ST AN3248 Application note

AN3248

Application note

Using STM32L1 analog comparators in application cases

Introduction

This document describes six application cases of the two analog comparators embedded in the ultra low power STM32L1 product line. The application cases are:

●Analog voltage monitoring

●Analog watchdog during Stop mode

●Pulse width measurement

●Pulse width modulation (PWM) signal control

●Capacitance measurement

●Brightness control using a light dependent resistor (LDR)

The six application cases demonstrate the usefulness of analog comparators and show how they are integrated with other peripherals, for example, the digital-to-analog-converter (DAC) and timers.

To ensure a quick start, four application cases presented in this document are implemented in C language and are available in Project\STM32L1xx_StdPeriph_Examples\COMP within the STM32L1xx_StdPeriph_Lib package.

Please note that this document is not intended to replace the routing interface (RI) and comparator sections in the product reference manual RM0038 (for STM32L1xx Ultra Low Power devices).

The peripheral power consumption should be consulted in the device datasheets.

February 2012

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Contents	AN3248

Contents

1	Analog voltage monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 4
2	Analog watchdog during Stop mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
3	Pulse width measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
4	PWM signal control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
5	Capacitance measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
6	Brightness control using a light dependent resistor
	(LDR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
7	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16

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AN3248	List of figures

List of figures

Figure 1. Sensor output connection to COMP2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 2. Sensor output connection to COMP1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 3. Power consumption in an analog voltage monitoring application . . . . . . . . . . . . . . . . . . . . . 5 Figure 4. COMP2 configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 5. Analog comparators combined in window mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 6. Analog watchdog during Stop mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 7. COMP2 with output redirection feature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 8. Pulse width measurement: COMP2 output redirection to timer . . . . . . . . . . . . . . . . . . . . . 10 Figure 9. PWM signal control: COMP2 output redirection to timer . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 10. RC network connection for capacitance measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 11. Capacitance measurement using COMP2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 12. Connecting an LDR resistor to an STM32L1 device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 13. Comparator output behavior versus light intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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Analog voltage monitoring	AN3248

1 Analog voltage monitoring

Ultra low power STM32L1 devices embed a 12-bit analog-to-digital converter (ADC) which is very fast with a sampling rate of 1 Msample/s. However, with a 1.45 mA typical consumption, it can jeopardize battery life time if left powered-on continuously. It is therefore recommended to use analog comparators in application cases when analog input voltage (sensor output) needs to be measured as soon as a pre-defined threshold is exceeded.

In STM32L1 devices, analog comparators are useful for monitoring the analog input voltage and powering on the ADC when it is required. While monitoring the analog voltage, the device can enter Stop mode at the same time that both comparators are still powered on. Consequently, better consumption is achieved and power is saved.

Note:

Analog comparators are powered by the internal reference voltage, VREFINT, which is still

powered on in Stop mode, and can be disabled by configuration. Once VREFINT is disabled,

the comparators can no longer be used.

In an analog voltage monitoring application, where the sensor output voltage is lower than

the threshold, the MCU remains in Stop mode thereby saving power. As soon as the sensor

output exceeds the threshold, the MCU is woken up, the ADC is powered on, and the analog

input voltage is measured. When the sensor output is under the threshold, the MCU re-

enters Stop mode.

Average power consumption is dramatically reduced when compared with an application

that continuously measures the analog voltage whatever the input value.

Figure 1 shows how to connect a sensor output (temperature sensor, pressure sensor,

pyroelectric infrared detector, photodiode sensor) to an STM32L1 device in an analog

voltage monitoring application using comparator 2 (COMP2). COMP2 monitors the analog

voltage in Stop mode while the ADC measures it in Run mode.

Figure 1. Sensor output connection to COMP2

STM32L1 device

Analog

Sensor

voltage

Amplifier (2)

ADC_CHx (1)

Filter (2)

COMP2_INP (1)

Ai17491

1.Legend for Figure 1 ADC_CHx: AC channel x

COMP2_INP: comparator 2 non-inverting input

2.Only if required.

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AN3248	Analog voltage monitoring

Figure 2 shows how to connect a sensor output to an STM32L1 device using comparator 1 (COMP1). COMP1 shares the same inputs as the ADC which reduces the number of required pins. Nevertheless, the threshold is fixed to VREFINT.

Figure 2. Sensor output connection to COMP1

						STM32L1 device
	Analog
Sensor	voltage	Amplifier (2)					COMP1_INP/
		Filter (2)					ADC_CHx (1)

Ai18734

1.Legend for Figure 2

COMP1_INP/ADC CHx: comparator 1 non-inverting input shared with ADC channel x

2.Only if required.

Figure 3 shows the gain in power consumption in an analog monitoring application.

Figure 3. Power consumption in an analog voltage monitoring application

Input analog voltage

Analog threshold

Time

MCU state

S	R	S	R	S	R	S
						Time

MCU current consumption

Few mA

Few μA

Time

ai17492

1.Legend for Figure 3 S: Stop mode

R: Run mode

The input analog voltage can be connected either to PB4 or PB5. The analog threshold can be provided internally through VREFINT and its submultiples or via an external pin through PB3. DAC channel 1 and channel 2 (DAC_OUT1 and DAC_OUT2 respectively) cannot be used in such application cases since the DAC channels are powered off in Stop mode.

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Analog voltage monitoring	AN3248

COMP2 wakes up the device from Stop mode through external interrupt line 22 (EXTI line 22).

Figure 4 shows the available configurations for inverting and non inverting inputs.

Figure 4. COMP2 configuration

	GR6		Wakeup
			EXTI line 22
	PB4	Input voltage	COMP2
	GR6		+
	PB5		CMP2OUT
		PB3	-
		DAC_OUT1
	Analog threshold:	DAC_OUT2
		VREFINT
	multiple sources
		3/4 VREFINT
		1/2 VREFINT
		1/4 VREFINT
			ai17493

1.Legend for Figure 4

DAC_OUT1: DAC channel 1 output DAC_OUT2: DAC channel 2 output

VREFINT: Internal reference voltage

CMP2OUT: Comparator 2 output (internal output)

When the device enters Stop mode, only COMP2 and the internal reference voltage, VREFINT, remain powered on.

Note: 1 Refer to the device datasheet for power consumption values.

2In High-density devices, pins PB6 and PB7 can be used also as COMP2 non-inverting inputs.

3If the analog threshold corresponds to the internal reference voltage, VREFINT (1.22 V), COMP1 can be used instead of COMP2 since it consumes much less power. In this case, the input analog voltage can be connected to any channel among the 24 ADC channels.

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