Motorola MVME172 - Manual (Page 152)

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VMEchip2
2
Tick Timer 1 Control Register
EN When this bit is high, the counter increments. When this
bit is low, the counter does not increment.
COC When this bit is high, the counter is reset to zero when it
compares with the compare register. When this bit is low,
the counter is not reset.
COVF The overflow counter is cleared when a one is written to
this bit.
OVF These bits are the output of the overflow counter. The
overflow counter is incremented each time the tick timer
sends an interrupt to the local bus interrupter. The
overflow counter can be cleared by writing a one to the
COVF bit.
Prescaler Counter
The VMEchip2 has a 32-bit prescaler that provides the clocks required by
the various timers in the chip. Access to the prescaler is provided for test
purposes. The counter is described here because it may be useful in other
applications. The lower 8 bits of the prescaler counter increment to $FF at
the local bus clock rate and then they are loaded from the prescaler adjust
register. When the load occurs, the upper 24 bits are incremented. When
the prescaler adjust register is correctly programmed, the lower 8 bits
increment at the local bus clock rate and the upper 24 bits increment every
microsecond. The counter may be read at any time.
ADR/SIZ $FFF40060 (8 bits of 32)
BIT 7 6 5 4 3 2 1 0
NAME OVF COVF COC EN
OPER R C R/W R/W
RESET 0 PS 0 PS 0 PS 0 PS
ADR/SIZ $FFF40064 (32 bits)
BIT 31 . . . 0
NAME Prescaler Counter
OPER R/W
RESET 0 P