Cirrus-logic CS4271 User Manual download pdf (Page 30)

CS4271

30 DS592F1

5.2.4 Internal Digital Loopback

In Control Port Mode, the CS4271 supports an internal digital loopback mode in which the output of the ADC is rout-

ed to the input of the DAC. This mode may be activated by setting the LOOP bit in the Mode Control 2 register (07h).

When this bit is set, the status of the DAC_DIF(2:0) bits in register 01h will be disregarded by the CS4271. Any

changes made to the DAC_DIF(2:0) bits while the LOOP bit is set will have no impact on operation until the LOOP

bit is released, at which time the Digital Interface Format of the DAC will operate according to the format selected in

the DAC_DIF(2:0) bits. While the LOOP bit is set, data will be present on the SDOUT pin in the format selected in

the ADC_DIF bit in register 06h.

5.2.5 Dither for 16-Bit Data

The CS4271 may be configured to properly dither for 16-bit data. To do this, the Dither16 bit in the ADC Control

Register (06h) must be set. When set, a half bit of dither is added to the least significant bit of the 16 most significant

bits of the data word. The remaining bits should be disregarded. See Figure 10. This function is useful when 16-bit

devices are downstream of the ADC. This bit should not be set when using word lengths greater than 16 bits.

It should be noted that this function is supported for all serial audio output formats, and may be activated in either

Master or Slave Mode.

5.2.6 Auto-Mute

The Auto-Mute function is controlled by the status of the AMUTE bit in the DAC Control register. When set, the DAC

output will mute following the reception of 8192 consecutive audio samples of static 0 or -1. A single sample of non-

static data will release the mute. Detection and muting are done independently for each channel. Auto-Mute detec-

tion and muting can become dependent on either channel if the MUTECA=B function is enabled. The common mode

on the output will be retained and the Mute Control pin for that channel will become active during the mute period.

The muting function is effected, similar to volume control changes, by the Soft and ZeroCross bits in the DAC Vol-

ume and Mixing Control register. The AMUTE bit is set by default.

5.2.7 High Pass Filter and DC Offset Calibration

The operational amplifiers in the input circuitry driving the CS4271 may generate a small DC offset into the A/D con-

verter. The CS4271 includes a high pass filter after the decimator to remove any DC offset which could result in

recording a DC level, possibly yielding "clicks" when switching between devices in a multichannel system.

The high pass filter continuously subtracts a measure of the DC offset from the output of the decimation filter. The

high pass filter can be independently enabled and disabled for channels A and B. If the HPFDisableA or HPFDis-

ableB bit is set during normal operation, the current value of the DC offset for the corresponding channel is frozen

and this DC offset will continue to be subtracted from the conversion result. This feature makes it possible to perform

a system DC offset calibration by:

1) Running the CS4271 with the high pass filter enabled until the filter settles. See the Digital Filter Characteristics

for filter settling time.

2) Disabling the high pass filter and freezing the stored DC offset.

16-Bit Word

23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Disregard Contents

½ Bit Dither

Figure 10. Example of Dither for 16-Bit Data with 24-Bit Left Justified Format

1 2 ... 25 26 27 28 29 30 31 32 33 34 35 ... 52 53

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Cirrus-logic CS4271 User Manual Page 30