Motorola MVME172 manual Control Word Byte Count

4

IP2 Chip

Each DMAC’s control is divided into two registers. The first register is only accessible by the processor. The second register can be loaded by the processor in the direct mode and by the DMAC in the command chaining mode.

There is a case when the byte count for a DMA (to local bus) operation is initially set larger than the actual received data. In this case, there is residual data in the internal data paths of the DMA controller. To flush the data, set the byte count register to 0. A normal termination of the DMA occurs after the byte count register has been initialized to zero. Do not use the DMAEND signal on the IP bus to terminate a DMA operation for this case. This would terminate the DMA process in such a way that the data could not be flushed from the fifo data path.

Once a DMAC is enabled, its counter and control registers should not be modified by software. When the command chaining mode is used, the list of commands must be in local (not IP), 32-bit memory and the entries must be aligned to a 16-byte boundary. That is, the address which is loaded into the DMA table address counter must have bits three through zero set to a zero. This is true for the initial value which is loaded by the processor or the subsequent values which are loaded by the command chaining logic. If they are not set to zero, the command chaining process will halt.

A DMAC command packet includes a control word that defines: single command interrupt enable, DMA transfer direction, IndustryPack data width, sDMA or aDMA selection, and the DMAEND direction and usage. The format of the control word is the same as control register 2. The command packet also includes a local bus address, an IP address, a byte count, and a pointer to the next command packet in the list. The end of a command is indicated by setting bit 0 or 1 of the next command address. The command packet format is shown in the following table.

Entry					Function
0	31			Address of Next Command Packet		0
1	31				Local Bus Address	0
2	31	Control Word	24	23	Byte Count	0
3	31		24	23	IndustryPack Address DMA	0

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