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Chapter 5 - Programming
the VMEbus Interface
Overview
The VMEbus is controlled by the Tundra Universe PCI-VME bridge chip. Active
access to the VMEbus is achieved by reading and writing to registers in the Universe
memory space. Because of dependencies between the PCI and VME sides of those
registers which are ported to both busses, considerable care must be exercised in both
reading and writing to these registers. For all but the most basic programming, try
using the programming interface provided by the RadiSys EPConnect software instead
of working directly with the VMEbus registers.
Register Initialization
At power-on-self-test (POST) time, a subset of the VME registers is initialized by the
BIOS. The Universe chip then initializes the remainder of the VME registers. This is
controlled by how the Universe chip is programmed.
Programming the Universe
The Universe chip can become system controller if the EPC-9 is in slot 1. It does this
by sampling the VME signal BG3IN*. If BG[3:0] is low after reset, which it is, due to
the Universe’s internal pull-down, then the EPC-9 is in slot 1 and the Universe
becomes system controller. When the Universe is the System Controller, its IACK
daisy chain driver and SYSCLK driver are also activated.
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