Getting Started with AC - Microchip Technologyww1.microchip.com/downloads/en/AppNotes/TB3211-Getting-Starte… · Figure 3-4. VREF.CTRLA - AC Voltage Reference Selection b A value

TB3211 Getting Started with AC

Introduction

Author: Cristian Pop, Microchip Technology Inc.

Microchip tinyAVR® 0- and 1-series, and megaAVR® 0-series devices features an Analog Comparator(AC) with flexible input selection, selectable hysteresis and configurable output (interrupts on rising/falling or both edges, event generation, and output inversion).

This technical brief explains the AC concepts and its implementation in tinyAVR® 0- and 1-series, andmegaAVR® 0-series devices with the following use cases:

• Level Crossing DetectorThis example shows how to use the AC to detect when a critical value of an analog signal is reached(for example, the battery level)

• Preventing False Spike DetectionExemplify how to use the hysteresis feature to minimize the number of false transitions in a noisyenvironment

• Analog Signal Pulse Duration/Frequency MeasurementThe example describes how to use the AC together with a timer to measure the pulse duration and/orthe period of analog signals with minimal intervention of AVR core

Note: The code examples were developed using ATmega4809 XPlained Pro (ATMEGA4809-XPRO).

© 2018 Microchip Technology Inc. DS90003211A-page 1

Table of Contents

Introduction......................................................................................................................1

1. Relevant Devices.......................................................................................................31.1. tinyAVR® 0-series......................................................................................................................... 31.2. tinyAVR® 1-series......................................................................................................................... 31.3. megaAVR® 0-series......................................................................................................................4

2. Overview....................................................................................................................5

3. Level Crossing Detector............................................................................................ 6

4. Preventing False Spike Detection............................................................................11

5. Analog Signal Pulse Duration/Frequency Measurements....................................... 12

6. References.............................................................................................................. 14

7. Appendix..................................................................................................................15

The Microchip Web Site................................................................................................ 19

Customer Change Notification Service..........................................................................19

Customer Support......................................................................................................... 19

Microchip Devices Code Protection Feature................................................................. 19

Legal Notice...................................................................................................................20

Trademarks................................................................................................................... 20

Quality Management System Certified by DNV.............................................................21

Worldwide Sales and Service........................................................................................22

TB3211


1. Relevant DevicesThis chapter lists the relevant devices for this document.

1.1 tinyAVR® 0-seriesThe figure below shows the tinyAVR 0-series, laying out pin count variants and memory sizes:

• Vertical migration is possible without code modification, as these devices are fully pin- and featurecompatible.

• Horizontal migration to the left reduces the pin count and, therefore, the available features.

Figure 1-1. tinyAVR® 0-series Overview

8 14 20 24Pins

Flash

ATtiny1607

ATtiny807

ATtiny1606

ATtiny806

ATtiny1604

ATtiny804

ATtiny402

ATtiny202

ATtiny404

ATtiny204

ATtiny406

32 KB

16 KB

8 KB

4 KB

2 KB

devices ATtiny~~ATtiny~~Legend:

common data sheet

Devices with different Flash memory size typically also have different SRAM and EEPROM.

1.2 tinyAVR® 1-seriesThe figure below shows the tinyAVR® 1-series devices, laying out pin count variants and memory sizes:

• Vertical migration upwards is possible without code modification, as these devices are pin compatibleand provide the same or more features. Downward migration may require code modification due tofewer available instances of some peripherals.

• Horizontal migration to the left reduces the pin count and therefore, the available features.

TB3211Relevant Devices


Figure 1-2. tinyAVR® 1-series Overview

48 KB

32 KB

16 KB

8 KB

4 KB

2 KB

8 14 20 24Pins

Flash

ATtiny816 ATtiny817ATtiny814

ATtiny417

ATtiny1616 ATtiny1617

ATtiny414 ATtiny416ATtiny412

ATtiny214ATtiny212

ATtiny1614

ATtiny3216 ATtiny3217

devicesATtiny~~

ATtiny~~Legend:

common data sheet


1.3 megaAVR® 0-seriesThe figure below shows the megaAVR 0-series devices, laying out pin count variants and memory sizes:

• Vertical migration is possible without code modification, as these devices are fully pin and featurecompatible.

• Horizontal migration to the left reduces the pin count and, therefore, the available features.

