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CS5535-UDC Datasheet(PDF) 88 Page - National Semiconductor (TI) |
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CS5535-UDC Datasheet(HTML) 88 Page - National Semiconductor (TI) |
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88 / 555 page  ACC Functional Description (Continued) www.national.com 88 Revision 0.8 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: 1) 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. 2) 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. 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. 4) 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. 5) 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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