AVR_Course_Day7 timers counters and interrupt programming

TimersCounters &
Interrupt
Programming

Contents
1. Introduction
2. 8 bit Timers
3. Interrupts.
4. 16 bit Timers

TimerCounters
1. Introduction(Time Terminology)
Frequency
Consider signal x(t) that repeats itself. This signal called a periodic with
period T if it satisfies:
x(t)=x(t+T).
To measure the frequency of a periodic signal, we count the number of
times a particular event repeats within a 1-s period.
Period
 The flip side of a frequency is a period.
 If an event occurs with a rate of 1 Hz, the period of that event is 1 s.
 To find a period, given a frequency, or vice versa, f=1/T.

TimerCounters
Duty Cycle
 In many applications, periodic pulses are used as control signals.
 A good example is the use of a periodic pulse to control a servo motor.
 To control the direction and sometimes the speed of a motor, a periodic
pulse signal with a changing duty cycle over time is used.

TimerCounters
Timing System Overview:
 The heart of the timing system is the crystal time base.
 The crystal’s frequency of a microcontroller is used to generate a
baseline clock signal.
 For a timer system, the system clock is used to update the contents
of a special register called a free-running counter.
 For input time-related activities, all microcontrollers typically have
timer hardware components that detect signal logic changes on one
or more input pins. Such components rely on a free-running counter
to capture external event times.

TimerCounters
 For output timer functions, a microcontroller uses a comparator, a
free-running counter , logic switches, and special-purpose registers
to generate time-related signals on one or more output pins.

TimerCounters
1. Introduction(Applications)
 Measure an input signal timing event.
 Count the number of external signal occurrences.
 Generate timed signals---termed output compare.
 Generate PWM signals.

TimerCounters
1. Introduction
TimerCounter used for the following:
1) Count an event (by connecting external source to increment the
counter registers).
2) Generate time delay (Oscillators or internal clock is used to
increment the counter registers).
There are three timers on Atmega328:
1) Timer (0) which is 8 bit timer.
2) Timer (1) which is 16 bit timer.
3) Timer (2) which is 8 bit timer

TimerCounters
2. 8 bit Timers(Timer 0)
Features:
• Two Independent Output Compare Units
• Double Buffered Output Compare Registers
• Clear Timer on Compare Match (Auto Reload)
• Phase Correct Pulse Width Modulator (PWM)
• Variable PWM Period
• Frequency Generator
• Three Independent Interrupt Sources (TOV0, OCF0A, and OCF0B)

TimerCounters
The architecture of Timer (0) shown in the following figure:

TimerCounters

TimerCounters
Normal Mode:the timer will continually
count from 0x00 (BOTTOM) to 0xFF
(TOP).
 When the TCNT0 returns to 0 on each
cycle of the counter, the Timer/Counter
Overflow Flag (TOV0) will be set.
 The normal mode is useful for
generating a periodic ‘‘clock tick’’ that
may be used to calculate elapsed real
time or provide delays within a system.

TimerCounters
Clear Timer on Compare Match:In the
CTC modem, the TCNT0 timer is reset to 0
every time the TCNT0 counter reaches the
value set in OCR0.
 The Output Compare Flag 0 (OCF0) is
set when this event occurs.
 The OCF0 flag is enabled by asserting
the Timer/Counter 0 Output Compare
Match Interrupt Enable (OCIE0) flag in
the TIMSK and when the I-bit in the
Status Register is set to 1
 The CTC mode is used to generate a
precision digital waveform such as a
periodic signal or a single pulse.

TimerCounters
Phase Correct PWM Mode: In the Phase
Correct PWM mode, the TCNT0 register
counts from 0x00 to 0xFF and back down
to 0x00 continually.
Every time the TCNT0 value matches the
value set in the OCR0 register, the OCF0
flag is set and a change in the PWM signal
occurs.

TimerCounters
Fast PWM:is used to generate a precision
PWMsignal of a desired frequency and duty
cycle.
 It is called the fast PWM because its
maximum frequency is twice that of the
Phase Correct PWM mode.
 When the TCNT0 register value reaches
the value set in the OCR0 register, it will
cause a change in the PWM output as
prescribed by the system designer.
 It continues to count up to the TOP
value, at which time the Timer/Counter 0
Overflow Flag(TOV0) is set.

