Motorola MVME172 - Manual (Page 146)

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VMEchip2
2
Prescaler Control Register
The prescaler provides the various clocks required by the counters and
timers in the VMEchip2. In order to specify absolute times from these
counters and timers, the prescaler must be adjusted for different local bus
clocks. The prescaler register should be programmed based on the
following equation. This provides a one MHz clock to the Tick timers.
prescaler register = 256 - B clock (MHz)
For example, for operation at 25 MHz the prescaler value is $E7, and at 32
MHz it is $E0.
Non-integer local bus clocks introduce an error into the specified times for
the various counters and timers. This is most notable in the tick timers. The
tick timer clock can be derived by the following equation.
tick timer clock = B clock / (256 - prescaler value)
If the prescaler is not correctly programmed, the bus timers do not generate
their specified values and the VMEbus reset time may be violated. The
maximum clock frequency for the tick timers is the B clock divided by
two. The prescaler register control logic does not allow the value 255
($FF) to be programmed.
ADR/SIZ $FFF4004C (8 bits of 32)
BIT 7 . . . 0
NAME Prescaler Adjust
OPER R/W
RESET $DF P