LM4935RLEVAL datasheet(27/91 Pages) TI1

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LM4935, LM4935RLEVAL

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SNAS296D – OCTOBER 2005 – REVISED MARCH 2006

4.2.11 ADC_2 CONFIGURATION REGISTER

This register is used to control the LM4935's audio ADC.

Table 4-12. ADC_2 (0x07h)

Bits

Field

Description

ULAW/ALAW

If COMPAND is set then the data across the PCM interface to the DAC and from the ADC is companded as

follows:

ULAW/ALAW

Commanding Type

µ-law

A-law

COMPAND

If set the 16 bit PCM data from the ADC is companded before the PCM interface and the PCM data to the

DAC is treated as companded data.

ADC_MUTE

If set the analog inputs to the ADC are muted.

5:3

AGC_FRAME_TIME This sets the frame time to be used by the AGC algorithm. In a given frame, the AGC's peak detector

determines the peak value of the incoming microphone audio signal and compares this value to the target

value of the AGC defined by AGC_TARGET (bits [3:1] of register (0x08h)) in order to adjust the microphone

preamplifiers gain accordingly. AGC_FRAME_TIME basically sets the sample rate of the AGC to adjust for a

wide variety of speech patterns. (Note 15)

AGC_FRAME_TIME

Time (ms)

0002

0012

128

0102

192

0112

256

1002

384

1012

512

1102

768

1112

1000

ADC_I2S_M

If set the DAC clock system is enabled to drive the I2S in master mode. The Point B frequency should be

double that at Point C. This bit should be set when using the I2S interface in master mode to read SAR

information whenever both the audio ADC and DAC are inactive.

AUDIO_IF_2_16BIT

If set the PCM and I2S interfaces are 16 bits per word in master mode. The 2 last clock cycles per word are

25% shorter to allow generation.

4.2.12 AGC_1 CONFIGURATION REGISTER

This register is used to control the LM4935's Automatic Gain Control. (Note 16)

Table 4-13. AGC_1 (0x08h)

Bits

Field

Description

AGC_ENABLE

If set the AGC controls the analog microphone preamplifier gain into the system. The microphone input must

be passed to the ADC.

3:1

AGC_TARGET

Programs the target level of the AGC. This will depend on the expected transients and desired headroom.

Refer to AGC_TIGHT (bit 7 of 0x09h) for more detail.

AGC_TARGET

Target Level

0002

−6 dB

0012

−8 dB

0102

−10 dB

0112

−12 dB

1002

−14 dB

1012

−16 dB

1102

−18 dB

1112

−20 dB

Application Information

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