ATMEL AVR1504 User Manual

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AVR1504: Xplain training - XMEGA Event

system

Prerequisites

• Required knowledge

Basic knowledge of microcontrollers and the C programming language
Completed AVR1500: Xplain training – XMEGA™ Basics
Recommended to have finished AVR1501: Xplain training – XMEGA

Timer/Counter

• Software prerequisites

Atmel
WinAVR/GCC 20100110 or later

• Hardware prerequisites

Xplain evaluation board
JTAGICE mkII

• Estimated completion time

2 hours

1 Introduction

The Event System is a set of features for inter-peripheral communication. It
enables the possibility for a change of state in one peripheral to automatically
trigger actions in other peripherals. What change of state in a peripheral, that will
trigger actions in other peripherals is configurable in software. It is a simple, but
powerful system as it allows for autonomous control of peripherals without any use
of interrupts or CPU and DMA resources.

®

AVR® Studio® 4.18 SP2 or later

8-bit
Microcontrollers

Application Note

The indication of a change of state in a peripheral is referred to as an event. The
events are passed between the peripherals using a dedicated routing network
called the Event Routing Network. This consists of eight multiplexers, where all
events are routed into all multiplexers.

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2 Introduction to the Event System

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This introduction is intended to give you a basic overview of the terminology and
behavior which is needed to understand the Event System and the tasks in this
training. The tasks in this training will show you how the Event System works in more
detail.

The figure below illustrates the Event System. The figure shows the different parts
that makes it operate; the event sources, the channel MUX’s and the event action
selection in the event user/peripheral.

The figure shows a simplified version with one timer/counter as event generator and
one ADC as an event user. The event channel MUX’s can select one of three
available sources to be routed though the corresponding event channel.

Events can be generated by the following peripherals:

• Timer/Counters (TCxn)

• Real Time counter (RTC)

• Analog to Digital converters (ADCx)

• Analog Comparators (ACx)

• Ports (PORTx)

• System clock (clksys)

Each of these peripherals has several sources for events. Examples of sources are
timer/counter overflow, pin change on a port or A/D conversion completed. The full list
of available event sources is shown in the register description for the Event System in
the Atmel XMEGA A manual.

The channel multiplexers (MUX) selects what source is routed into each of the 8
event system channels available. Each event system channel allows one source that
generates events to that channel. The EVSYS.CHxMUX registers controls the event
source for each channel.

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Events can be used by the following peripherals:

• Timer/Counters

• Analog to Digital Converters

• Digital to Analog Converters

• Direct Memory Access Controller (DMAC)

Usage of events is controlled on the individual peripherals. Configuration registers on
the individual peripheral allows you to select what event channel to use as input and
what the event action is for that channel. Several peripherals can be using the same
event channel as input. This is convenient for allowing several actions start at the
same time.

For example: starting input capture of a Timer/Counter at the same time as starting a
conversion in an ADC. The available event actions are shown in the register
description for each peripheral.

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3 Overview

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Here is a short overview of the tasks in this training:

Task 1: 32-bi t Timer/Counter

This task shows the basic Event System setup with event user and event generator
and how this can be used for making a 32-bit timer.

Task 2: Input capture with filtering

Input capture with a Timer/Counter is controlled with events in Atmel XMEGA, and
this task shows you how flexible this is.

Task 3: Synchronized triggering

More than one peripheral can use events from one event channel, and this can be
used to synchronize event actions in the peripherals.

Task 4: Manually generating events

Events can be generated from software, and this task gives you a basic example on
how to do this.

GOOD LUCK!

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4 Task 1: 32-bit Timer/Counter

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By using the overflow event from one Timer/Counter as the clock input/source to
another Timer/Counter, it is possible to use the Event System for making a 32-bit
Timer/Counter. In this setup it is also possible to do input capture in order to have 32­bit input capture. In the Timer/Counter hands-on session we use the Peripheral Clock
as input to the timer/counter TCC0, and the event system will be used as input to
timer/counter TCC1. The following figure shows this conceptually:

The AVR1001 – using the Atmel XMEGA event system application note contains a
code example on how to implement a 32-bit Timer/Counter with input capture.

