WM8948ECSSLASHR datasheet(95/189 Pages) WOLFSON

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Production Data

WM8948

PD, May 2011, Rev 4.1

If all 3 touch panel channels are selected (using TCH_X_ENA, TCH_Y_ENA and TCH_Z_ENA) then

all 3 Touch Data registers must be read before further measurements are permitted. If fewer

channels are selected, then only those selected channels need to be read before touch panel

measurements are enabled again.

The touch panel inhibit (preventing new touch panel measurements) commences when any of the

Touch Data registers is read. The touch panel inhibit ceases when all selected Touch Data registers

have been read.

The Touch Panel interrupts can be used to indicate when new data is available or if “Pen Down” is

detected - see “Interrupts”. A GPIO pin configured as “TCH_DONE” or “PDOWN” can also be used to

indicate these events - see “General Purpose Input / Output”.

The control fields associated with Touch Panel readback are defined in Table 61.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R57 (39h)

Touch Data X

TCH_PD1

Pen down status (indicates if the

Pen Down was detected prior to the

TP measurement)

0 = Pen Down not detected

1 = Pen Down detected

11:0

TCH_X [11:0]

000h

Touch panel X-axis data

R58 (3Ah)

Touch Data Y

TCH_PD2

Pen down status (indicates if the

Pen Down was detected prior to the

TP measurement)

0 = Pen Down not detected

1 = Pen Down detected

11:0

TCH_Y [11:0]

000h

Touch panel Y-axis data

R59 (3Bh)

Touch Data Z

TCH_PD3

Pen down status (indicates if the

Pen Down was detected prior to the

TP measurement)

0 = Pen Down not detected

1 = Pen Down detected

11:0

TCH_Z [11:0]

000h

Touch panel Z-axis data

Table 61 Touch Panel Readback

TOUCH PANEL OPERATING PRINCIPLES

A typical Touch Panel comprises two conductive sheets, connected via a switch matrix to the Touch

Panel supply voltage. When the Touch Panel is touched (usually with a pen-style pointer), an

electrical contact is made between the two sheets. The switch matrix is used to determine the

position of the pen contact by establishing a potential divider on one of the conductive sheets in

either the X-axis or Y-axis, and measuring the voltage on the other sheet.

Separate configuration is required for each axis measurement; these are configured one after the

other to determine the X and Y co-ordinate positions. Note that, due to the ratiometric measurement

method, the supply voltage does not affect the measurement accuracy in either axis.

Pen Down detection and Z-axis (pressure) measurements are achieved in a similar fashion, by

configuring the switch matrix and taking the appropriate voltage measurement via an ADC.

The Touch Panel interface connects to the Left / Right sides of one sheet and to the Top / Bottom

sides of the other sheet. The illustrations show the top sheet for X-axis and the bottom sheet for Y-

axis, but the reverse is also possible.

X-axis measurement is performed by applying a potential difference between the Left and Right sides

of the touch panel. When contact is made between the two sheets, the voltage present on the Top or

Bottom connections is a measure of the X-axis position of the contact. The configuration is illustrated

in Figure 35.

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