QDK PIC24/dsPIC-XC16 - Quantum Leaps

QDK
PIC24/dsPIC-XC16
www.state-machine.com/pic
NOTE: The QS_onGetTime() callback is always called with interrupts locked.
Figure 12 shows how the Timer2 is extended to 32 bits. The TMR2 register counts up from zero to the
reload value stored in the PR2 register. When TMR2 reaches PR2, the hardware automatically clears TMR2
on the subsequent clock tick. Simultaneously, the hardware sets the _T2IF flag, which triggers the
system clock tick interrupt.
The system clock tick ISR _T2Interrupt() keeps updating the “tick count” variable l_tickTime (see
Listing 4(5) and Listing 8(2)) by incrementing it each time by QS_tickPeriod_. The clock-tick ISR also
clears the _T2IF flag.
Figure 12: Using 16-bit Timer2 to provide 32-bit QS time stamp.
count
32-bit time stamp
returned from
QS_onGetTime()
count in
_T2Interrupt()
System
clock tick
period
0x00FFFFFF
Count in TMR2
Current Register
Timer2
Reload
Value
Register
System
clock-tick
time
Listing 12(11-15) shows the implementation of the function QS_onGetTime(), which combines all this
information to produce a monotonic time stamp.
(12) The QS_onGetTime() function tests the _T2IF flag to determine whether This flag being set means
that the TMR2 counter has rolled over to zero, but the _T2Interrupt() ISR has not run yet
(because interrupts are still locked).
(13) Most of the time the _T2IF flag is not set, and the time stamp is simply the sum of l_tickTime +
(uint32_t)TMR2).
(13) If the _T2IF flag is set, the TMR2 counter misses one update period and must be additionally
incremented by BSP_TMR2_PERIOD_, which is a compile-time constant.
Copyright © Quantum Leaps, LLC. All Rights Reserved.
32 of 35