WM9712L_11 Datasheet, PDF (47/78 Pages) Wolfson Microelectronics plc – AC’97 Audio and Touchpanel CODEC

Production Data
WM9712L
AUXILIARY ADC INPUTS
The ADC used for touchpanel digitisation can also be used for auxiliary measurements, provided that
it is enabled (register 78h, PRP = 11). The WM9712L has four pins that can be used as auxiliary ADC
inputs:
 COMP1 / AUX1 (pin 29)
 COMP2 / AUX2 (pin 30)
 BMON / AUX3 (pin 31)
 WIPER / AUX4 (pin 12)
Note that pin 12 connects to the wiper of a 5-wire touchpanel wiper function. Auxiliary measurements
taken on pin 12 are only meaningful when it is not connected to a touchpanel (i.e. a 4-wire
touchpanel, or no touchpanel at all, is used). Pins 29 and 30 are also used as comparator inputs (see
Battery Alarm and Battery Measurement), but auxiliary measurements can still be taken on these pins
at any time. For the use of pin 31 see the “Battery Alarm And Battery Measurement” section, note that
the measured value from the BMON/AUX3 pin will be 1/3 of the actual value due to the potential
divider on this pin. The ADCSEL control bits select between different ADC inputs, as shown below.
REGISTER
ADDRESS
76h
Touchpanel
Digitiser
Control
BIT LABEL
14:12 ADCSEL
DEFAULT
000
Table 34 Auxiliary ADC Measurements
DESCRIPTION
Touchpanel ADC Input Selector
000: No measurement
001-011: Touchpanel measurement (please
refer to Touchpanel Digitiser section)
100: COMP1 / AUX1 measurement (pin 29)
101: COMP2 AUX2 measurement (pin 30)
110: BMON / AUX3 measurement (pin 31)
111: WIPER / AUX 4 measurement (pin 12)
Auxiliary ADC measurements are initiated in the same way as touchpanel measurements, and the
data is returned in the same manner. Please refer to the “Controlling the Touchpanel Interface”
section.
BATTERY MEASUREMENT USING THE BMON/AUX3 PIN
BMON/AUX3 (pin 31) has the capability to take inputs up to 5 volts (Assuming AVDD=3.3V) by
dividing down the input signal. The internal potential divider has a total resistance of 30k. However,
it is only connected to the pin when an AUX3 measurement is requested, and remains connected for
the duration of one AC-Link frame (20.83s, assuming a 24.576MHz clock crystal is used). The
effective input impedance of BMON/AUX3 is therefore given by:
RBMON = 30k  48kHz / [BMON sampling rate]
For example, if BMON is sampled ten times per second, the effective input resistance is 30k 
48kHz / 10Hz = 144M.
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PD, Rev 4.6, November 2011
47