TimerCounters
TCNTn: TimerCounter register it counts up every pulse. Also it can
be loaded by a certain value.
In each TCNTn there are:
WGMnm: Wave generation mode (Normal mode ,CTC mode ,Fast
PWM ,..etc.).
CSnm: Prescalar(8,64,256,1024).
TOVn: Timer overflow flag, when a timer overflow a flag will be set.
TCCRn: TimerCounter control register ,its for setting mode of
operation by loading the proper value for
ex(Normal mode ,CTC mode ,Fast PWM ,..etc.)will be explained later.

TimerCounters
TIMSK :Timer/Counter Interrupt Mask Register register is used by all three
timer channels.
Timer 0 uses the Timer/Counter 0 Output Compare Match Interrupt Enable
(OCIE0) bit and the Timer/Counter 0 Overflow Interrupt Enable (TOIE0) bit.

TimerCounters
2. 8 bit Timers (Normal Mode)
There are two equation to calculate the value of TCNT0:
1) In Decimal
256-(TdTc)=TCNT0
2) In Hexa
FF+1-(TdTc)=TCNT0
Td: Time on desired from MCU.
Tc: Timer Clock of the MCU ((1Fosc)*Prescalar=Tosc*Prescalar).
OSC: Oscillators (the Crystal).
Prescalar: is used to divide this clock frequency and produce a clock for
TIMER.

TimerCounters
Ex(1):
Assuming xtal=8MHZ,and required to generate a square wave with period of
12.5 us.
Td=12.5us/2=6.25us.
Tc=1/8MHZ=0.125us.
256-(6.25/0.125)=206=0xCE.
TCNT0=0xCE.
EX(2):
Assuming xtal=8MHZ,Calculate the frequency of square wave knowing that the
TCNT0=0x3E
Td=(FF+1-3E)*0.125us=24.25us.
F=1/2*Td=12*24.25us=20.61KHZ.

TimerCounters
Prescaler and generating a large time delay:
To get the maximum delay time
Td=(256-0)*Tc. (for xtal=8MHZ) ,Td=32us
F=12*Td=12*32us=15KHZ
To maximize the period prescaler is used
Using prescaler=8 Td=256us.
Prescaler=64 Td=2048 us.
Prescaler=256 Td=8192us.
Prescaler=1024 Td=32768us.
Ex(3):
Assuming xtal=8Mhz generate a square wave of 125 Hz,using timer 0 and
prescaler of 256
T=1125=8ms Td=4ms.
Tc=18MHZ *256=32us.
256-(4ms32us)=256-125=131.
TCNT0=0x83.

TimerCounters
Steps of Programming Timer 0 for Normal mode :
1) Load the TCCR0 (A,B) with the proper value according to the mode of
operation.
2) Load the TCNTn with proper values according to the oscillator
frequency(1/f*Prescalar=Tc*Prescalar) and the desired (Td)time required with or
without the prescalar.
3) If the timer overflow set the TOV to clear the timer and to count again in
(normal mode) Clear the flag or OCF0.
5) Repeat it again.

Ex(4)
Find the time delay (Td) when the OCR0=9, assuming that
xtal=8Mhz
The TCNT0 will be cleared after 9+1=10 counts
Td=10*0.125us=1.25us.
EX(5)
Using the CTC mode find the OCR0 value to generate a time delay
of (25.6ms) using the prscaler 1024
Td=25.6ms
Tc=0.125us*1024=128us
Number of counts=Td/Tc=200
OCR0=200-1=99.
TimerCounters
2. 8 bit Timers (CTC mode)

TimerCounters
Steps of Programming Timer 0 for Normal mode :
1) Load the TCCR0 (A,B) with the proper value according to the mode of
operation.
2) Load the TCNTn with proper values according to the oscillator
frequency(1/f*Prescalar=Tc*Prescalar) and the desired (Td)time required with or
without the prescalar.
3) Set the OCF0 when the TCNT0 = OCR0 to clear the timer and count again.
4) Clear the flag or OCF0.
5) 5) Repeat it again.
2. 8 bit Timers (CTC Mode)

Interrupt
Interrupts are basically events that require immediate attention by the
microcontroller. When an interrupt event occurs the microcontroller pause
its current task and attend to the interrupt by executing an Interrupt
Service Routine (ISR) at the end of the ISR the microcontroller returns to
the task it had pause and continue its normal operations.
In order for the microcontroller to respond to an interrupt event the
interrupt feature of the microcontroller must be enabled along with the
specific interrupt. This is done by setting the Global Interrupt
Enabled bit and the Interrupt Enable bit of the specific interrupt.
Interrupt Types:
1. Internal Interrupt (e.g. Timer interrupt).
2. External Interrupt ( will be discussed in the next session).

