Motorola MVME172 manual Functional Blocks

Models: MVME172

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Functional Blocks

Using control register bits in the LCSR, the DMAC can be configured to provide the following VMEbus capabilities:

Addressing capabilities:

A16, A24, A32

Data transfer capabilities: D16, D32, D16/BLT, D32/BLT, D64/BLT (BLT = block transfer)

Using the DMA AM control register, the address modifier code that the VMEbus DMA controller places on the VMEbus can be programmed under software control. In addition, the DMAC can be programmed to execute block-transfer cycles over the VMEbus.

Complying with the VMEbus specification, the DMAC automatically terminates block-transfer cycles whenever a 256-byte (D32/BLT) or 2-KB (D64/BLT) boundary is crossed. It does so by momentarily releasing AS and then, in accordance with its bus release/bus request configuration, initiating a new block-transfer cycle.

To optimize VMEbus use, the DMAC automatically adjusts the size of individual data transfers until 64-bit transfers (D64/BLT mode), 32-bit transfers (D32 mode) or 16-bit transfers (D16 mode) can be executed.

Based on the address of the first byte, the DMAC transfers single-byte, double-byte, or a mixture of both, and then continues to execute transfer cycles based on the programmed data width. Based on the address of the last byte, the DMAC transfers single-byte, double-byte, or a mixture of both to end the transfer.

To optimize local bus use when the VMEbus is operating in the D16 mode, the data FIFO converts D16 VMEbus transfers to D32 local bus transfers. The FIFO also aligns data if the source and destination addresses are not aligned so the local bus and VMEbus can operate at their maximum data transfer sizes.

To allow other boards access to the VMEbus, the DMAC has bus tenure timers to limit the time the DMAC spends on the VMEbus and to ensure a minimum time off the VMEbus. Since the local bus is generally faster than the VMEbus, other local bus masters may use the local bus while the DMAC is waiting for the VMEbus.

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Page 89
Image 89
Motorola MVME172 manual Functional Blocks