Taking timing further Chapter Nine 9.1 – 9.8 Dr. Gheith Abandah1.

Taking timing further

Chapter Nine9.1 – 9.8

Dr. Gheith Abandah 1

Outline

• Timer 1• Timer 2• Capture/Compare/PWM• Pulse Width Modulation (PWM)• Digital to Analog Conversion (DAC)• Summary

Timing Issues

• Maintaining continuous counting functions• Recording (‘capturing’) in timer hardware the

time an event occurs• Triggering events at particular times• Generating repetitive time-based events• Measuring frequency, e.g., motor speed

The PIC 16 Series

Device Pins Features

16F84A 18 1 8-bit timer1 5-bit port1 8-bit port

16F873A16F876A

28 3 parallel ports,3 counter/timers,2 capture/compare/PWM,2 serial, 5 10-bit ADC, 2 comparators

16F874A16F877A

40 5 parallel ports,3 counter/timers,2 capture/compare/PWM,2 serial, 8 10-bit ADC, 2 comparators

PIC 16F84A Timer 0 Module

Option Register

• T0CS: Clock source select• T0SE: Source edge select• PSA: Prescaler assignment bit• PS2:PS0: Prescaler rate select

PIC 16F87XA Timer 1 Module

Timer 1 Registers16-bit register:•TMR1L (0Eh)•TMR1H (0Fh)

Control Register: •T1CON (10h)

Timer 1 control register

• T1CKPS1:T1CKPS0: Input Clock Prescale Select, 1:1-1:8

• T1OSCEN: Oscillator Enable Control• T1SYNC’: External Clock Input Synchronization

Control• TMR1CS: Clock Source Select• TMR1ON: Timer1 On

Derbot odometer

• Timers 0 and 1 are used to count pulses generated by the optical sensors mounted on the shaft encoders.

• The program drives the Derbot forward for 1m. It then completes a 180◦ turn on the spot and runs forward for 1m again. The program loops continuously in this manner.

Derbot odometer circuit

Odometer Example – Page 1

;Initialization

movlw B'01000100' ;set port A for right

movwf adcon1 ;analog/digital mix

movlw B'11101000' ;T0: external input,

movwf option_reg ;low to high transition,

;no prescale

movlw B'00000011' ;T1: no prescale,

movwf t1con ;oscillator disabled,

;external sync input

opto_move

clrf tmr0 ;clear timers

clrf tmr1l

clrf tmr1h

clrf flags

btfss portc,0 ;increment T1 if ip is zero,

incf tmr1l ;as 1st rising edge isn’t

;detected

call leftmot_fwd ;start motors running

call rtmot_fwdDr. Gheith Abandah 14

Because the counter must first have a falling edge before it starts to count.

opto_loop

movlw D'91' ;test for 1m

subwf tmr0,0

btfsc status,z

bcf porta,mot_en_left ;disable motor if =

movlw D'91'

subwf tmr1l,0

btfsc status,z

bcf porta,mot_en_rt ;disable motor if =

goto opto_loopDr. Gheith Abandah 15

Generating a ‘clock tick’ – a repetitive interrupt stream

Example: ‘clock tick’ generation

• Assuming an oscillator frequency of 4MHz, what is the slowest ‘clock tick’ rate that can be obtained from Timer 0 and Timer 1?

• T0: slowest interrupt rate with prescaler ÷256.• The input frequency to T0 is 1 MHz/256, or

3.906 kHz.• The 8-bit timer divides this frequency by 256 to

produce the clock tick frequency, which will be 3.906 kHz/256, or 15.26 Hz.

Example: ‘clock tick’ generation

• T1: slowest interrupt rate with prescaler ÷8.• The input frequency to T1 is 1 MHz/8, or 125

kHz.• The 16-bit timer divides this frequency by 216

to produce the clock tick frequency, which will be 125 kHz/ 216, or 1.91 Hz.

PIC 16F87XA Timer 2 Module

Timer 2 Registers8-bit register:•TMR2 (11h)

Control Register: •T2CON (12h)•PR2 (92h), period register

Timer 2 control register

• TOUTPS3:TOUTPS0: Output Postscale Select, 1:1-1:16

• TMR2ON: Timer 2 On• T2CKPS1:T2CKPS0: Clock Prescale Select, 1:1,

1:4, 1:16

The PR2 register, comparator and postscaler

Example: Timer 2 interrupt rate• Assuming an oscillator frequency of 4MHz, what is

the slowest ‘clock tick’ rate that can be obtained from Timer 2?

• Slowest interrupt rate with prescaler ÷16 and postscaler ÷16 .

• The input frequency is 1 MHz/16, or 62.5 kHz.• If PR2 is preset to 255, the frequency is divided by 256

to produce the reset frequency, which will be 62.5 kHz/256, or 244.14 Hz.

• With postscaler of 16, then the interrupt frequency will be 15.26 Hz.

