Embedded Microcomputer Systems
Lecture 16.1
Dan, I was going use input
capture and build you a
"Universal Remote"
yippie
But the thought of you
having universal control
over anything is too much
for me to handle.
oh, man!
JwV
Lecture 16 objectives are to use:
• Input capture to generate edge based interrupts;
• Input capture to measure period;
• Input capture to measure pulse width;
CCPx pins used for input capture: e.g., CCP0=PD4
See book section 8.1
Period Measurement
Select clock period, t because measurement resolution
TIMER0_TAILR_R = 0xFFFF
by Jonathan W. Valvano
Embedded Microcomputer Systems
Lecture 16.2
Choose edge (rise/fall)
Arm interrupt on capture
ISR
Poll to see which channel (if needed)
Now = captured time
Period = Now-Last
Last = Now
Acknowledge interrupt
Save/process Period
Figure 6.1. Ping))) sensor signals. See PingDocs.pdf for an explanation of this figure.
1) make the SIG pin an output;
2) issue a 5 s output pulse;
3) switch the SIG pin to back to an input; and
4) measure the tIN.
Rising edge, record TAR
Falling edge, calculate tIN.
For HCSR04, skip steps 1,3
by Jonathan W. Valvano
Embedded Microcomputer Systems
Lecture 16.3
+3.3V TLC2274 or OPA2350
Microcontroller
+3.3V
5k
V1
R2
V2
+
Input capture
QRB1134
2V
+3.3V
100
R1
light
V2
V1
3.3V
2V
0V
Use AddPeriodicThread to detect period too long (speed to slow)
Tachometer data collected with a 15-slot disk powered at +9V. IR sensor interface on left
by Jonathan W. Valvano
Embedded Microcomputer Systems
Lecture 16.4
What does this circuit do?
100k
3.3V
10k
Vin
Vout
10k
3.3V
10k
See two examples
InputCapture_4F120 count edges
PeriodMeasure_4F120 16-bit period measurement.
How to choose the resolution
Determine the minimum and maximum robot speed
Convert speed to tachometer period
For example
Period
P resolution
Speed
Speed
7100
4
10
3.521127
211.2676
holes/rotation
usec
rps
RPM
by Jonathan W. Valvano
Embedded Microcomputer Systems
Lecture 16.5
Should you filter the tachometer?
Reduce noise (good)
Increase lag (very very bad)
Raw
IIR filter, y(n) = (y(n-1)+x(n))/2
FIR filter, y(n) = (x(n)+x(n-1))/2
Median of x(n),x(n-1),x(n-2)
7600
7550
Period (10  s)
7500
7450
7400
7350
7300
7250
7200
7150
7100
1
3
5
7
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49
Sample number
How to detect a speed too slow (period too large)?
Clear a counter on each tachometer edge
Increment the counter on each rollover 0000 to FFFF
If counter >= 2, then wheel is stopped
by Jonathan W. Valvano