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 Department
Mechatronic

Timers and PWM
Bibliography
Microcontroller PIC18F4550 Data Sheet
39632c.pdf - Chapters 11 to 15
MPLAB_C18 libraries documentation
MPLAB_C18_Libraries_51297f.pdf - Chapters 2.7 and 2.9
MPLAB_C18 header files
timers.h and pwm.h
MPLAB_C18 C source files
mcc18srctraditionnalpmc

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 electronics
DC
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
left
distance
done
right
distance
done
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

Lecture 2 timers, pwm, state machine IN PIC

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