Motorola MVME172 - Manual (Page 206)

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MC2 Chip
3
Prescaler Clock Adjust Register
This register adjusts the prescaler so that it maintains a 1 MHz clock source
for the tick timers. To provide a 1 MHz clock, the prescaler adjust register
should be programmed based on the following equation:
Prescaler Clock Adjust Register = 256 - processor clock (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 processor clocks introduce an error into the specified times for
the tick timers. The tick timer clock can be derived by the following
equation:
Tick clock = processor clock / (256 - Prescaler Value)
The maximum clock frequency for the tick timers is the processor clock
divided by two. The value $FF is not allowed to be programmed into this
register. If a write with the value of $FF occurs to this register, the cycle
terminates correctly but the register remains unchanged.
Tick Timer 1 and 2 Control Registers
Each tick timer has a control register. The control registers for timers one
and two are defined in this section. Control registers for timers three and
four are described in a later section.
ADR/SIZ $FFF42014 (8 bits)
BIT 23 ... 16
NAME Prescaler Clock Adjust
OPER R/W
RESET $DF P