Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Clocks and timers
Clock interrupt sources
That’s the end of the timer five-cent tour.
Actually, there’s a little bit more to it than first meets the eye.
So where does the clock interrupt come from? Here’s a diagram that shows the
hardware components (and some typical values for a PC) responsible for generating
these clock interrupts:
1.1931816
MHz clock
Time of
day
82C54
÷ 11931
Clock
handler
Timers
Applications
Applications
PC clock interrupt sources.
As you can see, there’s a high-speed (MHz range) clock produced by the circuitry in
the PC. This high-speed clock is then divided by a hardware counter (the 82C54
component in the diagram), which reduces the clock rate to the kHz or hundreds of Hz
range (i.e., something that an ISR can actually handle). The clock ISR is a component
of the kernel and interfaces directly with the data structures and code of the kernel
itself. On non-x86 architectures (MIPS, PowerPC), a similar sequence of events
occurs; some chips have clocks built into the processor.
Note that the high-speed clock is being divided by an integer divisor. This means the
rate isn’t going to be exactly 10 ms, because the high-speed clock’s rate isn’t an
integer multiple of 10 ms. Therefore, the kernel’s ISR in our example above might
actually be interrupted after 9.9999296004 ms.
Big deal, right? Well, sure, it’s fine for our 15-second counter. 15 seconds is 1500
timer ticks — doing the math shows that it’s approximately 106 μs off the mark:
15 s - 1500 × 9.9999296004 ms
= 15000 ms - 14999.8944006 ms
= 0.1055994 ms
= 105.5994 μs
Unfortunately, continuing with the math, that amounts to 608 ms per day, or about
18.5 seconds per month, or almost 3.7 minutes per year!
You can imagine that with other divisors, the error could be greater or smaller,
depending on the rounding error introduced. Luckily, the kernel knows about this and
corrects for it.
April 30, 2009 Chapter 3 • Clocks, Timers, and Getting a Kick Every So Often 139