ST AN3147 Application note

AN3147

Application note

STM8L family power management

Introduction

This document is intended for application designers who require a hardware implementation overview of low power modes in the STM8L product families. It shows how to use the STM8L devices in these modes, explains differences between different modes and gives several examples of how to select the best mode for your application needs.

This document is not intended to replace STM8L datasheets. All values given in this document are for guidance only. Please, refer to the related datasheet to get guaranteed values.

March 2010

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Contents	AN3147

Contents

1	Power consumption factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		4
2	Power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
	2.1	Internal supply structure for STM8L101xx devices . . . . . . . . . . . . . . . . . .	5
	2.2	Internal supply structure for STM8L15x devices . . . . . . . . . . . . . . . . . . . .	6
		2.2.1 Brownout detector (BOR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
3	Clock management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7

3.1 STM8L101xx clock system overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2 STM8L15x clock system overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.3 Clock configuration and power management . . . . . . . . . . . . . . . . . . . . . . . 9 3.4 Clock selection vs. power consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4	Run and low power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10
	4.1	Flash memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
	4.2	Overview of low power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10

4.2.1 STM8L101xx low power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.2.2 STM8L15x low power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.3 Slowing down the clock frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.4 Peripheral clock gating (PCG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.5 Execution from RAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.6 Wait mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.6.1 Entering Wait mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.6.2 Exiting Wait for interrupt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.6.3 Exiting Wait for event mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.7 Low power run mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.7.1 Entering Low power run mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.7.2 Exiting Low power run mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.8 Low power wait mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.9 Halt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.9.1 Entering Halt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.9.2 Exiting Halt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.9.3 Active-halt mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

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AN3147			Contents
	4.10	Activation level control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . . 16
5	Power management tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. . . . 17
	5.1	Choosing the optimal low-power mode for your application . . . . . . .	. . . . 17
	5.2	GPIO initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . 17
	5.3	Dynamic control of pull-up resistor . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . 17
	5.4	Waiting loops / delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . 18
	5.5	Minimizing power consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . 19
6	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. . . . 20
7	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. . . . 21

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Power consumption factors	AN3147

1 Power consumption factors

The STM8L microcontrollers are digital logic devices using the complementary metal oxide semiconductor (CMOS) technology. In this 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. It is calculated from the formula: Dynamic power = C x V² x f, 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

IDD = f × IDynamicRun[ μA ⁄ MHz] + IStatic[ μA]

Consequently, power consumption depends on:

●The microcontroller unit (MCU) chip size: technology used, number of transistors, 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 squared power supply voltage (V²). Thus, power consumption may be reduced by lowering the MCU supply voltage. This is less critical for STM8L devices than for other microcontrollers, as an internal voltage regulator is used but this could have an influence for 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 used MCU features (CSS, BOR, PVD,...): the greater the number of active peripherals or features, the greater the amount of consumed power.

●The operating mode: Power consumption depends on which mode a particular application is running (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.

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AN3147	Power supply

2 Power supply

The STM8L family embeds two regulators which provide a supply voltage (VCORE) for the core and internal peripherals.

Figure 1. Power supply overview

										STM8 MCU Core
VDD				1.65 V-3.6 V				1.8 V domain 1)		CPU
						Main voltage regulator
						(MVR)				RAM
										Flash
						Low Power voltage regulator
						(LPVR)
						I/Os

1. If VDD is lower than 1.8 V, the 1.8 V domain is supplied by the voltage on the VDD input. For low power

	modes where the 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 own consumption of this regulator is
	higher than the own consumption of the LPVR. Consequently, the MVR is used during a
	standard operation only.
	The own consumption of the low power regulator (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 (VCORE) to the internal digital parts of the
	microcontroller. Depending on the functional mode, the MVR can be switched off. In this
	case, the LPVR continues to provide the VCORE voltage.
	The power supply is monitored by the POR/PDR (power-on reset / power-down reset) and
	and BOR (brown-out reset) system. This system ensures a proper startup and reset of the
	MCU, while VDD is rising above the POR threshold. It resets the MCU when VDD is falling
	below the PDR threshold.
Note:	Analog peripherals require a power supply voltage above the PDR threshold (VDD > 1.8 V,
	while VPDR = 1.5 V). In this case, it is recommended to use the BOR (Brownout reset) to
	guarantee a proper functionality.
	The BOR is available on STM8L15x devices only.

2.1Internal supply structure for STM8L101xx devices

The STM8L101xx devices are able to operate from 1.65 V up to 3.6 V, connected to one pair of supply pins.

There are no dedicated supply pins for the analog voltage domain. It is recommended to use

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Power supply	AN3147

a decoupling ceramic capacitor placed close to the supply pins.

●In Run and Wait modes, both MVR and LPVR provide the VCORE.

●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.

2.2Internal supply structure for STM8L15x devices

STM8L15x devices require a supply voltage of 1.8 V to 3.6 V. Once the supply voltage reaches 1.8 V, the MCU can work down to 1.65 V with a limitation in the functionality of analog peripherals.

The medium density STM8L15x devices have several supply pins, depending on the package pincount. In addition to the main supply pair VSS/VDD, another supply pin (VDDA) could be present on the package.

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 VCORE.

●In Low power run mode and Low power wait mode, the MVR has to be switched off. Only the LPVR provides the VCORE.

●In Active-halt mode, during the Halt phase, either the MVR or the LPVR can provide the VCORE. The user can select the regulator to be used.

●In Halt mode, the LPVR is automatically used, once MVR is switched off.

2.2.1Brownout detector (BOR)

	The brownout detector is used to guarantee a proper functionality of all peripherals, mainly
	analog ones. By default, this detector is enabled but the user can disable it through option
	byte to decrease the MCU consumption by ~2.8 µA.
	The BOR is mandatory to ensure a proper functionality of the product at power-down, to
	cover a supply voltage area between VDD min. and VPDR.
Note:	The BOR is available on STM8L15x devices only.

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3 Clock management

3.1STM8L101xx 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” section of the STM8L101xx reference manual (RM0013) for more details.

3.2STM8L15x clock system overview

Several different clock sources can be used to drive the system clock, depending on the product and package you use:

●1-16 MHz high speed external clock from the crystal oscillator (HSE crystal)

●Up to 16 MHz high speed external clock provided by the user (HSE bypass)

●16 MHz high speed internal RC oscillator (HSI)

●32.768 kHz low speed external crystal oscillator (LSE)

●38 kHz low speed internal RC oscillator (LSI)

Each clock source can be switched on or off, depending on the actual needs of the application. Each peripheral clock can be switched on or off independently, in order to optimize power consumption when it is not used. This is done by using the peripheral clock gating (PCG) feature. See the ‘Clock control’ section in the STM8L15x reference manual (RM0031) for more details.

Several clock sources can be enabled in one time. The following table shows the main features of each clock source. STM8L devices offer a complete range of clock sources to meet customer application requirements in terms of cost, accuracy and consumption.

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