
See the top level README for information on where to find documentation
for the raspberry pi and the ARM processor inside.  Also find information
on how to load and run these programs.

This example is for the older pi boards, see other directories for
other flavors of raspberry pi.

This example uses the ARM timer, not in free running mode like blinker03
but the other mode.

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 having 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.

