Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Learning Objectives

• Why are timer peripherals important?
• Can we do timing using software?
• Why do we need timers in the project?
• How does a timer peripheral work?
• What different timer peripheral configurations are there?
• How do we characterize timer peripherals?
• Learning to configure the timers in the ATmega328P
• Learning to develop code to use the timers in the ATmega328P
• Understanding pulse width modulation (PWM)

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Lecture Quiz

• The lecture quiz is now available on Canvas
  • Quiz is available for 3 days and allows 3 attempts
  • Best of the 3 attempts taken as the final score

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

When is Timing Required?

• Often we want to perform some action after a certain period of time
  • This could be a one time (single) action, or a repeated (periodic) action
  • As an example consider blinking an LED every 1s
• We also want to monitor activities, and want to determine for how long or how often they occur
  • As an example consider having to measure how long a button is pressed for
• All microcontrollers have a notion of time based on its clock, as each clock period is a function of the system clock frequency $T_{system\_clk} = \frac{1}{f_{system\_clk}}$
• Timers are peripherals which enable us to convert the $T_{system\_clk}$ either into actions in real time or to measure events in real time

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Timing Without Timer Peripherals

• The microcontroller processor also has a notion of time from the system clock since the duration of each execution cycle is also $T_{cpu\_clk} = \frac{1}{f_{cpu\_clk}}$
• We could implement timing in software by using for example a 'dummy' block of code that uses up execution cycles
  • This is at the expense of taking up the processor time
• We also need to know exactly how many clock cycles it took to execute the block of code used for timing
  • This can be tricky and the functionality is limited
• As an example we can use the assembly instruction NOP to generate a single execution cycle delay
  • In the C code having asm("NOP"); repeated a number of times can create a given delay
  • Alternatively a loop may also be used

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Example: Timing in Software

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Timing in Your Project

• The smart energy monitor needs to measure voltage and current to determine power drawn by the load
• We will need to measure these signals at regular time intervals as per project specifications
  • We may also want to for example measure time between zero crossings to determine the frequency
• We will need to update the display periodically and send data over the UART periodically
• We will use the timer peripherals on the ATmega328P to perform each of these timing tasks
  • You will need to think about the most suitable timer peripheral for each function taking in to account their characteristics and interrupt priorities

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Operating Principles of a Timer Peripheral

• A timer peripheral is implemented using a counter that counts up and/or down when its input changes
  • If counting up, the count is increased every time the input changes from low to high
  • If counting down, the count is decreased every time the input changes from low to high
• This input is often derived from the system clock directly or through a prescaler

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Timing Using a Timer Peripheral

• If the counter size is 8-bits and it is counting up, it will reset to 0 at the next increment after 255
• Alternatively, the counter can be made to reset every time it reaches a specific count, referred to as TOP, and TOP should be less than largest number the counter can hold (referred to as the MAX)
• The event generated when a counter resets, referred to as an overflow, can be used for precise timing
  • Either using polling or interrupts

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Using Compare Match for Timing

• A compare value can be used to detect when a set count value is reached
• The event generated when a compare match occurs can be used for precise timing
  • Either using polling or interrupts
• Overflow and compare match events can also generate a pulse width modulated (PWM) signal
  • We will talk about PWM briefly later in the lecture

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Using External Clock

• Rather than using the system clock to increment the count, we can use an external clock input
• Each time the external input changes from low to high, the count will increase
  • However, the timer is still synchronized to the system clock, so the count will only increase on the rising edge of the system clock

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Example: Timing with Timer Peripheral

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Characteristics of a Timer (PI)

• Prescaler
  • The clock divider which divides the system clock to create the timer clock $f_{timer\_clk} = \frac{f_{system\_clk}}{\text{Prescaler}}$
• Bits
  • The number of bits allocated to the count register $0 \leqslant \text{count} < 2^{bits}$
• Resolution
  • This is the minimum time interval the timer can measure and is equal to one timer clock period $\text{Resolution} = \frac{1}{f_{timer\_clk}}$

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Characteristics of a Timer (PII)

• Range
  • This is the maximum time interval the timer can measure $\text{Range} = \text{Resolution} \times \left( 2^{bits} - 1 \right)$
• Top
  • The count value at which the count is reset
  • Top must be less than or equal to the maximum possible count value which can be stored with the available bits
• Period
  • The time interval taken for the count to return to the starting value
  • As the transition from top to ‘0’ takes one cycle, we must add 1 to the top to calculate the period $\text{Period} = \text{Resolution} \times \left( \text{Top} + 1 \right)$

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Example: ATmega328P 8-Bit Timer

