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Computer Group Literature Center Web Site
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
Содержание MVME172
Страница 1: ...MVME172 VME Embedded Controller Programmer s Reference Guide VME172A PG2 Edition of February 1999 ...
Страница 6: ...Place holder ...
Страница 18: ...xviii ...
Страница 78: ...1 60 Computer Group Literature Center Web Site Board Description and Memory Maps 1 ...
Страница 332: ...5 42 Computer Group Literature Center Web Site MCECC 5 ...
Страница 336: ...A 4 Related Documentation A ...
Страница 340: ...VMEchip2 Tick Timer 1 Periodic Interrupt Example B 4 Computer Group Literature Center Web Site B ...
Страница 352: ...Index IN 12 Computer Group Literature Center Web Site I N D E X ...
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