WM9705 datasheet(48/55 Pages) WOLFSON

48 / 55 page

WM9705

Production Data

PD Rev 4.4 August 2006

BUSY SIGNAL

The BUSY signal is derived from the pen state machine and is an indication of the AUXADC

being active on a conversion cycle. When an AUXADC conversion is requested BUSY will go high

61 BCLKs after the rising edge of SYNC in the frame that the conversion was requested. BUSY

will remain high until the data from the conversion is written into register 7A as the state machine

sees the register write as the last stage of the conversion.

During a conversion that does not have the WAIT bit set, the BUSY negative pulse between

conversions (signifying data written to Reg 7Ah) lasts 3 BCLKs and starts 11 BCLKs before the

next rising SYNC edge. At the end of a conversion (or set of conversions) BUSY goes low 11

BCLKs before SYNC rises.

The WAIT bit instructs the device to wait until the previous conversion result has been read from

multiple conversion cycle in which WAIT is set the busy signal may operate differently. If the

device it set to perform 3 simultaneous conversions with WAIT set the timing of busy is

dependant on when the results are read from register 7A. If at the end of conversion 2 the result

from conversion 1 has not been read from the register the conversion sequence can not

complete. In this case BUSY will remain high until 61 BCLKs after the rising SYNC edge at the

beginning of the frame in which register 7A was read. However if the result is read from register

7A before the end of conversion 2 the busy signal will operate as described in the paragraph

above.

In the case of single conversions with WAIT set the operation of BUSY would be the same as that

described above if the data was not read from Reg 7Ah before the next conversion requested had

completed.

PRESSURE MEASUREMENT

Pen pressure is implied by determining the resistance between the two plates.

Pressure measurements are sometimes used to determine whether the pen is in contact with the

screen. However in this system the pen-down detect method is a far less power hungry way of

determining whether the pen is ‘down’ as it does not require continuous driving of the screen and

ADC conversions. This has the supplemental benefit of minimising audio signal interference. Pen

pressure measurements may be made if required, perhaps for reasons other than simply

determining pen-down.

This is done by injecting a known current into one end of one of the two plates (Y+), and

grounding one end of the other plate (X-). Then the voltage at the other two plate terminals is

converted (Y- and X+), the difference being a direct measure of the injected current value

multiplied by the Pen contact resistance. Figure A4 shows this schematically.

Requesting a pressure measurement automatically initiates a pair of conversions, the difference

in conversion values being returned as the pressure measurement. Delay between switching the

appropriate screen and ADC mux switches, and performing the conversions, is set in the DEL[3-

0] registers.

Figure 24 Pressure Measurement

Y+

X+

X-

Y-

Rpen

A/D

converter

Vrswxn

Vrswyp

Xswn

Yswp

MXyp 1st conv

MXyn 2nd conv

Prsw

Ipress

Yswp

Vrswap

Vrswan

WM9705 bảng dữ liệu(PDF) 48 Page - Wolfson Microelectronics plc

WM9705 bảng dữ liệu(HTML) 48 Page - Wolfson Microelectronics plc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

bảng dữ liệu tải về

Link URL