Motorola MVME172 User Manual
Page 268
4-32
Computer Group Literature Center Web Site
IP2 Chip
4
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