Figure 1-3. megaAVR® 0-series Overview

48 KB

32 KB

16 KB

8 KB

28/32 48Pins

Flash

ATmega3208

ATmega4808

ATmega3209

ATmega4809

ATmega808

ATmega1608 ATmega1609

ATmega809


TB3211Relevant Devices


2. OverviewThe AC is a module that compares two analog input voltages and outputs a signal level indicating whenone of the inputs is higher than the other. An AC is basically an amplifier without feedback and thus hasvery high gain.Figure 2-1. Analog Comparator Block Diagram

+

AC-

Hys

tere

sis

AINN0

AINNn

Inve

rt

Enab

le

AINP0

Controllerlogic

OUT

Voltagedivider

AINPn

...

...

Event out

CTRLAMUXCTRL

CMP(Int. Req)

DACREF

VREF

Figure 2-1 shows the block diagram of the analog comparator in tinyAVR® 0- and 1-series, andmegaAVR® 0-series devices. It compares the voltage levels on two inputs and gives a digital outputbased on this comparison.

The dynamic behavior of the AC can be adjusted by a hysteresis feature. The hysteresis can becustomized to optimize the operation for each application. The input selection includes analog port pinsand internally generated inputs.

The comparator has one positive input and one negative input. The positive input may be chosen from aselection of analog input pins. The negative input may be chosen from a selection of analog input pins orinternal inputs, such as a band gap reference voltage (DACREF). The digital output from the comparatoris ‘1’ when the difference between the positive and the negative voltage is positive, and ‘0’ otherwise.The AC can be configured to generate interrupt requests and/or events upon several differentcombinations of input change. The AC output state can also be the output on a pin for use by externaldevices.

TB3211Overview


3. Level Crossing DetectorThis example shows a basic initialization and set up for the AC module. The comparator can be used tomonitor battery voltage (or any other DC level), and to notify the application/user via interrupt and outputpin.Figure 3-1. Analog Comparator as Voltage Monitor

DACREFVREF

VBAT

V1

AINP0[PD2]

+

-

OUT[PA7]

To interrupt system

0.8V

To monitor an external voltage, the AC input should be connected to this voltage using an I/O pin. Thispin needs to have the digital input buffer and the pull-up resistor disabled, in order to have the highestpossible input impedance. For the ATmega4809, PORTD pin 2 (PD2/AINP0) is used as AC positive input:Figure 3-2. AC Positive Input

QFN48/TQFP48

Pin name (1,2) Special ADC0 AC0

20 PD0 AIN0

21 PD1 AIN1 P3

22 PD2 AIN2 P0

23 PD3 AIN3 N0

This translates to the following code:PORTD.PIN2CTRL = PORT_ISC_INPUT_DISABLE_gc;

In the selected application, the AC uses the analog pin as positive input and an internal reference for itsnegative input. The Voltage Reference module (VREF) should be configured prior to using the internalvoltage reference on negative input.

TB3211Level Crossing Detector


Figure 3-3. VREF Block Diagram

Reference se lect

Bandgap ReferenceGenerator

InternalReferenceBUF

1.1V1.5V2.5V4.3V

0.55V

Reference enable

Reference request

Bandgapenable

The VREF provides control registers for selecting between multiple internal reference levels. The internalreferences are generated from an internal band gap.Figure 3-4. VREF.CTRLA - AC Voltage Reference Selection

b

A value of 1.5 volts is selected as reference voltage:VREF.CTRLA = VREF_AC0REFSEL_1V5_gc;

To enable VREF voltage generation, the output buffer must be enabled after selection of reference voltage.This can be done by setting the AC0REFEN bit from the VREF Control B register:



Figure 3-5. VREF.CTRLB - Enable AC0REFEN bit

b

This translates to the following code:VREF.CTRLB = VREF_ADC0REFEN_bm;

After configuring the peripherals and the modules required by the AC, the selection of inputs is doneusing MUXCTRLA register as follows:Figure 3-6. AC Input Selection

b

b

The positive input pin 0 (AINP0) and internal DAC reference are used as AC inputs:AC0.MUXCTRLA = AC_MUXPOS_PIN0_gc | AC_MUXNEG_DACREF_gc;



The analog value used by AC (VDACREF) is derived from internal reference using DACREF register andthe output voltage is defined by:�� = ��256 × ��The DACREF value can be calculated using the following macro:#define DACREF_VALUE (VDACREF * 256 / VREF)

, where VDACREF represents the desired value on the analog comparator input and VREF represents thevalue selected as internal reference (1.5 Volts).

