This is NOT for the pi3 nor pi2, look in the boards directory or in my
separate raspberrypi-three repo.  If you are new to interrupts
I recommend first buying a pi-zero and learning there.  Then aarch32
mode on the pi3 then lastly aarch64 mode on the pi3.

---------

See the top level README for information on where to find the
schematic and programmers reference manual for the ARM processor
on the raspberry pi.  Also find information on how to load and run
these programs.

This example uses the ARM timer, not the free running ARM timer which
counts up (see blinker03) but the other timer that counts down.

A few more typos in the datasheet.  It is an SP804 not AP804 (page 196).
Page 197, bit 1 32 bit counter not 23 bit counter.  Page 198 neither
the raw nor masked IRQ registers are at address 0x40C.

So if you want to poll an arbitrary time with a free running timer you
take a reference count, and then take a sample, wait for the difference
between the sample and reference count.  You have to know if it is an
up or down counter.  You can turn this timer into a free running timer
by setting the load registers to all ones.  What you can also do with
this kind of timer that you cannot with a free runing timer is set the
load registers to some number of ticks (minus 1) and it will roll over
the counter at that rate.  Often these kinds of timers have an interrupt
as does this one.  And you can poll the interrupt bit without havingt to
actually configure an interrupt.  This example sets the prescaler to
divide by 250.  The clock as running here is 250Mhz, so this takes it
down to 1Mhz, 1000000 clocks per second.  Now if we set the load registers
to 4 million counts (minus 1) then every 4 million counts at 1 million
per second is 4 seconds.  Every 4 seconds the timer interrupt will fire.
Which you can read in the raw irq (not masked) register.  Once it is
set you have to write anything to the clear interrupt register.  If you
were to set up an interrupt service routine, you would clear that
interrupt in the interrupt handler.