WM9705_06 WOLFSON [Wolfson Microelectronics plc], WM9705_06 Datasheet - Page 19

• Manufacturer Part Number: WM9705_06
• Manufacturer: WOLFSON [Wolfson Microelectronics plc]

WM9705
PEN DIGITIZER AND AUXILIARY ADC
3-D STEREO ENHANCEMENT
w
A 4 wire input pen digitiser function is included on WM9705. This circuit comprises driver circuits
to drive typical resistive touch screens of the type used on PDA’s, plus a 12 bit resolution ADC to
convert pen input values. This ADC may also be used to perform additional auxiliary ADC
conversions of the levels present on the AUXADC, BMON, PCBEEP or PHONE pins. A control bit
(PHIZ in register 78h) is provided to allow PCBEEP and PHONE inputs to be made high
impedance (internally disconnected so signal paths are cut) if required.
Operation of the pen digitiser function is controlled from digitiser control registers 76h and 78h.
The ADC conversion result is obtained by reading from the contents of bits [11-0] in register 7Ah,
or optionally by enabling the AC link SLOT transfer method, when results are sent back in the
AC’97 slot data format. The pen digitiser ADC is a 12bit successive approximation type converter
with excellent differential non-linearity performance.
The pen digitiser ADC may be used to convert either pen input data, or the voltages present on
the AUXADC, BMON, PCBEEP or PHONE pins. Such functions as battery monitoring or
temperature measurement might therefore be implemented.
The following pen digitiser features are available:
Support for wake-on-pen-down
Pen down detection, pressure measurement, auxiliary conversions
MASK conversion delay override or synchronous operation option
SLOT or R/W register data transfer
Programmable screen drive to sample taken delay
Programmable Pen-down detection threshold
Details of pen digitiser operation are available in the Pen Digitiser description section. Note that
the pins allocated for X/Y screen connections are those that would normally be used for AUX and
VID stereo inputs in a conventional AC ’97 codec. In WM9705 these pins remain connected to the
MIXER and ADC inputs, and may be used as analogue inputs, with the restriction that gain
through the mixer input is fixed at 0dB. The normal MUTE function is provided using bit 15 in the
appropriate register. It is recommended that these MUTE bits are left ‘mute’ whenever the screen
is driven. Reading back the registers will report 0dB gain, and the MUTE value as programmed.
ADC gain control on the AUX and VID inputs works as normal.
This device contains a stereo enhancement circuit, designed to optimise the listening experience
when the device is used in a typical PC operating environment. That is, with a pair of speakers
placed either side of the monitor with little spatial separation. This circuit creates a difference
signal by differencing left and right channel playback data, then filters this difference signal using
lowpass and highpass filters whose time constants are set using external capacitors connected to
the CX3D pins 33 and 34. Typically the values of 100nF and 47nF set highpass and lowpass
poles at about 100Hz and 1kHz respectively. This frequency band corresponds to the range over
which the ear is most sensitive to directional effects.
The filtered difference signal is gain adjusted by an amount set using the 4-bit value written to
Register 22h bits 3 to 0. Value 0h is disable, value Fh is maximum effect. Typically a value of 8h
is optimum. The user interface would most typically use a slider type of control to allow the user to
adjust the level of enhancement to suit the program material. Bit D13 3D in Register 20h is the
overall 3D enable bit. The Reset Register 00h reads back the value 11000 in bits D14 to D10.
This corresponds to decimal 24, which is registered with Intel as Wolfson Stereo Enhancement.
Note that the external capacitors setting the filtering poles applied to the difference signal may be
adjusted in value, or even replaced with a direct connection between the pins. If such adjustments
are made, then the amount of difference signal fed back into the main signal paths may be
significant, and can cause large signals which may limit, distort, or overdrive signal paths or
speakers. Adjust these values with care, to select the preferred acoustic effect. There is no
provision for pseudo-stereo effects. Mono signals will have no enhancement applied (if the signals
are in phase and of the same amplitude). Signals from the PCM DAC channels can have stereo
PD Rev 4.4 August 2006
Production Data
19