Important: To implement this application, the DACREF register is configured to select 0.8V onthe negative input. The user must select a proper combination of R1 and R2 resistors (see Figure 3-1) in such way that when battery voltage low threshold is reached, voltage V1 shouldbe 0.8V:�1 = 0.8� = �� × �2�1 + �2Assuming VBAT = 3V and R2=10 kΩ, the value of R1 should be 27.5 kΩ

The output of the comparator is present on the external pin and the interrupts are enabled on thenegative edge to notify the application when the critical level is reached. That is done using CTRLAregister:Figure 3-7. AC0.CTRLA - Set AC, Enable Interrupts and Output

b

b

b

These settings are translated into following code:AC0.CTRLA = AC_ENABLE_bm | AC_INTMODE_NEGEDGE_gc | AC_OUTEN_bm;

To complete the setup and make selected interrupt available to the application, the AC peripheralinterrupt must be enabled. This is done by using the CMP bit from INTCTRL register:



Figure 3-8. AC0.INTCTRL - Enable AC Interrupt

b

Tip: The full code example is available in the Appendix section.



https://github.com/MicrochipTech/TB3211_Getting_Started_with_AnalogComparator

4. Preventing False Spike DetectionThis example demonstrates the hysteresis features of the AC module that helps in avoiding frequenttoggling of the AC when the positive input oscillates very close to the negative input level. This applicationis almost similar to the voltage level detector application. Additionally, it has the Hysteresis mode enabled.Figure 4-1. Analog Comparator Response With and Without Hysteresis Enabled

AINNn

AOUT without Hysteresis

AOUT with Hysteresis

AINNn + 25 mV

AINNn - 25 mVTime

AINPn

The hysteresis is configured by writing to the Hysteresis Mode Select bit field (HYSMODE[1:0]) in theControl A register:Figure 4-2. AC0.CTRLA - Configure Hysteresis Mode

b

A medium hysteresis is used (25 mV) and that is translated in the following sequence of code:AC0.CTRLA |= AC_HYSMODE_25mV_gc;

Tip: The full code example is also available in the Appendix section.

TB3211Preventing False Spike Detection



5. Analog Signal Pulse Duration/Frequency MeasurementsThe tinyAVR® 0- and 1-series, and megaAVR® 0-series devices feature Event System (EVSYS), a simplebut powerful system that allows autonomous control of peripherals without any use of interrupts, CPU, orDMA resources. It allows a change in one peripheral (the Event Generator) to trigger actions in otherperipherals (the Event Users) through Event Channels. It provides short and predictable response timesbetween peripherals, and can reduce the complexity, size, and execution time of the software, to savepower.Figure 5-1. Analog Signal Duration/Frequency Measurement Block Diagram

DACREFVREF

V1

AINP0[PD2]

+

-

OUT[PA7]

0.8V

Analog signal

Event channel 0CNT

CCMP

TCB

The next application example shows an implementation of duration/frequency measurement for ananalog input signal, with minimal usage of microcontroller power. It uses the Event System (EVSYS) toroute the signals from the AC output through an Event Channel to Timer Counter B (TCB) Event Input. Todo this, the Event System must be configured properly.

The first step in the Event System configuration is to set the AC output as Event Generator for channel 0:Figure 5-2. EVSYS.CHANNEL - Set AC output as event generator for channel 0

b

For EVSYS channel 0, the channel generator selection register must be loaded with 0x20 to enableanalog comparator as Event Generator:EVSYS.CHANNEL0 = EVSYS_GENERATOR_AC0_OUT_gc;

To trigger events on TCB input, the TCB Event User must be connected to channel 0:EVSYS.USERTCB0 = EVSYS_CHANNEL_CHANNEL0_gc;

TB3211Analog Signal Pulse Duration/Frequency Measurement...


To enable pulse and period measurements, the TCB is configured in Pulse-Width Measurement mode,having as input the Event System (EVSYS), which is used to route AC output through Event channel 0 toTCB event input.

