AD1845 datasheet(33/40 Pages) AD

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AD1845

–33–

REV. C

The AD1845 performs a sequenced power-down that eliminates

audible effects from the DAC’s output, and saves the codec’s

internal operating state. Clearing the bits (writing a “0” to the

control bits) returns the AD1845 from the power-down state

and begins the initialization sequence. The AD1845 exits the

power-down mode within 1 sample period. However, an

additional 128 sample periods are required to unmute the out-

puts and restore the internal settings to the pre-Power-Down

operating state.

•

Standby: Entering the Standby mode places the ADC, DAC

and the Mixer into a low power state, and forces all outputs

to be muted. Standby turns off all internal digital and analog

circuitry with the exception of the digital interface and the

voltage reference. All ADC and DAC data is flushed includ-

ing data in the capture and playback FIFOs.

•

Mixer Power-Down: Entering the Mixer Power-Down mode,

causes both the mixer and the DAC circuitry to be turned

off. All DAC data is flushed including data in the playback

FIFO. In this mode the mixer is off and the AD1845 is

muted, but the ADC remains functional.

•

Mixer Only: The Mixer Only mode is initiated by powering

down both the ADC and DAC, leaving the analog mixer and

the digital interface active. MIC, LINE, AUX1, AUX2, and

M_IN can be mixed in the analog domain on the AD1845

outputs. All ADC and DAC data is flushed including data in

the capture and playback FIFOs.

•

ADC Power-Down: Entering the ADC Power-Down mode,

causes the ADC digital and analog engines to be turned off.

All ADC data is flushed including data in the capture FIFO

and the AD1845 is rendered deaf. The input programmable

gain amplifier (PGA) is also shut down. The DAC and

mixer remain active allowing the AD1845 to continue to

playback and mix samples.

•

DAC Power-Down: Entering the DAC Power-Down mode

suspends the DAC digital and analog engines, and all DAC

data is flushed including data in the playback FIFO. How-

ever, the mixer and ADC are functional allowing the

AD1845 to continue to capture and mix samples.

AUTOCALIBRATION

The AD1845 calibrates the ADCs and DACs for greater accu-

racy by minimizing dc offsets. Upon power-up or after

RESET,

the AD1845 automatically performs an autocalibration after the

first return from the Mode Change Enable state, regardless of

the state of the ACAL bit. Autocalibration can be forced when

the AD1845 returns from the Mode Change Enable state and

the ACAL bit in the Interface Configuration register has been

set. If the ACAL bit is not set, the RAM normally containing

ADC and DAC offset compensations will be saved, retaining

the offsets of the most recent autocalibration.

The completion of autocalibration can be determined by polling

the Autocalibrate-In-Progress (ACI) bit in the Test and Initial-

ization Register, which will be set during autocalibration. Trans-

fers enabled during autocalibration do not begin until the

completion of autocalibration.

The following summarizes the procedure for autocalibration:

• Set the Mode Change Enable (MCE) bit.

• Set the Autocalibration (ACAL) bit.

• Clear the Mode Change Enable (MCE) bit.

• The Autocalibrate-In-Progress (ACI) bit will remain HI for

384 sample periods. Poll the ACI bit until it transitions from

HI to LO.

• Set desired gain/attenuation/mute and digital mix values.

During the autocalibration sequence, data output from the

ADCs is meaningless. Inputs to the DACs are ignored. Even if

the user specified the muting of all analog outputs, near the end

of the autocalibration sequence, dc analog outputs very close to

VREF will be produced at the line output.

CHANGING SAMPLE RATES

In MODE1 the AD1845 can change sample rates by entering

the Mode Change Enable state or writing directly to the Clock

and Data Format Register. In MODE2, the AD1845 changes

sample rates by writing directly to the Upper and Lower Fre-

quency Select Register. Please refer to the following examples

for changing the sample rate.

To change the selection of the current sample rate by entering

the Mode Change Enable state requires the sequence which is

summarized as follows (this is the same sequence used by the

AD1848, AD1846, CS4248, and CS4231):

• Set the Mode Change Enable (MCE) bit.

• In a single write cycle, change the Clock Frequency Divide

Select (CFS2:0) and/or the Clock Source Select (CSS).

• The AD1845 now needs to resynchronize its internal states to

the new clock. Writes to the AD1845 will be ignored. Reads

will produce “1000 0000 (80h)” until the resynchronization is

complete. Poll the Index Register until something other than

this value is returned.

• Clear the Mode Change Enable (MCE) bit.

• If ACAL is set, follow the procedure described in

“Autocalibration” above.

• Wait 128 sample cycles or poll the ACI bit until it transitions

LO.

• Set to desired gain/attenuation values, and unmute DAC

outputs (if muted).

Alternatively, the AD1845 can be programmed to change the

sample rate selection “on the fly” without entering the Mode

Change Enable Sequence. The following sequence applies to

the AD1845 operating in MODE1 or MODE2.

• In a single write cycle, change the Clock Frequency Divide

Select (CFS2:0) and/or the Clock Source Select (CSS). For

compatibility reasons, the AD1845 will send out “1000 0000

(80h)” for approximately 200

µs. Even this short wait can be

disabled by setting the INITD bit. When the INITD bit is set,

the AD1845 is ready immediately after changing the sample

rate using CFS and CSS.

• The AD1845 now needs to resynchronize its internal states to

the new clock. Writes to the AD1845 will be ignored. Reads

will produce “1000 0000 (80h)” until the resynchronization is

complete. Poll the Index Register until something other than

this value is returned.

• Set to desired gain/attenuation values, and unmute DAC

outputs (if muted).

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