1. Time Measurement
Capture Mode of the Capture/Compare/PWM module is used for time measurement.
TMR1 or TMR3 16-bit value transferred to 16-bit capture register on edge detect.
Rising or falling edge detect, with interrupt flag set.
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2. Measuring pushbutton pulse width
PIC18
RC2/CCP1
Pulse Width
Capture TMR1 value on falling edge (Tf) in CCPR1
Capture TMR1 value on rising edge (Tr) in CCPR1
Pulse width = Tr – Tf (Elapsed Timer1 Tics)
Use interrupt to save timer values.
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3. Computing Pulse Width
© Charles River Media
In overflow case, the value can be greater > 16 bits so need to use a LONG type to hold TimerDelta value.
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4. ISR for capturing pulse width.
tmr1_ov variable keeps track of timer1 overflows.
After falling edge, reconfigure for rising edge capture.
After rising edge, compute delta timer tics
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6. After pulse width is captured, the capture_flag semaphore is set and the
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7. swdetov.c (configuration)
#define FOSCQ #define PRESCALE 2.0 #define TMR1TIC 1.0/(FOSCQ/4.0)*PRESCALE double pulse_width_float; long pulse_width; main(void){ serial_init(95,1); // in HSPLL mode ptr = (char *) &this_capture; // initialize timer 1 // prescale by 2 T1CKPS1 = 0; T1CKPS0 = 1; T1OSCEN = 0; // disable the oscillator TMR1CS = 0; //use internal clock FOSC/4 T1SYNC = 0; // set CCP1 as input bitset(TRISC,2); // enable interrupts IPEN = 0; PEIE = 1; GIE = 1;
Initialize Timer1
CCP1 used as input pin.
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8. swdet.c (main loop)
configure for falling edge, enable CCP1 interrupt, and timer1 interrupt
wait for capture
print pulse width
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9. swdet.c (main loop) First (falling) edge capture
for (;;) { // loop forever err_flag = 0; CCP1CON = 0; //turn off when changing modes CCP1CON = 0x4; // capture every falling edge edge_type = 0; // looking for falling edge capture_flag = 0; TMR1IF = 0; // clear timer 1 interrupt flag TMR1IE = 1; // allow timer 1 interrupts TMR1ON = 1; // enable timer while(!capture_flag) { asm("clrwdt"); //wait forever for button press } capture_flag = 0; CCP1CON = 0; /* turn off when changing modes */ CCP1CON = 0x5; /* capture every rising edge */ edge_type = 1; /* looking for rising edge */ while(!capture_flag) { asm("clrwdt"); // wait for rising edge }
First (falling) edge capture
Second (rising) edge capture
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10. swdet.c (main loop, cont.)
// continued from code on previous slide
if (err_flag) { // multiple overflows printf("**Timer 1 overflowed more than once! **"); pcrlf(); } else { // scale to microseconds pulse_width_float = (delta * ONE_TIC)*1.0e6; pulse_width = (long) pulse_width_float;
printf ("Switch pressed, timer ticks: %ld, pwidth: %ld (us)", delta,pulse_width); pcrlf(); } }
based on Fosc, pre-scale
// pre=8, crystal = MHz HSPLL #define ONE_TIC e-6
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11. interrupt service Timer1 overflow no overflow during button press no
TMR1IF?
CCP1IF?
yes
CCP1F = 0; Get 16-bit capture value (CCPR1H:CCPR1L)
overflow++; clear TMR1IF
1 (rising)
0 (falling)
edge_type?
last_capture = timer value overflow = -1 -1
Timer1 overflow
overflow?
delta_time = (0 – last_capture) + this_capture;
delta = this_capture – last_capture
no overflow during button press
disable TMR1 interrupt
return from interrupt
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12. Timer1 Overflow Timer1 is free running, captured at any time 0x0000
Second capture (B)
First capture (A)
First capture (A)
Second capture (B)
0xFFFF
0xFFFF
no overflow case (B > A) delta = B - A
overflow case delta = (0 – A) + B
Note that B can occur after ‘A’, so value can be > 0xFFFF, so need a ‘long’ for delta value!!!
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13. Interrupt Service pack 2 bytes into one integer. First edge
volatile unsigned int last_capture, this_capture; volatile long delta; volatile signed char tmr1_ov, err_flag; timer_isr(void){ if (TMR1IF) { tmr1_ov++; // increment timer1 overflow TMR1IF = 0; } if (CCP1IF) { CCP1IF = 0; //clear capture interrupt flag this_capture = (CCPR1H << 8) | CCPR1L ; if (edge_type == 0) { last_capture = this_capture; tmr1_ov = -1; // clear timer 1 overflow count } else {
pack 2 bytes into one integer.