The goal for this task is that you:

• Understand the basics of using the Event System, and how to configure an event
channel

• Know how to use an event channel in a peripheral module

• Understand how to use an event channel to clock a timer

TASK:

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1. Locate the Atmel XMEGA-EventSystem folder and open the

32bitTimerCounter.aps project file in AVR Studio

2. Spend some time to understand the code, how it works, and ensure you know the

basics of how the Event System is set up

3. Build the project, ensure there are no errors (you can ignore the warning) and start

a debug session

4. Run the code, and you will see that the LEDs are counting upwards with the clock

tick rate of the most significant TC which is clocked from the event system

5. Break and place a breakpoint as indicated below:

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6. Run the code again and see that it breaks when the least significant timer is close

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to overflow. If you single step a few times, the timer will overflow, and you will see
that the LED is counting. If you expand the IO view for TCC1, you can see how
CNT increase when TCC0 overflows

7. If you wish to write some code in this task, you can add code to make a 48- or 64-

bit timer ☺

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5 Task 2: Input Capture with Filtering

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In Task 1 we used the Event System to trigger events that were the clock source to a
Timer/Counter (TC). In general, a peripheral may perform different actions when
receiving an event. For instance, for the TC, see the timer event action list in the
CTRLD register shown in the register description of the Timer counter in the Atmel
XMEGA A Manual. Take a look at this table to understand what event actions the
timer can use.

By having the Event System able to trigger input capture is much more flexible than
having one or a few capture pins. Because a pin change on any I/O pin can be used
to generate an event, this means that any I/O pin can be used as an input capture
pin.

In fact, any event can trigger an input capture, not just pin changes. In this task we
will stick to the basics and we will use a pin change event to do input capture. To
make this work we need to configure the following:

• Timer TCC0 to perform input capture on capture channel A (CCA) when getting an
event on channel 0

• Event System routing to route PORTE pin 0 events into event channel 0

• Input sensing on PORTE pin 0 to specify if rising edge, falling edge, both edges or

the level of the pin generates events

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The goal for this task is that you:

• Understand how to set up input capture using a timer and the Event System

• Know how filtering on the I/O pins is handled by the Event System

TASK:

1. Locate the InputCapture.aps Atmel AVR Studio project file. Open the task2.c file

and familiarize yourself with the code

2. The code is almost done, but you need to configure the event channel 0 MUX to

use pin 0 of PORTF as input to this event channel. If you need help, see how this
is done in Task 1

3. Build the project, ensure there are no errors (you may ignore the warning) and

open the debug file in AVR Studio

4. Run the code; press the switch a few times

5. Each press will generate an event (or in fact two events) that trigger the input

capture. The capture values are continuously read and output to the LEDs

Why does each press generate two events?

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6. Let’s look at the Error flag. The Error flag is set when there is a buffer overflow for

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the input captures registers in the TC. You can study Section 14.5 (double
buffering) in the Atmel XMEGA A manual to see how this works

7. Add error_count to the Watch window so you can follow the number of overflow

errors

8. Run the code, press the switch many times with short intervals, break and see if

any errors occurred. You should be able to generate some

9. We are using noise from the buttons to generate quickly enough to get a buffer

overflow. If it is difficult to press the switch to generate overflow, you can add a
delay (for example _delay_ms(200); ) in the while(1) loop so the CCA register is
not read that often

10. While the debug session is still stopped, locate the Event System in the IO view

and expand it to see the current configuration

11. The first instance of the Digital Filter, represents the filter for event channel 0

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12. Change the digital filter to set how many pin change events must be sampled by

the peripheral clock before the event is passed through

13. Run the code again, press the switch and see how the different filter value is able

to reduce the error_count

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6 Task 3: Synchronized Triggering

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It is possible to let different peripherals use the same event channel as input, and by
doing this several peripherals can use the same event. This can for example be used
to synchronize actions. Here are a few examples where this is useful:

• For doing an input capture and start an ADC conversion at the same time, in order
to give the conversion a time stamp

• For starting conversions on two ADCs at the same time

• Other combinations of ADC, DAC, Timer/counter and DMA

In this task we are going to keep it simple and use the Event System to initiate input
capture on 3 timers (TCC0, TCD0 and TCE0) simultaneously. Timer/counter TCF0
overflow is used as the event trigger source (generator).

