wm9708 Wolfson Microelectronics plc, wm9708 Datasheet - Page 13

• Manufacturer Part Number: wm9708
• Description: Ac?97 Revision 2.1 Audio Codec
• Manufacturer: Wolfson Microelectronics plc

Production Data
DEVICE DESCRIPTION
INTRODUCTION
VARIABLE SAMPLE RATE SUPPORT
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The WM9708 comprises a stereo 18-bit Codec, (that is, 2 ADCs and 2 DACs) a comprehensive
analogue mixer with 2 sets of stereo inputs, phone, microphone, and PC-beep inputs. Additionally,
on-chip reference generation 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 WM9708 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 WM9708 will process the 16-bit word along
with 0 padding bits in the 2 LSB locations (to make 18-bit). At the ADC output, WM9708 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 WM9708 is interrogated at Register 00h, it responds
indicating it is an 18-bit device.
The WM9708 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 I/Os are 5V tolerant when
the analogue supplies are 5V, so the WM9708 may be connected to a controller running on 5V
supplies, but use 3.3V for the digital section of WM9708. WM9708 is also capable of operating with a
3.3V supply only (digital and analogue).
An internally generated midrail reference is provided at pin CAP which is used as the chip reference.
This pin should be heavily decoupled. Refer to Figure 15 for more details.
The WM9708 is not limited to PC-only applications. The ability to power down 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 telecoms, may be anticipated.
The DACs and ADCs on this device support all the recommended sample rates specified in the Intel
Revision 2.1 specification for audio rates. The default rate is 48ks/s. If alternative rates are selected
and variable rate audio is enabled (Register 2Ah, bit 0), the AC’97 interface continues to run at 48k
words per second, but data is transferred across the link in bursts such that the net sample rate
selected is achieved. It is up to the AC’97 Revision 2.1 compliant controller to ensure that data is
supplied to the AC link, and received from the AC link, at the appropriate rate.
The device supports on demand sampling. That is, when the DAC signal processing circuits need
another sample, a sample request is sent to the controller which must respond with a data sample in
the next frame it sends. For example, if a rate of 24ks/s is selected, on average the device will
request a sample from the controller every other frame, for each of the stereo DACs. Note that if an
unsupported rate is written to one of the rate registers, the rate will default to the nearest rate
supported. The Register will then respond when interrogated with the default sample rate.
The left and right channels of the ADCs and DACs always sample at the same rate.
PD, Rev 4.2, August 2009
WM9708
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