MXB7846EEE MAXIM [Maxim Integrated Products], MXB7846EEE Datasheet - Page 19

• Manufacturer Part Number: MXB7846EEE
• Description: 2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor
• Manufacturer: MAXIM [Maxim Integrated Products]

with Internal Reference and Temperature Sensor
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.
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.
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.
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.
2.375V to 5.25V, 4-Wire Touch-Screen Controller
Connection to Standard Interface
______________________________________________________________________________________
Touch-Screen Settling
Hardware Power-Down
MICROWIRE Interface
QSPI/SPI Interface
Figure 13. MICROWIRE Interface
Figure 14. QSPI/SPI Interface
Figure 15. TMS320 Serial 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.
MICROWIRE
TMS320LC3x
QSPI/SPI
INTERRUPT
MASKABLE
MASKABLE
INTERRUPT
MISO
MOSI
CLKX
CLKR
SCK
MISO
MOSI
FSR
I/O
SCK
DX
DR
XF
I/O
TMS320LC3x Interface
CS
DCLK
DOUT
DIN
BUSY
CS
SCLK
DIN
DOUT
BUSY
CS
DCLK
DOUT
DIN
BUSY
MXB7846
MXB7846
MXB7846
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