Lecture 2 timers, pwm, state machine IN PIC

Microcontroller Lab.Eng.Khaled Tamziz

Project Pedagogy approach of Microcontroller – Palestinian Robotic Cup 1

PPU IUT Cachan

Mechanical DepartmentMechatronic

Timers and PWM

Bibliography

Microcontroller PIC18F4550 Data Sheet

39632c.pdf - Chapters 11 to 15


MPLAB_C18 libraries documentationMPLAB_C18_Libraries_51297f.pdf - Chapters 2.7 and 2.9

MPLAB_C18 header files

timers.h and pwm.h

MPLAB_C18 C source filesmcc18\src\traditionnal\pmc

Introduction

• In this course, we will talk about :

– Timers

– DC motor speed control (shortly)

– Pulse width modulation

– Basic state machine diagrams


– Basic state machine diagrams

Timers

• 4 timers are available :

– Timer0

– Timer1

– Timer2

– Timer3


• Each of them has general and special features.

• We will study general features.

• Then we’ll use special features with Timer2.

• It is necessary to understand the microcontroller data sheet to be able to use the C hardware libraries.

Basic timer block diagram

8 or 16-bit counterClock

TMRxON bit

TMRxIF bit

set on

overflow

---

cleared by

software


Clock selection

Multiplexer

Programmable Prescaler

Sync with internal clock

Internal

or

external

clock

sources

software

Microcontroller internal data bus

read or write the counter

register

.

.

.

Timers configuration and use

• C18 timers libraries allow :

– to access configuration registers and start the timer (OpenTimerx functions).

– to read the value of the counter register (ReadTimerx functions).

– to write a new value to the counter register


– to write a new value to the counter register (WriteTimerx functions).

– to stop the timer (CloseTimerx functions).

• To test overflow, we need to read TMRxIF bit directly in its own register.

• The Microcontroller Data Sheet gives information about the concerned registers.

Rotation measurement

signal

LEFT_ENCODER

R1 R2

VCC

4

31

2

U3

H21A1

VCC

Simplified schematic


time

Each edge corresponds to a rotation of a known angle of the axis of the motor.

We know the reducer rate and the diameter of the wheel, so it is possible to calculate

the distance accomplished by the robot.

We have to count edges : we ‘ll use a timer with an external clock.

Example : use of Timer1 to count events


Configure and start Timer1

OpenTimer1(

TIMER_INT_OFF &

T1_16BIT_RW &

T1_SOURCE_EXT &

T1_PS_1_1 &

T1_OSC1EN_OFF &


T1_OSC1EN_OFF &

T1_SYNC_EXT_OFF

);

Disable the interrupt generation from

Timer1. We will see this later.

Use Timer1 as a 16-bit counter.

Select an external clock.

Choose the prescale value.

Disable the internal Timer1 oscillator.

Disable the synchronization with the

processor clock.

Then, clear the counter register, clear TMR1IF and start the timer.

Read and write the Timer1 value

• Read the value :

– prototype

unsigned int ReadTimer1( void );

– code example

unsigned int result;

result = ReadTimer1();


• Write a new value :

– prototype

void WriteTimer1( unsigned int timer );

– code example

WriteTimer1(0);

Test the Timer1 overflow

• TMR1IF is the bit 0 of PIR1 register.

• Code example :

if (PIR1bits.TMR1IF != 0) {

// timer1 overflow detected


// timer1 overflow detected

PIR1bits.TMR1IF = 0; // reset flag

}

Timer2 special features

• Timer2 is an 8-bit counter.

• The values of TMR2 and PR2 are compared on each clock cycle.

• When the two values are equal, the comparator generates an output signal.

• This signal also resets the value of TMR2 to 00h on the next cycle and drives the output counter/postscaler.


DC motor speed control

• First, we assume the motor speed depends on the average voltage. Thus, we need to change the average voltage on the motor.

• The power electronics block contains a H-Bridge designed for motion

Power electronicsDC

motor

sign

pwm

POWER

Vmotor

from

microcontroller


• The power electronics block contains a H-Bridge designed for motion control applications.

• The signal sign controls the direction of the rotation.

• The signal pwm controls the average voltage on the motor.

0

1

t

T

α.T(1 - α).T

pwm

– T is the period.

– The duty cycle (α) is generally given in percent.

– The average value is α.

– For a power supply of U volts, Vmotor = α.U.

Capture/Compare/PWM (CCP) Modules

• Two CCP modules are available (CCP1 and CCP2).

• In PWM mode, the two modules use Timer2 as a time base. Thus PWM frequency is the same for CCP1 and CCP2.

• Each module has its own duty cycle.