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

ATmega328P Timer Peripherals

• The ATmega328P has 3 timer peripherals
  • 8-bit Timer/Counter0 (TC0)
  • 16-bit Timer/Counter1 (TC1)
  • 8-bit Timer/Counter2 (TC2) with Asynchronous Operation
• In this lecture we will focus only on Timer/Counter0 (TC0)
  • 8-bit resolution
  • Two compare units
  • Clear on match (auto-reload)
  • PWM generation
  • Three independent interrupt sources
• We will learn to use the timer with polling
• For the project you will have to use interrupts as there are tasks that need to happen in parallel
  • You may also decide to use more than 1 timer peripheral

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

ATmega328P TC0 Pins

• In the ATmega328P there are 3 pins associated with TC0
  • PD4 (T0) is used to provide an external clock source while PD5 (OC0B) and PD6 (OC0A) are used to generate waveforms based on compare match events

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

ATmega328P TC0 Implementation

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

ATmega328P TC0 Registers

• There are 7 registers associated with TC0

• TCCR0A and TCCR0B are used to configure TC0 peripheral
• TCNT0 is an 8-bit register that holds the count value
• OCR0A and OCR0B are used to set the compare match value (and in some modes the Top)
• TIMSK0 is used to configure interrupts and TIFR0 contains flags

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

TCCR0A Register

• COM0A[1..0]: Compare Match Output A Mode where 00 = Disconnect, 01 = Toggle on Match, 10 = Clear on Match, 11 = Set on Match
• COM0B[1..0]: Compare Match Output B Mode where 00 = Disconnect, 01 = Toggle on Match, 10 = Clear on Match, 11 = Set on Match
• WGM0[1..0]: Waveform Generation Mode together with WGM02 in TCCR0B selects 000 = Normal, 001 = Phase Correct PWM with 0xFF as Top, 010 = Clear on Compare Match with OCR0A as Top, 011 = Fast PWM with 0xFF as Top, 101 = Phase Correct PWM with OCR0A as Top, 111 = Fast PWM with OCR0A as Top

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

TCCR0B Register

• FOC0A: Force Output Compare A
• FOC0B: Force Output Compare B
• WGM0[2]: Waveform Generation Mode with WGM0[1..0]
• CS0[2..0]: Clock Select where 000 = None (i.e. Timer/Counter Stopped), 001 = I/O Clock, 010 = I/O Clock/8, 011 = I/O Clock/64, 100 = I/O Clock/256, 101 = I/O Clock/1024, 110 = External Clock on Falling Edge, 111 = External Clock on Rising Edge

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

TCNTO Register

• TCNT0[7..0]: Timer/Counter Register can be read to obtain the counter value but when writing the count value while the timer is running it can introduce the risk of missing compare matches

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

OCR0A & OCR0B Registers

• OCR0A[7..0]: Output Compare Register A is either used as Top, or compared with TCNT0 and a match can be used to generate an interrupt or to generate a waveform on the OC0A pin
• OCR0B[7..0]: Output Compare Register B is compared with TCNT0 and a match can be used to generate an interrupt, or to generate a waveform on the OC0B pin

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

TIMSK0 Register

• OCIE0B: Output Compare Match B Interrupt Enable executes corresponding interrupt when OCR0B match TCNT0
• OCIE0A: Output Compare Match A Interrupt Enable executes corresponding interrupt when OCR0A match TCNT0
• TOIE0: Overflow Interrupt Enable executes corresponding interrupt when TCNT0 overflows

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

TIFR0 Register

• OCF0B: Output Compare Match B Flag is set when OCR0B match TCNT0
• OCF0A: Output Compare Match A Flag is set when OCR0A match TCNT0
• TOV0: Overflow Flag is set when TCNT0 overflows

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Clear Timer on Compare Match

• Before using a timer we need to decide on what mode we want to use it in
  • In this lecture we are going to operate TC0 in Clear Timer on Compare Match (CTC) mode
• In CTC mode, TCNT0 counts up starting from 0, where TCNT0 is incremented by 1 every timer clock cycle
• TCNT0 value is compared with OCR0A value we will define and when they match, TCNT0 will be set to 0 (i.e. cleared)
  • OCF0A flag will be set when a match occurs and is cleared either by writing a 1 to OCF0A or executing the corresponding interrupt

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Configuring TC0 in CTC Mode

• First we need to configure TC0 as per our needs
  • Need to set the bits of the TCCR0A and TCCR0B registers
  • Do this in an initialization function
• In the lectures, as an example, we will configure the TC0 in CTC mode with a prescaler of 8
• We want the timer to have period of 1ms so we can create our own millisecond delay
  • Since the ATmega328P uses a 2MHz system clock, to achieve this we need to load 249 to OCR0A

//This function configures TC0 to operate in CTC mode with a period of 1ms
//The prescaler of 8 needed is not loaded to keep timer stopped until it is used 
void tc0_init(void){
  TCCR0A = 0b00000010;    //WGM0[2..0] should be set to 010 for CTC mode
  TCCR0B = 0b00000000;    //Initialize CS0[2..0] to 000 so that timer is stopped until needed
                          //When running we need to load 010 to CS0[2..0] for a prescaler of 8
  OCR0A = 249;            //Loading OCR0A with 249 to get a period of 1ms
}

