProxSense is not switched off by reseting the PXS_EN bit in the
PXS control register 1 (PXS_CR1) or by setting the LOW_POWER bit, there is a static
consumption which depends on the number of enabled receivers. To reduce this
consumption, it is worth disabling all the receivers by resetting the PXS receiver enable
register (PXS_RXENR).
This is particularly relevant for applications that need a fast response time and which cannot
use the LOW_POWER bit option due to the stabilization time.
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Low power mode management by the STM8TL5x Touch Sensing Firmware Library AN3404
7 Low power mode management by the STM8TL5x
Touch Sensing Firmware Library
7.1 Configuration available in the stm8_tsl_conf.h file
This section presents the parameters of the library, corresponding to the features presented
in
Section 6: ProxSense and low power modes.
7.1.1 Acquisition time
The acquisition time depends on the:
1. Frequency of the acquisition
2. Up and pass length
3. Average value targetted when there is no touch
Frequency of the acquisition
The frequency of the acquiqition is set in the HSI_PXS (the high-speed internal clock that is
dedicated to the analog ProxSense system). The possible frequencies are:
● 16 MHz (HSI_PXS 16000)
● 8 MHz (HSI_PXS 8000)
● 4 MHz (HSI_PXS 4000)
● 2 MHz (HSI_PXS 2000)
● 1 MHz (HSI_PXS 1000)
● 500 kHz (HSI_PXS 500)
● 250 kHz (HSI_PXS 250)
● 125kHz (HSI_PXS 125)
Up and pass length
Up and Pass are the two active phases of a ProxSense acquisition cycle. Their length must
be selected in order to ensure a correct charge and discharge of the projected capacitance
between each receiver/transmitter pair.
If the Up and Pass lengths are set for a value of less than five by UP_LENGTH and
PASS_LENGTH definitions, each phase lasts this value (UP_LENGTH/ PASS_LENGTH) +
0.5 cycles. For a value greater than or equal to five, each Up and Pass length is
2^(UP_LENGTH-2)+0.5 cycles and 2^(PASS_LENGTH-2)+0.5 cycles.
If the UP_LENGTH and PASS_LENGTH are equal, Ta b le 3 gives the correspondance
between the Up and Pass length value and the number of cycles of a complete phase (Up +
Pass + deadtimes).
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AN3404 Low power mode management by the STM8TL5x Touch Sensing Firmware Library
M
Table 3. ProxSense cycle length
UP_LENGTH and PASS_LENGTH value ProxSense cycle length (in HSI_PXS period)
14
26
38
410
518
634
766
Average value targetted when there is no touch
The average value targetted when there is no touch is set in the
KEY_TARGET_REFERENCE definition. This value defines how long the acquisition lasts. If
this value is set to 1000, the acquisition of a receiver set lasts approximately 1000 times the
ProxSense cycle length. The lower this value, the lower the consumption but, the lower the
key sensitivity.
7.1.2 LOW_POWER bit
The LOW_POWER bit is set if LOW_POWER_MODE is defined as 1.
The firmware library detects if several acquisitions are launched sucessively. In this case,
the LOW_POWER bit is reset until the last acquisition is launched. This avoids having to
wait for the stabilization time at each acquisition.
7.1.3 Stabilization time
The stabilization time is defined by the STAB definition. It can be set to:
● LONG_STAB (default stabilization time) for 1.7 ms
● MEDIUM_STAB for 500 µs
● SHORT_STAB for 250 µs
7.1.4 Bias parameter
The bias parameter is defined by the BIAS definition. It can be set to:
● HIGH_BIAS for 10 µA (default bias)
● MEDIUM_BIAS for 7.5 µA
● LOW_BIAS for 5µA
● VERY_LOW_BIAS for 2.5 µA
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Low power mode management by the STM8TL5x Touch Sensing Firmware Library AN3404
7.1.5 Receiver configuration when disabled
The receivers are configured, when disabled, according to the INACTIVE_RX definition. The
respective bit is reset in the receive enable register (PXS_RXENR). The INACTIVE_RX
definition can be defined as 0 to drive the receivers to ground or it can be set to 1 to keep
them floating.
When the receiver group A is selected, all group B receivers are configured according to the
INACTIVE_RX definition.
The receiver PXS_RX pins are fully dedicated to ProxSense acquisition, i.e. they cannot be
used for general purposes and are configured and driven according to the ProxSense
configuration even if they are not used by any of the acquisition banks.
7.1.6 Transmitter configuration when disabled
The transmitters, which are defined in an acquisition bank and not enabled in the Transmit
enable register (PXS_TXENR), are configured according to the INACTIVE_TX definition.
The INACTIVE_TX definition can be defined as 0 to drive the transmitters to ground or it can
be set to 1 to keep them floating.
