Getting Started with QNX Neutrino - QNX Software Systems

Advanced topics
© 2009, QNX Software Systems GmbH & Co. KG.
Real Time Neutrino Time Activity
11:22:05 11:22:00 Wake up at “now” + 00:00:30 (see below)
11:22:15 11:22:15 Clock gets adjusted as before
11:22:20 11:22:20 Power management turns off CPU
11:22:30 11:22:30 CLOCK_REALTIME wakes up
11:22:35 11:22:35 CLOCK_MONOTONIC wakes up
11:45:07 11:45:07 Power management turns on CPU, and
CLOCK_SOFTTIME wakes up
Using different clock sources
There are a few things to note here:
• We precomputed our wakeup time as “now” plus 30 seconds and used an absolute
timer to wake us up at the computed time. This is different from waking up in 30
seconds using a relative timer.
• Note that for convenience of putting the example on one time-line, we’ve lied a
little bit. If the CLOCK_REALTIME thread did indeed wake up, (and later the same
for CLOCK_MONOTONIC) it would have caused us to exit out of power
management mode at that time, which would then cause CLOCK_SOFTTIME to
wake up.
When CLOCK_SOFTTIME “over-sleeps,” it wakes up as soon as it’s able — it doesn’t
stop “timing” while the CPU is powered down, it’s just not in a position to wake up
until after the CPU powers up. Other than that, CLOCK_SOFTTIME is just like
CLOCK_REALTIME.
To specify one of the different clock source, use a POSIX timing function that accepts
a clock ID. For example:
#include
int
clock_nanosleep (clockid_t clock_id,
int flags,
const struct timespec *rqtp,
struct timespec *rmtp);
The clock_nanosleep() function accepts the clock_id parameter (telling it which clock
source to use), a flag (which determines if the time is relative or absolute), a
“requested sleep time” parameter (rqtp), as well as a pointer to an area where the
function can fill in the amount of time remaining (in the rmtp parameter, which can be
NULL if you don’t care).
162 Chapter 3 • Clocks, Timers, and Getting a Kick Every So Often April 30, 2009