Motorola MVME1X7P manual Using Bus Timers

Using Bus Timers 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:

Local bus timer	Measures the time an access to an onboard resource
	takes
VMEbus access	Measures the time from when the VMEbus request has
timer	been initiated to when a VMEbus grant has been
	obtained
Global VMEbus	Measures the time from when a VMEbus cycle begins to
timer	when it completes

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 μsec.

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

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