MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
18 ______________________________________________________________________________________
byte (see Tables 3 and 4). Once the processor instructs
the MXB7846 to initiate a conversion, the MXB7846
biases the touch screen through the internal switches at
the beginning of the acquisition period. The voltage
transient at the touch screen needs to settle down to a
stable voltage before the acquisition period is over.
After the acquisition period is over, the A/D converter
goes into a conversion period with all internal switches
turned off if the device is in single-ended mode. If the
device is in differential mode, the internal switches
remain on from the start of the acquisition period to the
end of the conversion period.
Power-On Reset
When power is first applied, internal power-on circuitry
resets the MXB7846. Allow 10µs for the first conversion
after the power supplies stabilize. If CS is low, the first
logic 1 on DIN is interpreted as a start bit. Until a con-
version takes place, DOUT shifts out zeros. On power-
up, allow time for the reference to stabilize.
Power Modes
Save power by placing the converter in one of two low-
current operating modes or in full power down between
conversions. Select the power-down mode through
PD1 and PD0 of the control byte (Tables 3 and 4).
The software power-down modes take effect after the
conversion is completed. The serial interface remains
active while waiting for a new control byte to start a con-
version and switches to full-power mode. After complet-
ing its conversion, the MXB7846 enters the programmed
power mode until a new control byte is received.
The power-up wait before conversion period is depen-
dent on the power-down state. When exiting software
low-power modes, conversion can start immediately
when running at decreased clock rates. Upon power-
on reset, the MXB7846 is in power-down mode with
PD1 = 0 and PD0 = 0. When exiting software shutdown,
the MXB7846 is ready to perform a conversion in 10µs
with an external reference. When using the internal ref-
erence, allow enough time for reference to settle to 12-
bit accuracy when exiting full power-down mode, as
shown in the
Typical Operating Characteristics
.
PD1 = 1, PD0 = 1
In this mode, the MXB7846 is always powered up and
both the reference and the ADC are always on. The
device remains fully powered after the current conver-
sion completes.
PD1 = 0, PD0 = 0
In this mode, the MXB7846 powers down after the cur-
rent conversion completes or on the next rising edge of
CS, whichever occurs first. The next control byte
received on DIN powers up the MXB7846. At the start
of a new conversion, it instantly powers up. When each
conversion is finished, the part enters power-down
mode, unless otherwise indicated. The first conversion
after the ADC returns to full power is valid for differen-
tial conversions and single-ended measurement con-
versions when using an external reference.
When operating at full speed and 16 clocks per conver-
sion, the difference in power consumption between
PD1 = 0, PD0 = 1, and PD1 = 0, PD0 = 0 is negligible.
Also, in the case where the conversion rate is
decreased by slowing the frequency of the DCLK input,
the power consumption between these two modes is
not very different. When the DCLK frequency is kept at
the maximum rate during a conversion, conversions are
done less often. There is a significant difference in
power consumption between these two modes.
PD1 = 1, PD0 = 0
In this mode, the MXB7846 is powered down. This
mode becomes active after the current conversion
completes or on the next rising edge of CS, whichever
occurs first. The next command byte received on the
DIN returns the MXB7846 to full power. The first conver-
sion after the ADC returns to full power is valid.
PD1 = 0, PD0 = 1
This mode turns the internal reference off and leaves
the ADC on to perform conversions using an external
reference.