The GPIOs which support the transmitter (PXS_TX) alternate function, but which do not
belong to any acquisition bank or acquisition transmitter definition (example, BANKxx_TX)
are not configured by the STM8TL5x Touch Sensing Library. It is the responsibility of the
application code developer to configure the GPIOs which are not used for ProxSense
acquisition.
7.2 Practical code example
Section 7.2 presents two code examples. Both are based on the STM8TL5x Touch Sensing
Library. The first example (see Section 7.2.1: Low power management with all acquisition
banks) is a general case using all the acquisition banks as defined by default. The second
example (see Section 7.2.2: Very low power management with proximity detection)
configures the system in a very low power mode using Active-halt mode, with the AWU and
ProxSense waiting for a proximity detection.
7.2.1 Low power management with all acquisition banks
This example focuses on the TSL_Action management and respects the following two
definitions in the stm8_tsl_conf.h file:
● #define AUTOMATIC_CHAINING (1)
● #define FAST_RESPONSE_TIME (1)
This example shows how to perform all bank acquisitions with a minimum number of calls to
TSL_Action and where the rest of the time is spent in low power mode. If a communication
does occur, the application should call TSL_Action() in order to start the pending acquisition
after which the acquisitions chain themselves. In this example, it is assumed that the
communication is managed by interrupt, the STM8TL53xx being slave in this
communication protocol.
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AN3404 Low power mode management by the STM8TL5x Touch Sensing Firmware Library
Code example 1
Clock_init();//Initialize the CPU clock (user code)
GPIO_Init(); //Initialize the General purpose I/Os(user code)
TSL_Init(); //Initialize the Touch Sensing library
ExtraCode_Init(); // other user initialization code
for (;;) //endless loop
{
/*
At this stage TSLState is always TSL_IDLE_STATE.
*/
TSL_Action();//switch from TSL_IDLE_STATE to TSL_ALLKEYS_PXS_ACQ_STATE
// with AcqState equals NO_ACQUISITION
/*
The following loop is used in case a communication occurs during
the acquisitions (communication being processed by interrupt)
If communication occurs, the Activation level bit must be reset
once finished in order to resume the processing
*/
do
{
TSL_Action();//Launch the acquisitions AcqState equals ACQUISITION_ON_GOING
CFG->GCR |= CFG_GCR_AL; // set Activation level to stay in low power
// till the last acquisition
wfi(); //halt() can also be used
}
while (AcqState != ALL_ACQUISITION_COMPLETED); // this test allows to relaunch
the acquisition in case a communication has stopped their chaining
/*
When the main resumes all the acquisition has been performed
at this stage, TLSState equals TSL_SCKEY_PXS_PROC_STATE (if SCKey defined)
or TSL_MCKEY_PXS_PROC_STATE(if only MCKeys are defined)
*/
TSL_Action();//perform the key processing (switch to TSL_ECS_STATE)
TSL_Action();// perform the environment change system
// (switch to TSL_IDLE_STATE)
ExtraCode_StateMachine(); //application processing
}
7.2.2 Very low power management with proximity detection
This example shows how to manage a proximity detection by scanning the key each 512
ms. This is done by alternating Active-halt mode with the AWU and Active-halt mode with
ProxSense. Once a proximity is detected, a scan of the touch keys is performed. This last
step is almost identical to the previous example in
with all acquisition banks.
Entry into Proximity detection mode is conditioned by the value of the “proxy_mode” variable
which is managed in a user function (ExtraCode_StateMachine() in this example). This
mode is exited by checking the STM8TL5x Touch Sensing Library global variable
“TSL_GlobalState.b.DETECTED”.
The response time depends on the scanning frequency but, also on the debounce value
which is defined by the “DetectionIntegrator” global variable. If needed, this variable can be
modified by the user application when entering Proximity detection mode and it can be
restored when exiting it (this variable is not modified in the example given below).
Doc ID 018847 Rev 2 25/30
Section 7.2.1: Low power management
Low power mode management by the STM8TL5x Touch Sensing Firmware Library AN3404
Code example 2
Clock_init();
GPIO_Init();
TSL_Init();
ExtraCode_Init();
AWU_Init(AWU_Timebase_512ms);
tick_delay = 512 * 2;
AcquisitionBankSorting[4] = 0; //Only the first four acquisition banks are
acquired, the fifth one being for proximity
for (;;) //endless loop
{
if (proxy_mode)
{
AcquisitionBankSorting[0] = 5; //Configure bank #5 to be acquired first
AcquisitionBankSorting[1] = 0; //Only the first acquisition is performed
while (proxy_mode)
{
TSL_Action();// switch from TSL_IDLE_STATE to TSL_ALLKEYS_PXS_ACQ_STATE
//only PXS and TIM4 are clocked
// This assumes there will be no communication while in low power
RestorePCKENR = CLK->PCKENR1;
CLK->PCKENR1 = CLK_PCKENR1_PXS + CLK_PCKENR1_TIM4;
//Launch the acquisition AcqState equals ACQUISITION_ON_GOING
TSL_Action();
halt(); //entering in Active-halt with wake-up by ProxSense
// PXS acquisition estimated between 2 and 2.5ms with 16MHz
// and UP/PASS_LENGTH = 3 for 1 bank
TSL_Timer_Adjust(5);
CLK->PCKENR1 = RestorePCKENR;
// When the main resumes all the acquisition has been performed.
