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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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Measuring pushbutton pulse width

RC2/CCP1

PIC18

Pulse Width

1. Capture TMR1 value on falling edge (Tf) in CCPR1

2. Capture TMR1 value on rising edge (Tr) in CCPR1

3. Pulse width = Tr – Tf (Elapsed Timer1 Tics)

Use interrupt to save timer values.

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Computing Pulse Width

In overflow case, the value can be greater > 16 bits so need to use a LONG type to hold TimerDelta value.

© Charles River Media

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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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After pulse width is captured, the capture_flag semaphore is set and the

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#define FOSCQ 29491200#define PRESCALE 2.0#define TMR1TIC 1.0/(FOSCQ/4.0)*PRESCALEdouble pulse_width_float;long pulse_width;main(void){ serial_init(95,1); // 19200 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;

swdetov.c (configuration)

Initialize Timer1

CCP1 used as input pin.

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swdet.c (main loop)

wait for capture

configure for falling edge, enable CCP1 interrupt, and timer1 interrupt

print pulse width

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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 1 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

}

swdet.c (main loop)First (falling) edge capture

Second (rising) edge capture

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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 = 7.3728 MHz HSPLL#define ONE_TIC 1.08492e-6

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last_capture = timer valueoverflow = -1

CCP1F = 0;Get 16-bit capture value (CCPR1H:CCPR1L)

interrupt service

edge_type?

yes

overflow?

1 (rising)

delta_time = (0 – last_capture) + this_capture;

overflow++;clear TMR1IF

TMR1IF?

0 (falling)

noCCP1IF?

-1

delta = this_capture – last_capture

Timer1 overflow

disable TMR1interrupt

return from interrupt

no overflow during button press

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Timer1 Overflow

0x0000

0xFFFF

First capture (A)

Second capture (B)

Timer1 is free running, captured at any time

no overflow case (B > A)delta = B - A

0x0000

0xFFFF

First capture (A)

Second capture (B)

overflow casedelta = (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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Interrupt Servicevolatile 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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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 1 TMR1IE = 0; // disable timer 1 interrupts TMR1IF = 0; // clear flag just in case } capture_flag = 1; }}

2nd edge

no overflow case

overflow case

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Timer1 Scaling

Precaling options are 1,2,4,8. However, can be clocked by an source independent of main oscillator.

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Implementing a Clock/Calendar

Use 32.768KHz crystal on T1OS0/T1OS1 pins, this crystal frequency good for accurate time keeping

32.768KHz = 32768 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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Intensity Based Infrared

emitter receiver

Rout (dc volt)

Vref

Vdd

Vout

Vout = Vdd, IR present (Rout > Vref)

Vout = 0v, IR absent (Rout < Vref)

+

Problem: value for Vref changes depending on ambient light!

time

time

Increase in ambient light raises DC bias

Vref

Vref

volt

age

volt

age

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How to block Ambient Light?

Vdd

Transmitter

~~

Receiver

amplifier

capacitor blocks DC

Open/Close switch at particular frequency

DC voltage here depends only how fast transmitter is switched

Transmitter

Waveform

switch opening/closing

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Integrated IR Receiver

Actual IR receiver a bit more complicated

All of this is in here

1 23Out Gnd

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IR WaveformWaveform produced by receiver when stimulated by a universal remote control depends on function and manufacturer.

Start ‘0’ ‘1’

# of bits depends on IR remote function

Stop

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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Space-Width Encoding Examples

T 2T T 3T T 2T

‘0’ ‘1’ ‘0’

Bit Time

Bit Value

REC-80 code, Panasonic: ‘0’ period = 3T, ‘1’ period = 4T

T T 2T T T

‘0’ ‘1’ ‘0’

Bit Time

Bit Value

T

Sony code: ‘0’ period = 2T, ‘1’ period= 3T

From: “A Primer on Remote Control Technology”, Innotech Systems, Inc.

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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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Example Waveform

‘0’ ‘0’ ‘0’ ‘0’ ‘0’ ‘0’ ‘0’‘1’ ‘1’ ‘1’ ‘1’ ‘1’ ‘1’ ‘1’ ‘0’ ‘0’

assume LSB first

First Byte Second Byte

11101000 = 0xE8 00110010 = 0x32

Print first two bytes of frame to screen.

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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 high-to-low is a ‘0’ transition low-to-high is a ‘1’

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Experiment 12 NotesEnsure 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’.