Motorola MVME172 - Manual (Page 91)

Functional Blocks
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transfers which are not an even byte count or start at an odd address, with
respect to the port size. A 16-bit device should respond with VA<1> high
or low. Devices on the local bus should respond to any combination of
LA<3..2>. This is required to support the burst mode on the MC68060 bus.
Normally when the non-increment mode is used, the starting address and
byte count would be aligned to the port size. For example, a DMA transfer
to a 16-bit FIFO would start on a 16-bit boundary and would have an even
number of 16-bit transfers. If the starting address is not aligned or the byte
count is odd, the DMA controller will increment the lower address lines.
This is required because the lower order address lines are used to define
the size of the transfer and the byte lanes.
The VMEbus uses VA<2..1>, LWORD*, and DS<1..0>* to define the
transfer size and byte lanes. If the VMEbus port size is D32, then VA<1>,
LWORD* and DS<1..0>* are used to define the transfer size and byte
lanes. During D16 transfers, the VMEbus address line VA<1> toggles. If
the VMEbus port size is D64, then VA<2..1>, LWORD* and DS<1..0>*
are used to define the transfer size and byte lanes. Local bus address
LA<3..0> and SIZ<1..0> are used to define the transfer size and byte lanes
on local bus. During local bus transfers, LA<3..2> count.
The DMA controller internally increments the VMEbus address counter
and if the transfer mode is BLT, the DMA controller generates a new
address strobe (AS*) when a block boundary is crossed.
DMAC VMEbus Requester
The chip contains an independent VMEbus requester associated with the
DMA Controller. This allows flexibility in instituting different bus tenure
policies for the single-transfer oriented master, and the block-transfer
oriented DMA controller. The DMAC requester provides all the signals
necessary to allow the onchip DMA Controller to request and be granted
use of the VMEbus.
Requiring no external jumpers, the chip provides the means for software to
program the DMAC requester to request the bus on any one of the four bus
request levels, automatically establishing the bus grant daisy-chains for the
three inactive levels.