In Pulse-Width Measurement mode, the TCB will start counting when a positive edge is detected on theevent input signal. On the next falling edge, the count value is captured. The counter stops when thesecond rising edge of the event input signal is detected, and this will set the Interrupt flag. Reading thecapture will clear the Interrupt flag. When the Capture register is read, or the Interrupt flag is cleared, andthe TCB is ready for a new capture sequence. Therefore, it is recommended to read the Counter registerbefore the Capture register since it is reset to zero at the next positive edge:Figure 5-3. TCB - Input Capture Frequency and Pulse-width Measurement

CNT

MAX

BOTTOM

"Interrupt"

Startcounter

Copy CNT toCCMP

Stop counter andinterrupt

CPU reads the

Ignore tillCapture is read

Trigger nextcapture sequence

Event Input

Edge detector

CCMP register

The following code provides a basic initialization for TCB in Pulse-Width Measurement mode with EventSystem as input:int8_t TIMER0_init(){

TCB0.CTRLB = TCB_CNTMODE_FRQPW_gc; TCB0.EVCTRL = TCB_CAPTEI_bm;

TCB0.INTCTRL = TCB_CAPT_bm; TCB0.CTRLA = TCB_CLKSEL_CLKDIV2_gc | TCB_ENABLE_bm | TCB_RUNSTDBY_bm;

}

Tip: The full code example is also available in the Appendix section.

TB3211Analog Signal Pulse Duration/Frequency Measurement...



6. References1. ATmega4809 web page: https://www.microchip.com/wwwproducts/en/ATMEGA48092. DS40002015A - MegaAVR® 0-series User Manual: http://ww1.microchip.com/downloads/en/

DeviceDoc/40002015A.pdf3. ATmega4809 Device Manual: http://ww1.microchip.com/downloads/en/DeviceDoc/40002016A.pdf4. AN2451 - Getting Started with Core Independent Peripherals on AVR® Microcontrollers

(DS00002451)

TB3211References


https://www.microchip.com/wwwproducts/en/ATMEGA4809http://ww1.microchip.com/downloads/en/DeviceDoc/40002015A.pdfhttp://ww1.microchip.com/downloads/en/DeviceDoc/40002015A.pdfhttp://ww1.microchip.com/downloads/en/DeviceDoc/40002016A.pdfhttp://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en598462http://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en598462

7. AppendixExample 7-1. Level Crossing detector source code

#include #include

/* set DACREF to 0.8 Volts for Vref = 1.5Volts */#define DACREF_VALUE (0.8 * 256 / 1.5)

void PORT0_init (void);void AC0_init(void);

ISR(AC0_AC_vect){ /* Insert AC interrupt handling code here */ /* The interrupt flag has to be cleared manually */ AC0.STATUS = AC_CMP_bm;}

void PORT0_init (void){ /* Positive Input - Disable digital input buffer */ PORTD.PIN2CTRL = PORT_ISC_INPUT_DISABLE_gc; /*Enable output buffer on PA7*/ PORTA |= PIN7_bm;}

void AC0_init(void){

/* Negative input uses internal reference - voltage reference should be enabled */ VREF.CTRLA = VREF_AC0REFSEL_1V5_gc; /* Voltage reference at 1.5V */ VREF.CTRLB = VREF_AC0REFEN_bm; /* AC0 DACREF reference enable: enabled */ AC0.DACREF = DACREF_VALUE; /* Set DAC voltage reference */

/*Select proper inputs for comparator*/ AC0.MUXCTRLA = AC_MUXPOS_PIN0_gc /* Positive Input - Analog Positive Pin 0 */ | AC_MUXNEG_DACREF_gc; /* Negative Input - DAC Voltage Reference */ AC0.CTRLA = AC_ENABLE_bm /* Enable analog comparator */ | AC_OUTEN_bm; /* Output Buffer Enable: enabled */ AC0.INTCTRL = AC_CMP_bm; /* Analog Comparator 0 Interrupt enabled */ }

int main(void){ PORT0_init(); AC0_init(); sei(); /* Global interrupts enabled */ while (1) { ; }}

TB3211Appendix


Example 7-2. Preventing false spike detection source code

#include #include

/* set DACREF to 0.8 Volts for Vref = 1.5Volts */#define DACREF_VALUE (0.8 * 256 / 1.5)

void PORT0_init (void);void AC0_init (void);

