Cirrus Logic AN199 User Manual

AN199

Application Note

DAI Interface for Playing MP3 Music

Introduction

The Maverick family of ARM7 core processors was designed to meet the needs of the "MP3" market to facilitate the design of a cost-effective digital audio decoder. MP3 is the general name used for this market, which does include other compressions schemes such as WMA, AAC, and ADPCM.

The Cirrus Logic 7209/12/73XX series microprocessors were all designed to support this market. Each one is unique, but all possess the necessary hardware to effectively support an "MP3" solution. Each can be used in a design to play a range of encoded music all based on different compression formats and sample rates.

This note will serve as a guide to initializing the Maverick processor to begin decoding music. Software decompression is beyond of the scope of the note since it involves a discussion of the compression schemes of the above mentioned formats.

Due to the changes in the DAI interface, a new hardware configuration is required to insure that the external CODECs receive the appropriate signals. This is discussed in the hardware interface section.

General Discussion

The DAI (Digital Audio Interface) is the buffering and synchronization mechanism for sending decoded digital music frames to the external CODEC(s) to be converted into audio. The DAI provides three synchronization clocks that sync the data, frame by frame, for the external CODEC. The DAI FIFO buffer is eight samples deep and is always filled with the decoded music frames.

The DAI FIFO controls the branch to the interrupt handler (FIQ). The FIFO can be configured a number of ways, but based on the state of the FIFO, the FIQ will be triggered at which time the information will be transferred to or from the FIFO. The information is either played or recorded.

The rule-of-thumb used in our reference player code is an interrupt rate that is ¼ of the sample rate. For a 44.1 kHz file, the FIQ will be hit at an approximate rate of 11kHz. This is accomplished by configuring the DAI FIQ to assert whenever the DAI FIFO is half-empty or contains only four frames.

To accomplish this, the DAI machine will need to be initialized properly and the FIQ Handler will need to process the buffered data correctly. The following discussion will insure the interrupt rate and integrity of the information passed to and from the DAI.

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DAI Initialization

The DAI machine shares the output pins with two other devices via an internal MUX. The DAI machine is selected, output pins are assigned to the machine, then enabled before exiting the routine. See the figure below.

DAI

CODEC

SSI2

SCLK

MCLK

LRCLK

SDIN

SDOUT

• To program the MUX shown above for DAI control, set bit 3 in the System Control Register 3 (SYSCON3).

• If an external clock will generate *MCLK, enable clock.

• The DAI Control Register (DAIR) is programmed with the lower 2 bytes 0404 required. The Right Channel

Transmit FIFO Interrupt Mask (RTCM) is enabled. This is the only interrupt which will call the FIQ for processing music. External clock source (ECS) bit should also be enabled if external MCLK generator is used.

• Clear DAI Status Register (DAISR) of any overrun or underrun bits. A write of 0xFFFFFFFF to this location is sufficient.

• Turn on the DAI by asserting pin 9 of SYSCON3.

• Enable the left and right channel FIFOs in the DAI Data Register 2 (DAIDR2). This will require checking the

DAI status register for each left and right channel enable to make sure the FIFO bit is set.

• Unmask the DAI Interrupt at the DAIINT register. Once unmasked, the DAI FIQ will assert since the RTCM is already half empty or more and you will immediately enter the FIQ Handler. There is no music to play so the handler must process the first entry with no data (refer to the FIQ Handler discussion).

Note: *The designer has the option of using an internally generated MCLK or an external MCLK for synchronization with the external

CODECs.

Programming the DAI for the 73xx

The 73xx chips have been enhanced with a DAI interface which allows for MCLK to be clocked at 64Fs (64 x sampling rate) as well as the traditional 128Fs provided by the 72xx series. There are CODECs on the market that support 64Fs mode.

The sample rate is also programmable. This allows for greater flexibility in the code design for individual songs digitized and compressed at different rates. The diagram and table below illustrate the DAI control.

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Programmable Divide

(AUDIV)

MUX

(AUDCLKSRC)

PLL

(73.728MHz)

EXTCLK

(11.2896)

Clock Source

(MHz)

73.728 32 4.0960 2.0480 9

11.2896 44.1 5.6448 2.8224 2

73.728 48 6.1440 3.0720 6

Sample Frequency

(kHz)

128Fs Audio Bit Clock

128/64(Fs)

(MHz)

Digital Audio Clock

Generation

7-bit

/32

Audio Bit Clock 128/64(Fs)

/128

/64

64Fs Audio Bit Clock

counter

fixed at 4

(MHz)

Audio

Sample

Frequency

(Fs)

SCLK

LRCLK(Fs)

MCLK (BUZ)

Audio Data

FIFO

Control

128/64 Divisor

(AUDDIV)

• Either 64Fs or 128Fs must be used for the 73XX. With one selected, the other must be turned off. For 128Fs, SYSCON3 bit 9 will be set, and bit 0 of the DAI64Fs control register will be cleared. For 64Fs, the converse must be applied.

