wm9704m Wolfson Microelectronics plc, wm9704m Datasheet - Page 11
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wm9704m
Manufacturer Part Number
wm9704m
Description
Amc97 Audio Modem Codec
Manufacturer
Wolfson Microelectronics plc
Datasheet
1.WM9704M.pdf
(37 pages)
Production Data
DEVICE DESCRIPTION
INTRODUCTION
OPERATIONAL MODES
WOLFSON MICROELECTRONICS LTD
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.
The WM9704M is pin programmable to operate in the following modes:
Table 1 Operating Modes
MODE1
PIN
0
1
MODE0
PIN
0
1
OPERATIONAL
Modem mode
2-channel
MODE
RESULTING DEVICE BEHAVIOUR
Rev 2.1 compliant codec (2-channel mode)
Modem DAC mode
PD Rev 3.2 January 2001
WM9704M
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