PWM period and duty cycle

• The main trap is that the PWM period is set by a 8-bit value and the duty cycle is controlled by a 10-bit value.

• Example :

– let’s assume the period value is 99, thus the TMR2

value (8-bit value) should grow from 0 to (99+1) = 100


value (8-bit value) should grow from 0 to (99+1) = 100

(reset value)

– so the TMR2 extended value (10-bit value) should

grow from 0 to (99+1) * 4 = 400

– and then the duty cycle value should vary from 0 (0%

duty cycle) to 400 (100% duty cycle).

PWM configuration and use

• C18 timers libraries allow :

– to configure Timer2 (OpenTimer2 function).

– to configure CCPx (OpenPWMx functions).

– to set the duty cycle (SetDCPWMx functions).

– to stop the PWM (ClosePWMx functions).


• The pins concerned must be configured as outputs

– by setting the appropriate TRIS register bit

– or by using SetOutputPWMx functions.

Example

• Let’s assume that we need a 20kHz PWM period.

– On our board, Fosc = 48MHz thus Fosc/4 = 12MHz.

– To have a full resolution (10-bit) we must choose the

biggest value for the 8-bit period register.

– If we choose 0xFF, the clock frequency will be divided

by 256 to obtain the Timer2 frequency.

– Now, we are able to calculate the prescaler :


– Now, we are able to calculate the prescaler :

(12.106 / 256) / 20000 = 2.34 < 4

– We have to reverse the calculus with a prescale value

of 4 and we’ll find the period register value :

PR2 = (12.106 / 4) / 20000 – 1 = 149

– Thus the duty cycle should be between 0 and 600.

Code example

#include <timers.h>

#include <pwm.h>

…

OpenTimer2( // Timer2 configuration

TIMER_INT_OFF & // No interrupt

T2_PS_1_4 & // Prescale 1:4

T2_POST_1_1 // Don’t use postscaler actually

);

OpenPWM1(149); // Use of CCP1 as PWM module




SetDCPWM1(0); // Set 0% duty cycle to PWM1


TRISCbits.TRISC2 = 0; // Make RC2 (PWM1) an output

TRISCbits.TRISC1 = 0; // Make RC1 (PWM2) an output

…



…

State machine

• We will use state machine diagram to program sequential comportment of the robot.

• We will translate the diagram into C language by using switch/case statements.

• We will study an example to understand clearly the principles.


principles.

• We would like the two wheels to cover the same distance.

• The simplest way to do it is exposed on the next slide.

Example of state machine diagram

0

1

set the distance to cover

BP0 pushed

two motors on

leftdistance

done

right

distancedone


32 left motor onright motor on

right

distance

done

left

distance

done

Code example…

void main(void)

{

//local variables definition and initialization

unsigned char State = 0; // Store the actual state

char Bp0; // Store 1 if BP0 is pushed

unsigned int LeftEncoder, // Store Timer1 value

RightEncoder, // Store Timer0 value

Distance; // Store "distance" to cover

//peripheral configuration

…

while(1) { // main loop

Bp0 = !(PORTBbits.RB3); // Read BP0 status


Bp0 = !(PORTBbits.RB3); // Read BP0 status

RightEncoder = ReadTimer0(); // Read right encoder value

LeftEncoder = ReadTimer1(); // Read left encoder value

switch (State) { // State machine

case 0 : // State 0

SetDCPWM1(0); // Set 0% duty cycle to left pwm

SetDCPWM2(0); // Set 0% duty cycle to right pwm

if (Bp0) { // BP0 is pushed

State = 1; // Go to State 1

WriteTimer0(0); // Reset encoders

WriteTimer1(0);

Distance = 1000; // Set distance to cover

}

break; // end case 0

case 1 :



if (LeftEncoder >= Distance) {

// Left wheel covers the distance


} else if (RightEncoder >= Distance) {

// Right wheel covers the distance


}


case 2 :



if (RightEncoder >= Distance) {





}


case 3 :



if (LeftEncoder >= Distance) {

// Left wheel covers the distance


}


} // end switch

} // end while

} // end main

Conclusion

• We have learnt

– how to use Timers

– how to control the speed of a DC motor

– how to generate PWM signals

– how to program a simple state machine


• We’ll use this knowledge

– to manage with distance measurement

– to control the speed of the robot

– to run a special trajectory

Now, practice

• Use timer1 to count events on left encoder

• Use timer0 to count events on right encoder

• Change speed of motors using PWM

• The robot covers a circle shaped trajectory

• The robot covers a house shaped trajectory


Technology

Lecture 2 timers, pwm, state machine IN PIC