First edge
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14. Interrupt Service (cont.)
} else { if (tmr1_ov == -1) { // no overflow delta = this_capture - last_capture ; } else { // overflowed once delta = 0 - last_capture; delta = delta + this_capture; } } /* disable timer1 interrupt */ TMR1ON = 0; // disable timer TMR1IE = 0; // disable timer 1 interrupts TMR1IF = 0; // clear flag just in case } capture_flag = 1; } }
no overflow case
overflow case
2nd edge
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15. Timer1 Scaling
Precaling options are 1,2,4,8. However, can be clocked by an source independent of main oscillator.
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16. Implementing a Clock/Calendar
Use KHz crystal on T1OS0/T1OS1 pins, this crystal frequency good for accurate time keeping
32.768KHz = Hz
 when 16-bit counter rolls over, then exactly 2 seconds
 when counter = 0x8000, exactly 1 sec
No error in time keeping, good for long term clock/calendar function.
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17. Intensity Based Infrared
Vdd
Rout (dc volt) +
Vout
Vref
emitter
receiver
Vout = Vdd, IR present (Rout > Vref)
Vout = 0v, IR absent (Rout < Vref)
Increase in ambient light
raises DC bias
voltage
Vref
Problem: value for Vref changes depending on ambient light!
time
voltage
Vref
time
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18. How to block Ambient Light?
amplifier
Vdd
DC voltage here depends only how fast transmitter is switched
~ ~
capacitor blocks DC
Transmitter
Receiver
Open/Close switch at particular frequency
switch opening/closing
Transmitter
Waveform
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19. Integrated IR Receiver
Actual IR receiver a bit more complicated
All of this is in here 3 2
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Out 1
Gnd

20. IR Waveform
Waveform produced by receiver when stimulated by a universal remote control depends on function and manufacturer.
# of bits depends on IR remote function
Stop
Start
‘0’
‘1’
Length of start period, ’0’ period, ‘1’ period will vary with function. ‘0’, ‘1’ waveforms may be inverted. Called space-width modulation.
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21. Space-Width Encoding Examples
REC-80 code, Panasonic: ‘0’ period = 3T, ‘1’ period = 4T T 2T T 3T T 2T
Bit Time
Bit Value
‘0’
‘1’
‘0’
Sony code: ‘0’ period = 2T, ‘1’ period= 3T
Bit Time T T 2T T T T
Bit Value
‘0’
‘1’
‘0’
From: “A Primer on Remote Control Technology”, Innotech Systems, Inc.
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22. Lab #12: Decoding IR Waveform
You will be assigned a function on the Universal Remote.
Use the scope and determine the periods of ‘start’, ‘0’, and ‘1’ (assume a ‘0’ the short period, a ‘1’ is the long period).
Using ‘swdet.c’ as a starting point, decode the first two bytes of a frame and print these two bytes out to the screen via Hyperterm. Choose a particular prescale for Timer1 to give you enough fidelity on Timer1 to distinguish ‘1’,’0’ periods.
Your particular function may have LOTs of bits in a frame, just decode the first two bytes (16 bits). If your function does NOT have at least 16 bits, then get the TA to assign you a different IR remote function (or find one, and show this to the TA).
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23. Example Waveform assume LSB first Second Byte First Byte
‘0’
‘0’
‘0’
‘1’
‘0’
‘1’
‘1’
‘1’
‘0’
‘1’
‘0’
‘0’
‘1’
‘1’
‘0’
‘0’
assume LSB first
Second Byte
First Byte
= 0xE8
= 0x32
Print first two bytes of frame to screen.
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24. Biphase Encoding
In a previous example, ‘1’ and ‘0’ were distinguished by having different periods.
Some Remote function/manufacturer use biphase encoding; ‘1’ and ‘0’ have same period, but use different transition in middle of the period (low-to-high or high-to-low).
‘0’
‘0’
‘0’
‘0’
‘1’
‘1’
‘1’
‘1’
transition low-to-high is a ‘1’
transition high-to-low is a ‘0’
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25. Experiment 12 Notes
Ensure that your assigned IR Remote function does NOT use biphase encoding (must use space-width encoding).
Simply measure the time between falling edges – the first period will be the start period, the periods after that will be either ‘1’ or ‘0’.
Compute the number of timer tics for a ‘1’ or ‘0’, and compare what is measured. Use some slack, if you compute 2000 timer ticks for a ‘0’, and 4000 for a ‘1’, then distinguish a ‘1’ as being greater than 3000 tics.
You must compute an appropriate prescale for timer1 so that your timer tics will have necessary fidelity to distinguish between ‘0’ and ‘1’.
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