Interrupt
Each Interrupt associated to vector number shown in the following
vector table (See other routines in the data sheet)

Interrupt
Why using Interrupt in
Timers??!!!

Interrupt VS Polling:
There are two methods to receive a service from microcontroller.
1) Polling method: the microcontroller continuously monitors the state of
the device ,when the state of the device met the condition the
microcontroller perform the service. After that it returns again to monitor
the state again.
2) Interrupt method: whenever any device needs a service, it notifies the
microcontroller by sending an interrupt signal. Upon receiving the signal
the microcontroller gets out the infinite loop and serves the device which is
associated with the interrupt it’s called interrupt service routine.
Interrupt

Advantages of Interrupts over Polling:
1) Every device can get the attention of the microcontroller by
sending an interrupt signal, when two devices send an interrupt signal
at same time the microcontroller serves the device with the high
priority.
2) Polling wastes the microcontroller time by polling a device(s)that
don’t need a service and tying it down from performing any other
tasks.
Interrupt

Interrupt
Steps of Executing an Interrupt:
1) The microcontroller finished the instruction that currently executing
and then saved the address of the next instruction (program counter) on
the stack.
2) It jumps to a fixed location in the memory (interrupt vector).The
interrupt vector directs the microcontroller to the address of ISR.
3) The microcontroller starts to executes the ISR until the it reaches the
last instruction RETI(return from interrupt).
4) The microcontroller returns to the place where its interrupted first, it
gets the program counter from the stack then starts to execute from that
address.

Interrupt
Enabling the Interrupts:
To enable the interrupt is should be unmasked as shown in the following
figure.
 SREG is the Status register ,to enable the interrupt (unmask the
interrupt) the bit D7(I) should be activated using sei() set interrupt then
use TMISK register to enable interrupts of Timer (0 or 1 or 2)this will
covered later.
 To disable the interrupt the bit D7(I) should be cleared by cli()clear the
interrupt.

Interrupt
Registers associated with Timer interrupt

TimerCounters
Assignment
Do the same programs
of timer 0 on timer 2

TimerCounters
3. 16 bit Timers
Features:
• True 16-bit Design (i.e., Allows 16-bit PWM)
• Two independent Output Compare Units
• Double Buffered Output Compare Registers.
• One Input Capture Unit.
• Input Capture Noise Canceler.
• Clear Timer on Compare Match (Auto Reload).
• Phase Correct Pulse Width Modulator (PWM).
• Variable PWM Period.
• Frequency Generator.
• External Event Counter.
• Four independent interrupt Sources (TOV1, OCF1A, OCF1B, and
ICF1).

TimerCounters
3. 16 bit Timers
The architecture of Timer (1) shown in the following figure:

TimerCounters
3. 16 bit Timers

TimerCounters
3. 16 bit Timers
ICNC1: Input Capture Noise Canceler Setting this bit (to one) activates the
Input Capture Noise Canceler. When the noise canceler is activated, the input
from the Input Capture pin (ICP1) is filtered.
ICES1: Input Capture Edge Select This bit selects which edge on the Input
Capture pin (ICP1) that is used to trigger a capture event. When the ICES1
bit is written to zero, a falling (negative) edge is used as trigger, and when the
ICES1 bit is written to one, a rising (positive) edge will trigger the capture.
ICIE1: Timer/Counter1, Input Capture Interrupt Enable When this bit is
written to one, and the I-flag in the Status Register is set (interrupts globally
enabled), the Timer/Counter1 Input Capture interrupt is enabled.
ICF1: Timer/Counter1, Input Capture Flag This flag is set when a capture
event occurs on the ICP1 pin

TimerCounters
3. 16 bit Timers
Assignment
Generate a square wave of
1 sec using the normal
mode and the CTC mode.
Repeat it using interrupt
method.

AVR_Course_Day7 timers counters and interrupt programming

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