ECE2049: Embedded Computing in Engineering Design
C Term – Spring 2016
Lecture #12: Real World Timers
Reading for Today:
Reading for Next Class:
HW #3 (on web):
Lab #2 (on web):
User's Guide Ch 17
Davies 9.2-9.3, 9.7, User's Guide 20
Due Tuesday 2/9/16 (in class)
Early sign-off by 5 pm Fri 2/12. Report due 2/16 (in class)
Last Class: Introduction to the MSP430F5529's timers.
>> Setting the control registers for Timer A2
Default Frequencies:
ACLK = 32768 Hz;
MCLK = 1.048576 MHz;
SMCLK = 1.048576 MHz;
Stop Watch example continued...
>> We want to implement Stop Watch with accuracy of 0.01 seconds.
Selected ACLK = 32768 Hz as TimerA2 source clock and Up count mode
--> Timer generates Interrupt after MAX COUNT reached 327
/*** This function configures and starts Timer A2 to count ~0.01
second intervals ***/
void runtimerA2(void)
{ // From Users Guide Ch 17
// Use ACLK (TASSEL_1), clock divider of 1 (ID_0)
// and start timer counting in Up mode (MC_1)
TA2CTL = TASSEL_1 + MC_1 + ID_0;
//
TA2CCR0 = 327;
TA2CCTL0 = CCIE;
// 327+1 ACLK tics = ~1/100 seconds
// TA2CCR0 interrupt enabled
}
What will it mean when the CPU gets an interrupt from TimerA2? What should the ISR
do?
#pragma vector=TIMER2_A0_VECTOR
// syntax for ISR
__interrupt void Timer_A2_ISR(void)
{
}
At the top of main() you must enable also interrupts ...
// Using msp430.h definitions
_BIS_SR(GIE);
// Global Interrupt enable
>> What does the CPU do when it receives the interrupt (assuming interrupts are enabled)
and between interrupts
unsigned int
timer;
// global timer count variable
. . .
_BIS_SR(GIE);
.
.
// Global Interrupt enable
.
// Inside main()
while(1) { // Forever loop of the stop watch
. . .
chkBtns();
// check for button press
if(button1)
{
timer_on=0;
stoptimerA2(1);
// reset timer
P1OUT =~BIT0;
// turn LED off
}
if(button2)
{
timer_on=1;
runtimerA2();
// starts timer
P1OUT =~BIT0;
// turn LED off
GrClearDisplay(&g_sContext);
}
if (timer_on)
if (timer%10==0)
// update display every 1/10th sec
displayTime(timer);
} /* End while(1) loop */
How accurate will our stop watch be? Is that accuracy ok?
>> Duration of 1 clock tic = 1/32768 Hz = 3.053E-5 sec
>> Our stop watch will actually run a bit slow. How long until it is off by .01 sec
>> Not unusual for clock crystals to run at odd-ball frequencies (32768 Hz is actually
reasonably nice!)
>> We can compensate in software – using leap counts!
#pragma vector=TIMER2_A0_VECTOR
__interrupt void Timer_A2_ISR(void)
{
// timer and leap_cnt are Global unsigned integers
}
>> Is this perfect? Now how long is will the stop watch be accurate to 0.01 sec?
>> Now that TimerA2 and its associated ISR are properly configured and running
perfectly how we use the resultant count??
-- What does the value of the variable timer represent?
What if we wanted 1 ms resolution? What would change in our TimerA2 settings?
Our ISR? How we use timer?
What about ½ second resolution?
Or 0.0001 second? Should we still use ACLK as the clock source here? What to do??