// Here, TLSState equals TSL_SCKEY_PXS_PROC_STATE,if SCKey defined,
// or TSL_MCKEY_PXS_PROC_STATE, only if MCKeys are defined
TSL_Action(); //perform the key processing (switch to TSL_ECS_STATE)
TSL_Action(); //perform the environment change system
// The following loop is to insure no TSL_state is missed
// in order not to be stuck in halt()
while (TSLState != TSL_IDLE_STATE)
{
TSL_Action();
}
disablePXS(); // ProxSense is off to decrease consumption
CLK_PCKENR1 &= ~CLK_PCKENR1_PXS;
if (TSL_GlobalState.b.DETECTED != 0)
{
proxy_mode = 0; //to exit proxy mode as a detection occured
}
else
{
// Disable the unused peripheral clock,
// only AWU and TIM4 clock are kept on
RestorePCKENR = CLK->PCKENR1;
CLK->PCKENR1 = CLK_PCKENR1_AWU + CLK_PCKENR1_TIM4;
// with AcqState equals NO_ACQUISITION
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AN3404 Low power mode management by the STM8TL5x Touch Sensing Firmware Library
// Prepare AWU
AWU_Cmd(ENABLE);
halt(); // enter in Active-halt mode with Auto wake-up
TSL_Timer_Adjust(tick_delay);
/*Restore the peripheral clocks */
CLK->PCKENR1 = RestorePCKENR;
//restart ProxSense
CLK_PCKENR1 |= CLK_PCKENR1_PXS;
enablePXS();
AWU_Cmd(DISABLE);
}
}
//restore the acquisition bank table to perform all the bank processing
AcquisitionBankSorting[0] = 1;
AcquisitionBankSorting[1] = 2;
}
while (!proxy_mode)//This loop is similar to the first example
//some parts can be factorized in order to save code size
{
/*
At this stage TSLState is always TSL_IDLE_STATE.
*/
TSL_Action();//switch from TSL_IDLE_STATE to TSL_ALLKEYS_PXS_ACQ_STATE
// with AcqState equals NO_ACQUISITION
/*
The following loop is used in case a communication occurs during
the acquisitions (communication being processed by interrupt)
If communication occurs, the Activation level bit must be reset
once finished in order to resume the processing
*/
do
{
TSL_Action();//Launch the acquisitions AcqState equals ACQUISITION_ON_GOING
CFG->GCR |= CFG_GCR_AL; // set Activation level to stay in low power
// till the last acquisition
wfi(); //halt() can also be used
}
while (AcqState != ALL_ACQUISITION_COMPLETED); // this test allows to relaunch
the acquisition in case a communication has stopped their chaining
/*
When the main resumes all the acquisition has been performed
at this stage, TLSState equals TSL_SCKEY_PXS_PROC_STATE (if SCKey defined)
or TSL_MCKEY_PXS_PROC_STATE(if only MCKeys are defined)
*/
TSL_Action();//perform the key processing (switch to TSL_ECS_STATE)
TSL_Action();// perform the environment change system
// (switch to TSL_IDLE_STATE)
ExtraCode_StateMachine();//application processing
//define if the proxy mode must be entered
//according to user conditions
}
}
Doc ID 018847 Rev 2 27/30
Conclusion AN3404
8 Conclusion
The STM8TL53xx devices offer many possibilities for developing low consumption
applications. The user can take advantage of various low power modes like Wait, Active-halt
or Halt modes. He can also reduce consumption by switching off peripherals when they are
not used. Another option for reducing consumption is to optimize the ratio between Run and
Halt modes due to the good performance of the CPU itself.
The ProxSense can be used in Wait or Active-halt modes where its consumption depends
mainly on the acquisition duration and on the number of enabled receivers.
The most important principles of low power mode management are described in this
application note, including hints on how and when to use it.
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AN3404 Revision history
9 Revision history
Table 4. Document revision history
Date Revision Changes
08-Jul-2011 1 Initial release.
STM8TL53xx product name update
04-Nov-2011 2
Changed references to associated documents
Updated Figure 1: Power supply overview on page 6
Doc ID 018847 Rev 2 29/30
AN3404
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