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Creating Custom Delay Function

• Lets develop our own millisecond delay function that uses TC0 with polling
• The function waits for OCR0A match by polling the OCF0A flag
  • Since an OCR0A match event occur every 1ms it repeats this for the number of ms requested

//This function creates a delay for duration requested in ms using polling 
void tc0_ms_delay(uint32_t milliseconds){
  uint32_t timer_overflows = 0;       //Counter to count the number of overflows
  TCNT0 = 0;                          //Reset the TCNT0 count
  TCCR0B |= 0b00000010;               //Start the timer with a prescaler of 8
  //Loop until the requested milliseconds have elapsed
  while (timer_overflows < milliseconds) {
    if((TIFR0 & (1 << OCF0A)) != 0){  //Check if the timer has overflowed
      timer_overflows++;              //Increase the overflow count
      TIFR0 |= (1 << OCF0A);          //Reset the overflow flag
    }
  }
  TCCR0B &= 0b11111000;               //Stop the counter
}

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Using the Delay Function

• Lets now use the delay function we wrote to toggle an LED at PB5 every 0.5s
• The two TC0 related functions we developed earlier are put in a source file named tc0.c and it has a corresponding header file called tc0.h

#include <avr/io.h>       //Needed for using the macros for register addresses
#include "tc0.h"          //Including our TC0 peripheral library
int main(void){
  tc0_init();             //Initializing TC0 to work in CTC mode with 1ms period 
  DDRB |= 1 << PINB5;     //Setting PB5 as output
  while (1){
    tc0_ms_delay(500);    //Create a 0.5s delay
    PORTB ^= 1 << PINB5;  //Toggle PB5 
  }
}

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Pulse Width Modulation

• We often want to produce a periodically pulsating signals that has a fixed time period (T_p)
  • The pulsating signal is high (logic 1) for a certain portion of T_p and this time is called the on-time (T_on)
  • For the remainder of T_p the pulsating signal is low (logic 0) and this time is called the off-time (T_off) $T_{p} = T_{on} + T_{off}$
• In many application we control the duration of T_on to control the “pulse width”
  • These signals are known as pulse-width modulated (PWM) signals

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

DC Value of a PWM Signal

• PWM signals produced by an MCU often control BJT/MOSFET switches to produce an amplified voltage (V_supply) during T_on
  • V_supply can be any voltage ranging from a few volts to thousands of volts
• Pulsating V_supply can be turned in to a DC signal by passing it through an analog low-pass filter
  • The DC signal can be controlled using T_on/T_p, which is referred to as the duty-cycle (D) $V_{DC} = V_{supply} \times T_{on}/T_{p} = D V_{supply}$

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Using Timer to Generate a PWM Signal

• In a timer, the count value is reset when it reaches the Top value, which sets the timer period
  • We can use this property to generate T_p of a PWM signal
• A compare match event can be used to drive the digital I/O pins connected to OCnA/OCnB when count value reaches the compare value
  • We can use this property to set T_on of the PWM, by setting the output state of OCnA/OCnB to low when every time a compare match is achieved
• The output state of OCnA/OCnB can be configured to be set to high when the count value resets
• Since we have to add 1 when calculating periods, this results in a PWM output where $T_{p} = \text{Resolution} \times \left( \text{Top} + 1 \right) = \left( \text{Top} + 1 \right) / f_{timer\_clk}$ $T_{on} = \text{Resolution} \times \left( \text{Compare} + 1 \right) = \left( \text{Compare} + 1 \right) / f_{timer\_clk}$
• The compare value can be changed between 0 and Top to change the duty-cycle between 0% and 100%

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

An Example PWM Signal

• As an example lets consider a counter that has the Top set to 5, and the compare value set to 3
  • Assume the resolution of the counter is 10μs (i.e. f_{timer_clk} is 100kHz)
• In this example, T_p will be 60μs and T_on will be 40μs (i.e. D is 66.6%)

Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2020)

Code for the Example PWM

• An initialization function can be developed for the ATmega328P to setup TC0 in the fast PWM mode with a Top of 5 and a compare value of 3

void tc0_PWM_init(void){
  DDRD |= 1 << PIND5;     //PD5, which is the OC0B pin is set to an output
  //Configure the timer to operate in the Fast PWM mode with OCR0A as Top and a prescaler of 8
  TCCR0A = 0b00100011;    //Set to clear OC0B on compare match and set OC0B at BOTTOM
  TCCR0B = 0b00001010;    //Setup prescaler to 8 and the mode to Fast PWM
  //Setup the period and the on-time of the PWM
  OCR0A = 5;              //Set the TOP value to 5
  OCR0B = 3;              //Set the compare value to 3
}

• The main function only has to call this initialization function at the start to run the PWM in the background (i.e. does not require processor cycles)