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VMEchip2
2
Tick Timers
2-16
Software is required to load the appropriate constant, depending
upon the local bus clock, following reset to ensure proper operation
of the prescaler.
The VMEchip2 includes two general purpose tick timers. These
timers can be used to generate interrupts at various rates or the
counters can be read at various times for interval timing. The timers
have a resolution of 1 µs and when free running, they roll over
every 71.6 minutes.
Each tick timer has a 32-bit counter, a 32-bit compare register, a
4-bit overflow register, an enable bit, an overflow clear bit, and a
clear-on-compare enable bit. The counter is readable and writable
at any time and when enabled in the free run mode, it increments
every 1µs. When the counter is enabled in the clear-on-compare
mode, it increments every 1µs until the counter value matches the
value in the compare register. When a match occurs, the counter is
cleared. When a match occurs, in either mode, an interrupt is sent
to the local bus interrupter and the overflow counter is
incremented. An interrupt to the local bus is only generated if the
tick timer interrupt is enabled by the local bus interrupter. The
overflow counter can be cleared by writing a one to the overflow
clear bit.
Tick timer one or two can be programmed to generate a pulse on
the VMEbus IRQ1 interrupt line at the tick timer period. This
provides a broadcast interrupt function which allows several VME
boards to receive an interrupt at the same time. In certain
applications, this interrupt can be used to synchronize multiple
processors. This interrupt is not acknowledged on the VMEbus.
This mode is intended for specific applications and is not defined in
the VMEbus specification.
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