The GND pin must be connected to a clean ground point. In
many cases, this is the analog ground. Avoid connections
which are too near the grounding point of a microcontroller or
digital signal processor. If needed, run a ground trace directly
from the converter to the power-supply entry or battery-
connection point. The ideal layout includes an analog ground
plane dedicated to the converter and associated analog
circuitry.
+VCC
100kΩ
Y+
PENIRQ
In the specific case of use with a resistive touch screen, care
should be taken with the connection between the converter
and the touch screen. Although resistive touch screens have
fairly low resistance, the interconnection should be as short
and robust as possible. Longer connections are a source of
error, much like the on-resistance of the internal switches.
Likewise, loose connections can be a source of error when
the contact resistance changes with flexing or vibrations.
X+
Y–
On
Y or X drivers on,
As indicated previously, noise can be a major source of error
in touch screen applications (for example, applications that
require a backlit LCD panel). This EMI noise can be coupled
through the LCD panel to the touch screen and cause
“flickering” of the converted data. Several things can be done
to reduce this error, such as using a touch screen with a
bottom-side metal layer connected to ground to shunt the
majority of noise to ground. Additionally, filtering capacitors,
from Y+, Y–, X+, and X– pins to ground can also help.
Caution should be observed under these circumstances for
settling time of the touch screen, especially operating in the
single-ended mode and at high data rates.
or TEMP0, TEMP1
measurements
activated.
FIGURE 15. ADS7846 PENIRQ Functional Block Diagram.
Furthermore, the PENIRQ output is disabled and low during
the measurement cycle for X-, Y-, and Z-Position. The PENIRQ
output is disabled and high during the measurement cycle for
battery monitor, auxiliary input, and chip temperature. If the last
control byte written to the ADS7846 contains PD0 = 1, the pen-
interrupt output function is disabled and is not able to detect
when the screen is touched. In order to re-enable the pen-
interrupt output function under these circumstances, a control
byte needs to be written to the ADS7846 with PD0 = 0. If the
last control byte written to the ADS7846 contains PD0 = 0, the
pen-interrupt output function is enabled at the end of the
conversion. The end of the conversion occurs on the falling
edge of DCLK after bit 1 of the converted data is clocked out
of the ADS7846.
PENIRQ OUTPUT
The pen-interrupt output function is shown in Figure 15. While
in power-down mode with PD0 = 0, the Y– driver is on and
connects the Y-plane of the touch screen to GND. The
PENIRQ output is connected to the X+ input through two
transmission gates. When the screen is touched, the X+ input
is pulled to ground through the touch screen. The PENIRQ
output goes low due to the current path through the touch
screen to ground, which initiates an interrupt to the processor.
During the measurement cycle for X-, Y-, and Z-Position, the
X+ input is disconnected from the external pull-up resistor.
This is done to eliminate any leakage current from the
external pull-up resistor through the touch screen, thus caus-
ing no errors.
It is recommended that the processor mask the interrupt
PENIRQ is associated with whenever the processor sends a
control byte to the ADS7846. This prevents false triggering
of interrupts when the PENIRQ output is disabled, as in the
cases discussed in this section.
ADS7846
SBAS125H
17
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