ISR(AC0_AC_vect){ /* Insert AC interrupt handling code here */ /* The interrupt flag has to be cleared manually */ AC0.STATUS = AC_CMP_bm;}


void AC0_init (void){

/* Negative input uses internal reference - voltage reference should be enabled */ VREF.CTRLA = VREF_AC0REFSEL_1V5_gc; /* Voltage reference at 1.5V */ VREF.CTRLB = VREF_AC0REFEN_bm; /* AC0 DACREF reference enable: enabled */ AC0.DACREF = DACREF_VALUE; /* Set DAC voltage reference */ /*Select proper inputs for comparator*/ AC0.MUXCTRLA = AC_MUXPOS_PIN0_gc /* Positive Input - Analog Positive Pin 0 */ | AC_MUXNEG_DACREF_gc; /* Negative Input - DAC Voltage Reference */ AC0.CTRLA = AC_ENABLE_bm /* Enable analog comparator */ | AC_HYSMODE_25mV_gc /* Enable hysteresis @25mV */ | AC_OUTEN_bm; /* Output Buffer Enable: enabled */ AC0.INTCTRL = AC_CMP_bm; /* Analog Comparator 0 Interrupt enabled */

}

int main(void){ PORT0_init(); AC0_init(); sei(); /*Global interrupts enabled */ while (1) { ; }}

Example 7-3. Analog signal pulse duration/frequency measurement source code

#include

/* set DACREF to 0.8 Volts for Vref = 1.5Volts */

TB3211Appendix


#define DACREF_VALUE (0.8 * 256 / 1.5)

void PORT0_init (void);void EVENT_SYSTEM_init (void);void AC0_init (void);void TIMER0_init (void);


void AC0_init (void){

/* Negative input uses internal reference - voltage reference should be enabled */ VREF.CTRLA = VREF_AC0REFSEL_1V5_gc; /* Voltage reference at 1.5V */ VREF.CTRLB = VREF_AC0REFEN_bm; /* AC0 DACREF reference enable: enabled */ AC0.DACREF = DACREF_VALUE; /* Set DAC voltage reference */ /*Select proper inputs for comparator*/ AC0.MUXCTRLA = AC_MUXPOS_PIN0_gc /* Positive Input - Pin 0 */ | AC_MUXNEG_DACREF_gc; /* Negative Input - DAC Voltage Reference */ AC0.CTRLA = AC_ENABLE_bm /* Enable analog comparator */ | AC_OUTEN_bm; /* Output Buffer Enable: enabled */ AC0.INTCTRL = 0; /* Analog Comparator 0 Interrupt disabled */

}

/*Init TCB in pulse width-frequency measurement mode, input from Analog Comparator through Event System */void TIMER0_init (void){

TCB0.CTRLB = TCB_CNTMODE_FRQPW_gc; /* Input Capture Frequency and Pulse-Width measurement */ TCB0.EVCTRL = TCB_CAPTEI_bm; /* Event Input Enable: enabled */ TCB0.INTCTRL = TCB_CAPT_bm; /* Capture or Timeout: enabled */ TCB0.CTRLA = TCB_CLKSEL_CLKDIV2_gc /* CLK_PER/2 (From Prescaler) */ | TCB_ENABLE_bm /* Enable: enabled */ | TCB_RUNSTDBY_bm; /* Run Standby: enabled */

}

/* Enable event generation from Analog comparator to TCB */void EVENT_SYSTEM_init (void){ EVSYS.CHANNEL0 = EVSYS_GENERATOR_AC0_OUT_gc; /* Analog Comparator 0 out linked to Event Channel 0 */ EVSYS.USERTCB0 = EVSYS_CHANNEL_CHANNEL0_gc; /* TCB uses Event Channel 0 */}

int main(void){ uint16_t signal_pulse = 0, signal_period = 0; PORT0_init(); AC0_init(); EVENT_SYSTEM_init(); TIMER0_init(); while(1)

TB3211Appendix


{ if (TCB0.INTFLAGS) { /** * First read the CNT register * The interrupt flag is cleared by writing 1 to it, or when the Capture register * is read in Capture mode */ signal_period = TCB0.CNT; signal_pulse = TCB0.CCMP; } }}

TB3211Appendix


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TB3211


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TB3211


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IntroductionTable of Contents1. Relevant Devices1.1. tinyAVR® 0-series1.2. tinyAVR® 1-series1.3. megaAVR® 0-series

2. Overview3. Level Crossing Detector4. Preventing False Spike Detection5. Analog Signal Pulse Duration/Frequency Measurements6. References7. AppendixThe Microchip Web SiteCustomer Change Notification ServiceCustomer SupportMicrochip Devices Code Protection FeatureLegal NoticeTrademarksQuality Management System Certified by DNVWorldwide Sales and Service

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Getting Started with AC - Microchip Technologyww1.microchip.com/downloads/en/AppNotes/TB3211-Getting-Starte… · Figure 3-4. VREF.CTRLA - AC Voltage Reference Selection b A value