Taking timing further
Chapter Nine
9.1 – 9.8
Dr. Gheith Abandah
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Outline
•
•
•
•
•
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Timer 1
Timer 2
Capture/Compare/PWM
Pulse Width Modulation (PWM)
Digital to Analog Conversion (DAC)
Summary
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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
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The PIC 16 Series
Device
Pins
Features
16F84A
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1 8-bit timer
1 5-bit port
1 8-bit port
16F873A
16F876A
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3 parallel ports,
3 counter/timers,
2 capture/compare/PWM,
2 serial, 5 10-bit ADC, 2 comparators
16F874A
16F877A
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5 parallel ports,
3 counter/timers,
2 capture/compare/PWM,
2 serial, 8 10-bit ADC, 2 comparators
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PIC 16F84A Timer 0 Module
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PIC 16F84A Timer 0 Module
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Option Register
•
•
•
•
T0CS: Clock source select
T0SE: Source edge select
PSA: Prescaler assignment bit
PS2:PS0: Prescaler rate select
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PIC 16F87XA Timer 1 Module
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Timer 1
Registers
16-bit register:
•TMR1L (0Eh)
•TMR1H (0Fh)
Control Register:
•T1CON (10h)
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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
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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.
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Derbot odometer circuit
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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
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Odometer Example – Page 2
opto_move
clrf tmr0
;clear timers
clrf tmr1l
Because the counter must
first have a falling edge
clrf tmr1h
before it starts to count.
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_fwd
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Odometer Example – Page 3
opto_loop
movlw D'91'
subwf tmr0,0
btfsc status,z
bcf porta,mot_en_left
movlw D'91'
subwf tmr1l,0
btfsc status,z
bcf porta,mot_en_rt
…
goto opto_loop
;test for 1m
;disable motor if =
;disable motor if =
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Generating a ‘clock tick’ – a
repetitive interrupt stream
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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.
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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.
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PIC 16F87XA Timer 2 Module
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Timer 2
Registers
8-bit register:
•TMR2 (11h)
Control Register:
•T2CON (12h)
•PR2 (92h), period
register
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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
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The PR2 register, comparator and
postscaler
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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.
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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 register
2. 16-bit Compare register
3. Pulse width modulation Master/Slave Duty Cycle
register
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CCP Registers
16-bit register:
•CCPR1L (15h)
•CCPR1H (16h)
•CCPR2L (1bh)
•CCPR2H (1ch)
Control Register:
•CCP1CON (17h)
•CCP2CON (1dh)
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CCP x control register
• CCPxX:CCPxY: PWM Least Significant bits
• CCPxM3:CCPxM0: Mode Select bits
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CCPx Mode Select bits
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Capture mode
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Compare mode
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Pulse width modulation
The current rises from
10 to 90% of its final
value in time 2.2L/R
Time constant small compared
to ‘on’ time.
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Pulse width modulation
Time constant large
compared to ‘on’ time,
narrow pulse.
Time constant large compared
to ‘on’ time, wide pulse
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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.
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Waveforms for the 16F873A PWM
generator
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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
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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.
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PWM applied in the Derbot for
motor control
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Example waveforms
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Derbot Motor Example – Page 1
;set up
movlw
movwf
movlw
movwf
movwf
movlw
movwf
...
PWM
B'00000100'
t2con
B'00001100'
ccp1con
ccp2con
0f9
pr2
;switch on Timer2,
;no pre or postscale
;enable PWM
;249
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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
return
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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.
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Generating PWM with timer
interrupt
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cblock assembler directive
cblock 20
var1
;reserve 1 byte for var1
var2
;reserve 1 byte for var2
…
endc
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PWM used for digital-to-analog
conversion (DAC)
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RC low-pass filter characteristics
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Derbot Example
fc = 1/(2π*100nF* 20kΩ)
= 80 Hz
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Generating a sine wave
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Lower: the PWM stream.
Upper: detail of analog output
T = 250 μs
f = 4 kHz
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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
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Sine wave example – Page 2
movwf
bcf
rrf
rrf
iorlw
movwf
incf
movf
temp
status,c
;adjust for CCP1CON
temp,f
temp,w
B'00001100' ;set some CCP1CON bits
ccp1con
pointer,f
pointer,w
call
goto
delay1
sin_loop
…
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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.
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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.
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Derbot speed measurement
program
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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
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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
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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 timebased activities be marshaled and harmonized?
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