The capture/compare/PWM (CCP) modules

• 2 CCP modules• Each CCP module contains a 16-bit register

which can operate as a:1. 16-bit Capture register2. 16-bit Compare register3. Pulse width modulation Master/Slave Duty Cycle

register

CCP Registers16-bit register:•CCPR1L (15h)•CCPR1H (16h)•CCPR2L (1bh)•CCPR2H (1ch)

Control Register: •CCP1CON (17h)•CCP2CON (1dh)

CCP x control register

• CCPxX:CCPxY: PWM Least Significant bits• CCPxM3:CCPxM0: Mode Select bits

CCPx Mode Select bits

Capture mode

Compare mode

Pulse width modulation

Time constant small compared to ‘on’ time.

The current rises from 10 to 90% of its final value in time 2.2L/R

Pulse width modulation

Time constant large compared to ‘on’ time, wide pulse

Time constant large compared to ‘on’ time,

narrow pulse.

PWM mode

Note 1: The 8-bit timer is concatenated with 2-bit internal Q clock, or 2 bits of the prescaler, to create 10-bit time base.

Waveforms for the 16F873A PWM generator

Calculations

T = (PR2 + 1) × (Timer 2 input clock period)= (PR2 + 1) × {Tosc × 4 × (Timer 2 prescale value)}

ton = (pulse width register) × (PWM timer input clock period),

= (pulse width register) × {Tosc × (Timer 2 prescale value)}

pulse width register = CCPR1L | CCP1CON<5:4> + 1

Example

PR2 is loaded with 249D. The clock oscillator frequency is 4 MHz. Neither pre- nor postscale. Find the PWM period.

T = (PR2 + 1) × {Tosc × 4 × (Timer 2 prescale value)}

= 250 × (250 ns × 4 × 1)= 250 μs

i.e. PWM frequency = 4.00 kHz.

PWM applied in the Derbot for motor control

Example waveforms

Derbot Motor Example – Page 1

;set up PWM

movlw B'00000100' ;switch on Timer2,

movwf t2con ;no pre or postscale

movlw B'00001100' ;enable PWM

movwf ccp1con

movwf ccp2con

movlw 0f9 ;249

movwf pr2

Derbot Motor Example – Page 2

leftmot_fwd ;run left motor forward

bsf porta,mot_en_left

movlw D'176'

movwf CCPR2L

return

leftmot_rev ;run left motor backward

bsf porta,mot_en_left

movlw D'80'

movwf CCPR2L

returnDr. Gheith Abandah 39

Generating PWM in software

• May use up all PWM resources or don’t have them in a low-cost microcontroller.

• PWM outputs can be generated based on software delay loops only.

• PWM outputs can also be generated based on timer interrupts.

Generating PWM with timer interrupt

cblock assembler directive

cblock 20

var1 ;reserve 1 byte for var1

var2 ;reserve 1 byte for var2

PWM used for digital-to-analog conversion (DAC)

RC low-pass filter characteristics

Derbot Example

fc = 1/(2π*100nF* 20kΩ) = 80 Hz

Generating a sine wave

Lower: the PWM stream.Upper: detail of analog output

T = 250 μsf = 4 kHz

Sine wave example – Page 1

clrf pointer

sin_loop

movf pointer,w

call sin_table ;get most significant byte

movwf ccpr1l ;move it to the PWM output

incf pointer,f ;increment the pointer

movf pointer,w

call sin_table ;get the MS byte

andlw B'11000000' ;we only use ms 2 bits

Sine wave example – Page 2

movwf temp

bcf status,c ;adjust for CCP1CON

rrf temp,f

rrf temp,w

iorlw B'00001100' ;set some CCP1CON bits

movwf ccp1con

incf pointer,f

movf pointer,w

call delay1

goto sin_loopDr. Gheith Abandah 49

Weaknesses of this method

• The output analog voltage is directly dependent on the logic levels of the PWM stream. These in turn are dependent on the accuracy of the power supply voltage.

• The low-pass filter cannot generate fast-changing signals..

• Running the PWM faster decreases the resolution.• There will always be some residual ripple on the

analog output.

Frequency measurement

Both a counter and a timer are needed, the timer to measure the reference period of time and the counter to count the number of events within that time.

Derbot speed measurement program

Speed measurement example – Page 1

Timer2_Int

decfsz int_cntr

goto int_end

;here if making a measurement

movf tmr0,w ;save counter values

movwf tmr0_temp

movf tmr1l,w

movwf tmr1_temp

clrf tmr0 ;clear counters

clrf tmr1l

Speed measurement example – Page 1

btfss portc,0 ;inc T1 if = 0, as first

incf tmr1l ;rising edge won’t be seen

movlw D'250' ;reload interrupt counter

movwf int_cntr

int_end

bcf pir1,tmr2if

retfie

Summary• Timing is an essential element of embedded system design –

both in its own right and to enable other embedded activities, like serial communication and pulse width modulation.

• A range of timers is available, with clever add-on facilities which extend their capability to capture, compare, create repetitive interrupts or generate PWM pulse streams.

• In applications of any complexity, a microcontroller is likely to have several timers running simultaneously, for quite different and possibly conflicting applications. The question remains open at this stage: how can these different time-based activities be marshaled and harmonized?

Taking timing further Chapter Nine 9.1 – 9.8 Dr. Gheith Abandah1.

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