SAF784X datasheet(13/93 Pages) NXP

13 / 93 page

SAF784X_2

Product data sheet

Rev. 02 — 9 May 2008

13 of 93

NXP Semiconductors

SAF784x

One chip CD audio device with integrated MP3/WMA decoder

that can decrease the quality of audio. The clocks related to audio DAC and LF ADC are

generated directly from the analog signal, instead of being derived from high frequency

PLLs. The clocking strategy for the digital core is shown in Figure 9.

6.3 General purpose analog inputs

The four general purpose ADC inputs, GPIO0 (pin 33), GPIO1 (pin 34), GPIO2 (pin 35),

and GPIO3 (pin 36), can be used for giving the ARM microprocessor access to external

analog sources, such as monitoring temperature and to provide simple resistor-ladder

keypad functionality. These inputs use an additional pair of sigma-delta ADCs identical to

those used for the LF diode inputs.

The general purpose analog inputs have separate interrupt request lines and use address

space in the servo registers for storing the converted digital values. The output of the

general purpose ADCs are low-pass ﬁltered and can have ﬁne-offset compensation

added before being passed to a decimation ﬁlter. The digital values output from the

decimation ﬁlter are then captured in the servo registers with a resolution of 10 bits per

channel.

There are only two ADCs for general purpose application and so each ADC is multiplexed

between two inputs: ADC1 between GPIO0 and GPIO2, and ADC2 between GPIO1 and

GPIO3. GPIO2 and GPIO3 inputs are selected by signal AuxControlandGPADC.

6.4 Auxiliary analog inputs

Analog inputs, AUX_L and AUX_R, are available, and have sufﬁcient resolution, for the

input of external audio sources, such as allowing ARM access to an external audio source

for sound processing algorithms. This allows audio processing of external audio sources

via the AUX pins, whilst simultaneously using the general purpose inputs for keyboard and

temperature inputs.

Since these two inputs share one pair of the LF sigma-delta ADCs used in the LF path (for

inputs D1 and D2) a multiplexer is used to control the data source into the ADCs.

Therefore, D1 and D2 cannot be used at the same time as AUX_L and AUX_R. This path

is designed for a tuner input where the THD speciﬁcation is ~0.3 % and the S/N is

< 60 dB. These performance values are lower than when the normal CD audio path is

used i.e. S/N > 80 dB and THD < 0.01 %.

The audio data is converted to a pulse-density modulated digital stream for both input

channels. This data is then low-pass ﬁltered and decimated to produce 10-bit

representations of the analog inputs.

The auxiliary inputs differ from the general purpose analog inputs because the parallel

data is converted to an I2S format stream and then sent to the I2S handler block to make

the data available to the ARM microprocessor. The I2S handler contains a 12-deep data

FIFO which allows the ARM microprocessor to service the audio data with a lower priority

than it would need if it were directly registered; see Figure 6.

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