Motorola MVME172 - Manual (Page 87)

Functional Blocks
http://www.mcg.mot.com/literature 2-9
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VMEbus to Local Bus Interface
The VMEbus to local bus interface allows an off-board VMEbus master
access to onboard resources. The VMEbus to local bus interface includes
the VMEbus slave, write post buffer, and local bus master.
Adhering to the IEEE 1014-87 VMEbus Standard, the slave can withstand
address-only cycles, as well as address pipelining, and respond to
unaligned transfers. Using programmable map decoders, it can be
configured to provide the following VMEbus capabilities:
Addressing capabilities: A24, A32
Data transfer capabilities: D08(EO), D16, D32, D8/BLT,
D16/BLT, D32/BLT, D64/BLT
(BLT = block transfer)
The slave can be programmed to perform write posting operations. When
in this mode, the chip latches incoming data and addressing information
into a staging FIFO and then acknowledges the VMEbus write transfer by
asserting DTACK. The chip then requests control of the local bus and
independently accesses the local resource after it has been granted the local
bus. The write-posting pipeline is two deep in the non-block transfer mode
and 16 deep in the block transfer mode.
To significantly improve the access time of the slave when it responds to
a VMEbus block read cycle, the VMEchip2 contains a 16 four-byte deep
read-ahead pipeline. When responding to a block read cycle, the chip
performs block read cycles on the local bus to keep the FIFO buffer full.
Data for subsequent transfers is then retrieved from the onchip buffer,
significantly improving the response time of the slave in the block transfer
mode.
The VMEchip2 includes an onchip map decoder that allows software to
configure the global addressing range of onboard resources. The decoder
allows the local address range to be partitioned into two separate banks,
each with its own start and end address (in increments of 64KB), as well
as set each bank’s address modifier codes and write post enable and snoop
enable.