Motorola MVME172 manual Prescaler Control Register, Prescaler register = 256 B clock MHz

2

VMEchip2

Prescaler Control Register

ADR/SIZ		$FFF4004C (8 bits of 32)
BIT	7	. . .	0
NAME		Prescaler Adjust
OPER		R/W
RESET		$DF P

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.

2-68 Computer Group Literature Center Web Site