Motorola MVME1X7P manual Prescaler clock adjust register = 256 Bclk MHz

Programming Model

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

3

http://www.motorola.com/computer/literature 3-21