Motorola MVME172 - Manual (Page 203)

Programming Model
http://www.mcg.mot.com/literature 3-15
3
Programming the Tick Timers
There are four programmable tick timers in the MC2 chip. These timers are
identical in function to the timers implemented in the PCCchip2 and the
VMEchip2.
Tick Timer 1 and 2 Compare and Counter Registers
The Tick Timer Counter is compared to the Compare Register. When they
are equal, an interrupt is sent to the local bus interrupter and the overflow
counter is incremented. If the clear on compare mode is enabled, the
counter is also cleared. For periodic interrupts, the following equation
should be used to calculate the compare register value for a specific period
(T).
T (
µ
s) = Compare Register
When programming the tick timer for periodic interrupts, the counter
should be cleared to zero by software and then enabled. If the counter does
not initially start at zero, the time to the first interrupt may be longer or
shorter than expected. The rollover time for the counter is 71.6 minutes.
The Tick Timer Counter, when enabled, increments every microsecond.
Software may read or write the counter at any time.