Motorola MVME172 manual Dmac VMEbus Requester

Functional Blocks

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.

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