• Register DAI64Fs must also be programmed to set the AUDCLKSRC, AUDIV, and to enable or disable 64Fs mode. MCLK256EN will be enabled. Use the table above or refer to the 73xx User Manual for more detail.

• Settings for 8-24 kHz sample frequencies are not included. The side-effect of using a sample frequency below 32 kHz is the raising of the noise floor at the external CODEC, thereby increasing the distortion of the original signal. This issue is external to the processor. In order to decrease noise, samples that fall below 24 kHz sample rate should pass through a sample rate conversion in software to bring it up to a higher rate. Conversion rates are as follows:

— 8 and 16 kHz to 32 kHz — 11 and 22 kHz to 44.1 kHz — 12 and 24 kHz to 48 kHz

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Programming the FIQHandler

MP3, AAC, WMA, etc., libraries decode left and right channel data which will both be stored at some designated address by the memory buffer. This address will be passed to the FIQ Handler so that data can be moved into the DAI left and right channel FIFOs.

• IMPORT the address of the buffers to the FIQ routine. This is the decompressed data to be used to fill the FIFOs.

• If data exists, write 4 samples to each left then right channel in that order until both FIFOs are half- full. The order here is important. Always start with the left channel and alternate with the right to make certain that both FIFOs are filled properly.

• If no data exists in the buffers, write four samples of silence to the FIFOs alternating between the left then right channels. Silence will consist of zeros. There is nothing decoded but the FIFOs are now half-full so the program can return from the handler and begin processing data.

• Read four samples from the receive buffer regardless of whether there is music to be played or not. If recording is not taking place at the time, discard the samples, otherwise store them into memory. This will insure that the DAI FIFO information is in sync. Otherwise the data becomes out of sync and will sound distorted.

• Clear overrun and underrun status bits in DAI status register in the same manner as mentioned above. This will insure we avoid an endless loop.

Hardware Interface

Generating MCLK has changed from the EP7212 to the EP7312 microprocessors. This issue is taken separately since the external hardware implementation is not described in detail in the EP73xx User’s Manual. Due to the enhancement of the DAI interface for the EP7312, this has lead to additional chip resources required to insure all signals are present at the external CODECs.

MCLK for the EP7212 is fed by an external oscillator (EXTCLK). That same signal is then passed to the external CODEC. The clock signals created internally are SCLK and LRCLK. MCLK is simply an input. The EP7212 is limited to 44.1 kHz sampled files, so the oscillator was fixed to 11.286 MHz or 256Fs. All music files, other than those sampled at 44.1 kHz, are sample rate converted in software to 44.1 kHz, then sent to the external DAC.

MCLK for the EP7312 is routed differently. Instead of a fixed sample rate, the DAI now allows internal clock adjustments to provide DAI clocking for all of the standard sample rates. To accommodate this change, MCLK is now provided by means of the BUZ pin. This can be referred to as MCLK (BUZ) in the User’s Manual, or as MCLKOUT.

MCLKIN (EXTCLK) is the MCLK signal pin on the DAI interface which can be programmed to receive either an external clock source or the internal PLL clock as the clock source. The EP7312 64DAIFS register allows the user to select what clock source will be used. This pin, as with the EP7212, is used as an input.

MCLK (BUZ) or MCLKOUT is also programmable by programming the 64DAIFS register. The signal, when enabled, will come out on the BUZ pin. Since the BUZ pin is multiplexed with the annuciator, this resource is dedicated either for that purpose, or for the DAI interface.

The diagram below shows the external hardware connections for MCLK. SCLK, LRCLK, SDTX, and SDRX are not shown since these signals remain the same across the entire family of processors.

MCLK Generation for the

EP7xxx Family

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7312

7212

MCLK

MCLKBUZ

Xtal(EXTCLK)

MCLKMCLK

DAC

Xtal(EXTCLK)

Conclusion

With the DAI initialization and the FIQ handler routines written correctly, you will achieve an interrupt rate that will improve the overall performance of the player code. The decoded information will also be shifted to the CODEC without losing information.

Our reference code, available under NDA, includes a complete initialization routine and interrupt handler which follows the method described above. This reference code can be used as a template for constructing routines.

Hardware interface examples for both processors are also available. Please contact epdapps@crystal.cirrus.com more information.

for

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• Notes •

Cirrus Logic AN199 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

Introduction

General Discussion

DAI Initialization

Programming the DAI for the 73xx

Programming the FIQHandler

Hardware Interface

Conclusion