Motorola MVME1X7P - Manual (Page 219)

Programming Model
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frequency is used for BCLK. To provide a 1 MHz clock to the tick timers,
the prescaler adjust register should be programmed based on the following
equation:
prescaler clock adjust register = 256 - BCLK (MHz)
For example, for operation at 20 MHz the prescaler value is $EC, at 25
MHz it is $E7, and at 33 MHz it is $DF.
Non-integer Local Bus clocks introduce an error into the specified times
for the tick timers. The tick timer clock can be derived by the following
equation.
tick timer clock = BCLK / (256 - prescaler value)
The maximum clock frequency for the tick timers is the BCLK frequency
divided by two. The value 255 ($FF) is not allowed to be programmed into
this register. If a write with the value of $FF occurs to this register, the
PCCchip2 terminates the cycle properly with TA*, but the register remains
unchanged.
ADR/SIZ $FFF42015 (8 bits)
BIT 23 . . . 16
NAME Prescaler Clock Adjust
OPER R/W
RESET $DF P