CS5535 datasheet(88/555 Pages) NSC

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ACC Functional Description (Continued)

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4.3.7.4

Programming Model

Audio Playback/Record

The following discussion explains, in steps, how to initiate

and maintain a bus master transfer between memory and

an audio slave device. In the steps, the reference to Exam-

ple refers to Figure 4-7:

Software creates a PRD table in system memory. The

last PRD entry in a PRD table must have the EOT or

JMP bit set.

Example - Assume the data is outbound. There are

three PRDs in the example PRD table. The first two

PRDs (PRD_1, PRD_2) have only the EOP bit set.

The last PRD (PRD_3) has only the JMP bit set. This

example creates a PRD loop.

Software loads the starting address of the PRD table

by programming the PRD Table Address register.

Example - Program the PRD Table Address register

with Address_3.

Software must fill the buffers pointed to by the PRDs

with audio data. It is not absolutely necessary to fill the

buffers; however, the buffer filling process must stay

ahead of the buffer emptying. The simplest way to do

this is by using the EOP flags to generate an interrupt

when an audio buffer is empty.

Example - Fill Audio Buffer_1 and Audio Buffer_2.

Ensure than an interrupt service routine is assigned to

the audio interrupt.

Read the IRQ Status register to clear the Bus Master

Error and End of Page flags (if set).

Program the AC97 codec properly to receive audio

data (mixer settings, etc.).

Engage the bus master by setting the Bus Master

Enable bit.

The bus master reads the PRD entry pointed to by the

PRD Table Address register. Using the address from

the PRD, it begins the audio transfer. The PRD Table

Address register is incremented by eight.

Example - The bus master is now properly pro-

grammed to transfer Audio Buffer_1 to a specific

slot(s) in the AC97 interface.

The bus master transfers data from memory and

sends it to the AC97 Codec. At the completion of each

PRD, the bus master’s next response depends on the

settings of the flags in the PRD.

Example - After transferring the data described by

PRD_1, an interrupt is generated because the EOP bit

is set, and the bus master continues on to PRD_2. The

interrupt service routine reads the Second Level Audio

IRQ Status register to determine which bus master to

service. It refills Audio Buffer_1 and then reads the

bus master’s IRQ Status register to clear the End of

Page flag and the interrupt.

After transferring the data described by PRD_2,

another interrupt is generated because the EOP bit is

set, and the bus master continues on to PRD_3. The

interrupt service routine reads the Second Level Audio

IRQ Status register to determine which bus master to

service. It refills Audio Buffer_2 and then reads the

bus master’s IRQ Status register to clear the End of

Page flag and the interrupt.

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. No interrupt is gen-

erated for PRD_3.

Figure 4-7. ACC 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

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