Motorola MVME1X7P - Manual (Page 77)

Using Bus Timers
http://www.motorola.com/computer/literature 1-51
1
Using Bus Timers
This section illustrates the use of bus timers by describing the sequence of
events when the MPU on one single-board computer accesses the Local
Bus memory on another single-board computer using the VMEbus. This
scenario involves three bus timers, which normally should be set to quite
different values:
The sequence begins when the MPU asserts a request for the Local Bus.
The MPU must wait until the Local Bus is released by the current bus
master before its cycle can begin. When the MPU is granted the Local Bus,
it begins its cycle and the Local Bus timer starts counting. It continues to
count until an address decode of the VMEbus address space is detected and
then the timer stops. This is normally a very short period of time. In fact,
all Local Bus non-error bus accesses are normally very short, such as the
time to access onboard memory. Therefore, it is recommended this timer
be set to a small value, such as 8
µ
sec.
The next timer to take over when one single-board computer accesses
another is the VMEbus access timer. This measures the time from when
the VMEbus has been address-decoded (and hence a VMEbus request has
been made) to when VMEbus mastership has been granted. Because
experience has shown that some VME systems can become very busy, we
recommend this time-out be set to a large value, such as 32 msec. For
debug purposes this value can also be set to infinity.
Once the VMEbus has been granted, a third timer takes over. This is the
global VMEbus timer. This timer starts when a transfer actually begins
(DS0 or DS1 goes active) and ends when that transfer completes (DS0 or
Local bus timer Measures the time an access to an onboard resource
takes
VMEbus access
timer Measures the time from when the VMEbus request has
been initiated to when a VMEbus grant has been
obtained
Global VMEbus
timer Measures the time from when a VMEbus cycle begins to
when it completes