MXB7846(2004) 데이터 시트보기 (PDF) - Maxim Integrated

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MXB7846
(Rev.:2004)

2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor

Maxim Integrated 

2.375V to 5.25V, 4-Wire Touch-Screen Controller

with Internal Reference and Temperature Sensor

Hardware Power-Down

CS also places the MXB7846 into power-down. When

CS goes HIGH, the MXB7846 immediately powers

down and aborts the current conversion. The internal

reference does not turn off when CS goes high. To dis-

able the internal reference, an additional command

byte is required before CS goes high (PD1 = 0). Set

PD1 = 0 before CS goes high.

Touch-Screen Settling

There are two key touch-screen characteristics that can

degrade accuracy. First, the parasitic capacitance

between the top and bottom layers of the touch screen

can result in electrical ringing. Second, vibration of the

top layer of the touch screen can cause mechanical

contact bouncing.

External filter capacitors may be required across the

touch screen to filter noise induced by the LCD panel

or backlight circuitry, etc. These capacitors lengthen

the settling time required when the panel is touched

and can result in a gain error, as the input signal may

not settle to its final steady-state value before the ADC

samples the inputs. Two methods to minimize or elimi-

nate this issue are described below.

One option is to lengthen the acquisition time by stopping

or slowing down DCLK, allowing for the required touch-

screen settling time. This method solves the settling time

problem for both single-ended and differential modes.

The second option is to operate the MXB7846 in the dif-

ferential mode only for the touch screen, and perform

additional conversions with the same address until the

input signal settles. The MXB7846 can then be placed

in the power-down state on the last measurement.

Connection to Standard Interface

MICROWIRE Interface

When using the MICROWIRE- (Figure 13) or SPI-com-

patible interface (Figure 14), set the CPOL = CPHA = 0.

Two consecutive 8-bit readings are necessary to obtain

the entire 12-bit result from the ADC. DOUT data transi-

tions occur on the serial clock’s falling edge and are

clocked into the µP on the DCLK’s rising edge. The first

8-bit data stream contains the first 8 bits of the current

conversion, starting with the MSB. The second 8-bit

data stream contains the remaining 4 result bits fol-

lowed by 4 trailing zeros. DOUT then goes high imped-

ance when CS goes high.

QSPI/SPI Interface

The MXB7846 can be used with the QSPI/SPI interface

using the circuit in Figure 14 with CPOL = 0 and CPHA

= 0. This interface can be programmed to do a conver-

sion on any analog input of the MXB7846.

I/O

SCK

MISO

MICROWIRE

MOSI

MASKABLE

INTERRUPT

Figure 13. MICROWIRE Interface

CS

DCLK

DOUT

MXB7846

DIN

BUSY

I/O

SCK

MISO

QSPI/SPI

MOSI

MASKABLE

INTERRUPT

Figure 14. QSPI/SPI Interface

CS

DCLK

DOUT

MXB7846

DIN

BUSY

XF

CLKX

CLKR

TMS320LC3x

DX

DR

FSR

CS

SCLK

MXB7846

DIN

DOUT

BUSY

Figure 15. TMS320 Serial Interface

TMS320LC3x Interface

Figure 15 shows an example circuit to interface the

MXB7846 to the TMS320. The timing diagram for this

interface circuit is shown in Figure 16.

Use the following steps to initiate a conversion in the

MXB7846 and to read the results:

1) The TMS320 should be configured with CLKX (trans-

mit clock) as an active-high output clock and CLKR

(TMS320 receive clock) as an active-high input

clock. CLKX and CLKR on the TMS320 are connect-

ed to the MXB7846 DCLK input.

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