The goal for this task is that you:

• Are able to configure the Event System

• Understand synchronized Triggering

TASK:

1. Locate the folder for Task 3 and open the SynchronizedTriggering.aps project.

Look at the task3.c file and familiarize yourself with the code

2. The code is almost complete, but we need to do some changes:

a. Add code that sets up the timer/counter TCD0 and TCE0 in the same

way as TCC0
b. Set up overflow of timer/counter TCF0 as input to event channel 0
c. Build the project, ensure there are no errors, and start debugging in

Atmel AVR Studio

3. Let’s run the code and make sure that the event is triggered as expected, and that
the input capture happens

4. Break the execution, and place a breakpoint as indicated below:

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5. Add a watch on the capture_values variable by right clicking on the variable and
selecting “Add watch : “capture_valu es”. This array contains capture values from
all three timers

6. Run the code and observe that the

You can notice that all the capture values are almost the same. Why are they not
exactly the same?

If you have time, add code to set all the count (CNT) values of the timer/counter to
zero before the while loop. Are the capture values the same now? Why?

capture_values variable gets updated

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7 Task 4: Manually Generating Events

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Events can be generated manually from software. This is done by STROBE registers
or by accessing the registers directly during on-chip debugging. Writing the STROBE
register triggers the operation.

It is possible to generate events on several channels at the same time by writing to
several bit locations at once. This can be useful for synchronizing event actions, for
on-chip debug or using events to keep track of program execution status.

Manually generated events last for one clock cycle and will overwrite events from
other event sources during that clock cycle.

The goal for this task is that you:

• Know how to generate events from software

• Understand when generating events from software can be useful

• Know how to synchronize several timer/counters

TASK:

1. Locate the ManuallyGeneratingEvent.aps project and open it in Atmel AVR Studio

2. Open the task4.c file and familiarize yourself with the code. The code is similar to
task3, but notice that timer/counter TCC0, TCD0 and TCE0 are now running with a
clock prescaler/divider of 1 (same speed as the CPU). In addition the
timer/counters are now configured to RESTART when an event is received

3. Build the project, ensure there are no errors, and start debugging the project

4. Place a breakpoint in the main loop

5. Add a watch to

6. Notice that the
happens on the exact clock cycle. This is like in Task 3 because the timers are
started at different clock cycles

capture_values so you can keep track of the compare values

capture_values are now different values even if the input capture

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7. The code is almost done, but you need to add code to generate events from
software on event channel 0

8. Place a breakpoint in the code so you can single step after the software events are
generated

9. Run the code and ensure that it stops at the breakpoint

10. Open the Event System in the IO view and single step to see that the STROBE
register is being written, and cleared again in the next cycle

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8 Summary

11. Use “Run to cursor” to see that the compare_values are updated with new values
after the event triggered the input capture. Notice that all the timers are now
perfectly synchronized

12. The STROBE register can be written during on-chip debug, for example by using
the IO view to set the bits. The bits that are written will be cleared in the next cycle.
You can test this, but keep in mind that the capture values for the timer are not
kept if the buffer is full. Instead you will get an error

In this hands-on we have learned how the Event System operates, how to configure it
and we have shown you potential uses for the Event System.

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9 Resources

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• Atmel XMEGA Manual and Datasheets

o http://www.atmel.com/xmega

• Atmel AVR Studio with help files

o http://www.atmel.com/products/AVR/

• WINAVR GCC compiler

o http://winavr.sourceforge.net/

• Atmel IAR Embedded Workbench

o http://www.iar.com/

10 Atmel Technical Support Center

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