WM9704M Просмотр технического описания (PDF) - Wolfson Microelectronics plc

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WM9704M

AMC’97 Audio and Modem Codec

Wolfson Microelectronics plc 

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WM9704M

DEVICE DESCRIPTION

INTRODUCTION

The WM9704M comprises a stereo 18-bit Codec, (that is, 2 ADCs and 2 DACs) plus two additional

DACs. The additional pair of DAC channels are used to convert the Modem Tx data mapped from the

Line1 interface slot, at the sample rate selected in the Modem Rate register. Additionally, a vendor

specific mode is provided to allow dual line support for line1 and line2.

Alternatively, the WM9704M can be configured as a standard AC’97 Rev 2.1 device. In this mode the

device includes line level outputs and support for the variable rate audio sample rates specified in

Rev 2.1.

The WM9704M is fully compliant with Rev 2.1 of the AC’97 specification. The WM9704M comprises

a stereo 18-bit Codec, (that is, 2 ADCs and 2 DACs) plus two additional DACs. The WM9704M also

has a comprehensive analogue mixer with 4 sets of stereo inputs, plus phone, 2 microphone, and

PC-beep inputs. Additionally, on-chip reference circuits generate the necessary bias voltages for the

device, and a bi-directional serial interface allows transfer of control data and DAC and ADC words to

and from the AC’97 controller. The WM9704M supports 18-bit resolution within the DAC and ADC

functions, but the AC’97 serial interface specification allows any word length up to 20-bits to be

written to, or read from, the AC’97 Codec. These words are MSB justified, and any LSBs not used

will simply default to 0. Normally it is anticipated that 16-bit words will be used in most PC type

systems. Therefore, for the DAC, 16-bit words will be downloaded into the Codec from the controller,

along with padding of 0s to make the 16-bit word up to 20-bit length. In this case, the WM9704M will

process the 16-bit word along with 0 padding bits in the 2 LSB locations (to make 18-bit). At the ADC

output, WM9704M will provide an 18-bit word, again with 0s in the two LSB locations (20-bit). The

AC’97 controller will then ignore the 4 LSBs of the 20-bit word. When the WM9704M is interrogated,

it responds indicating it is an 18-bit device.

The WM9704M has the ADC and DAC functions implemented using oversampled, or ‘sigma-delta’

converters, and uses on-chip digital filters to convert these 1-bit signals to and from the 48ks/s 16/18-

bit PCM words that the AC’97 controller requires. The digital parts of the device are powered

separately from the analogue to optimise performance, and 3.3V digital and 5V analogue supplies

may be used on the same device to further optimise performance. Digital IOs are 5V tolerant when

the analogue supplies are 5V, so the WM9704M may be connected to a controller running on 5V

supplies, but use 3.3V for the digital section of WM9704M. WM9704M is also capable of operating

with a 3.3V supply only (digital and analogue).

An internally generated midrail reference is provided at pin CAP2 which is used as the chip

reference. This pin should be heavily decoupled.

The WM9704M is not limited to PC-only applications. The ability to powerdown sections of the device

selectively, and the option to choose alternative master clock, and hence sample rates, means that

many alternative applications in areas such as telecomms, may be anticipated.

Additional features added to the Intel AC’97 specification, such as the EAPD (External Amplifier

Power Down) bit, internal connection of PC-beep to the outputs in the case where the device is reset,

are supported, along with optional features such as variable sample rate support.

OPERATIONAL MODES

The WM9704M is pin programmable to operate in the following modes:

MODE1 MODE0

PIN

PIN

OPERATIONAL

MODE

0

0

2-channel

1

1

Modem mode

Table 1 Operating Modes

RESULTING DEVICE BEHAVIOUR

Rev 2.1 compliant codec (2-channel mode)

Modem DAC mode

WOLFSON MICROELECTRONICS LTD

PD Rev 3.2 January 2001

11

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