Timer System Applications. Microcomputer Architecture and Interfacing Colorado School of Mines Professor William Hoff

Size: px

Start display at page:

Download "Timer System Applications. Microcomputer Architecture and Interfacing Colorado School of Mines Professor William Hoff"

Stuart Fletcher
5 years ago
Views:

1 Timer System Applications 1

pulse The distance can be determined by the time of flight to

2 Ultrasonic sensor An ultrasonic range sensor emits a high frequency sound pulse, then measures the time to the reflected pulse The distance can be determined by the time of flight to the object (and the speed of sound) Ultrasonic sensor mfd by Devantech Ltd 2

3 Ultrasonic sensor timing Note - The minimum width of the trigger pulse is 10 us, but make the pulse about 200 us wide so that it is easier to see on the oscilloscope. 3

4 Ultrasonic sensor (continued) Assume we want trigger the sensor to send a DT wide pulse at intervals of PSEND Sent pulse PSEND DT We will use output compare, with interrupts The interrupt service routine (ISR) simply adds the appropriate delay (DT or PSEND-DT) to the compare register See example in lecture slides on output compare 4

5 Ultrasonic sensor (continued) The sensor raises an output signal called ECHO when the sonar pulse is sent It lowers the signal when the sonar pulse is received We will use input capture to get the pulse width of the return pulse The time difference (between the time of the received pulse and the latest sent pulse) can be used to determine distance Input capture ISR: capture rising edge; capture falling edge, compute Dt = t2 t1 5

top dead center From http://dallasautosports.net/computerized_engine_control.

6 Application engine control module Modern automobile engines use a microcontroller to control spark timing (as well as other things) Want to generate a spark at a prescribed time when the piston has achieved a certain position A sensor generates a pulse when the piston has reached top dead center From The microcontroller generates a signal to output a spark at a pre-specified delay time after this signal The delay depends on RPM, engine temperature, load, etc Engine control module in Toyota Rav4 6

7 Generating a delayed pulse We can use input capture with output compare to generate a delayed pulse Let s say that we want to generate an output pulse a certain time after an input pulse HCS12 t a t b t c IC1 OC3 DELCNT PWCNT 7

8 Approach: generating a delayed pulse We ll set up input capture to look for a rising edge t a t b t c When found, the input capture ISR will set up output compare to make the output pin go high at time t b = t a + DELCNT When the output compare occurs, the output compare ISR will set up the output pin to go low at time t c = t b + PWCNT DELCNT PWCNT 8

9 Main program Initialize IC1 to look for low-to-high edge Enable IC1 interrupts Disable OC3 interrupts Turn on interrupt system Wait forever Pseudo code for example IC1ISR Clear interrupt flag Get captured time t a t b = t a + DELCNT Store t b into TC3 Set OC3 to go high on next match Clear OC3 flag Enable OC3 interrupt OC3ISR Disable OC3 interrupts t c = t b + PWCNT Store t c into TC3 Set OC3 to go low on next match 9

10 Summary / Questions Input capture and output compare can work together to achieve useful functionality; for example, generating an output signal at a precise time relative to an input signal Could you achieve this kind of precision using software alone? 10

Measuring Distance Using Sound

Measuring Distance Using Sound Distance can be measured in various ways: directly, using a ruler or measuring tape, or indirectly, using radio or sound waves. The indirect method measures another variable