AMD Geode SC1201 - Manual (Page 169)

AMD Geode™ SC1200/SC1201 Processor Data Book 169
Core Logic Module 32579B
4) Read the SMI Status register to clear the Bus Master
Error and End of Page bits (bits 1 and 0).
Set the correct direction to the Read or Write Control
bit (Command register bit 3). Note that the direction of
the data transfer of a particular bus master is fixed and
therefore the direction bit must be programmed
accordingly. It is assumed that the codec has been
properly programmed to receive the audio data.
Engage the bus master by writing a “1” to the Bus
Master Control bit (Command register bit 0).
The bus master reads the PRD entry pointed to by the
PRD Table Address register and increments the
address by 08h to point to the next PRD. The transfer
begins.
Example - The bus master is now properly pro-
grammed to transfer Audio Buffer_1 to a specific
slot(s) in the AC97 interface.
5) The bus master transfers data to/from memory
responding to bus master requests from the AC97
interface. At the completion of each PRD, the bus mas-
ter’s next response depends on the settings of the
flags in the PRD.
Example - At the completion of PRD_1 an SMI is gen-
erated because the EOP bit is set while the bus mas-
ter continues on to PRD_2. The address in the PRD
Table Address register is incremented by 08h and is
now pointing to PRD_3. The SMI Status register is
read to clear the End of Page status flag. Since Audio
Buffer_1 is now empty, the software can refill it.
At the completion of PRD_2 an SMI is generated
because the EOP bit is set. The bus master then con-
tinues on to PRD_3. The address in the PRD Table
Address register is incremented by 08h. The DMA SMI
Status register is read to clear the End of Page status
flag. Since Audio Buffer_2 is now empty, the software
can refill it. Audio Buffer_1 has been refilled from the
previous SMI.
PRD_3 has the JMP bit set. This means the bus mas-
ter uses the address stored in PRD_3 (Address_3) to
locate the next PRD. It does not use the address in the
PRD Table Address register to get the next PRD. Since
Address_3 is the location of PRD_1, the bus master
has looped the PRD table.
Stopping the bus master can be accomplished by not read-
ing the SMI Status register End of Page status flag. This
leads to a second EOP which causes a Bus Master Error
and pauses the bus master. In effect, once a bus master
has been enabled it never has to be disabled, just paused.
The bus master cannot be disabled unless the bus master
has been paused or has reached an EOT.
Figure 6-12. PRD Table Example
Address_1
Size_1
EOT = 0
Audio
Buffer_1
Audio
Buffer_2
Size_1
Size_2
Address_1
EOP = 1
JMP = 0
Address_2
Size_2
EOT = 0
EOP = 1
JMP = 0
Address_3
Don’t Care
EOT = 0
EOP = 0
JMP = 1
PRD_1
PRD_2
PRD_3
Address_2
Address_3
32-byte
boundary