SX8650 Datasheet, PDF (45/56 Pages) Semtech Corporation – World’s Lowest Power & Smallest Footprint 4-wire Resistive Touchscreen Controller with 15kV ESD

SX8650
World’s Lowest Power & Smallest Footprint 4-wire
Resistive Touchscreen Controller with 15kV ESD
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
6. Application Information
This section describes in more detail application oriented data.
6.1. Acquisition Setup
Prior to an acquisition, the SX8650 can be setup by writing the control registers. Registers are written by issuing the
register write command. They can be read by issuing the read command. Please refer to the section [4.8].
If no registers are written, the circuit will start in manual mode.
6.2. Channel Selection
The SX8650 can be setup to start a single channel conversion or to convert several channels in sequence. For a single
conversion, the channel to be converted is determined from the CHAN(2:0) field in the command word (defined in
Table 12).
Several channels can be acquired sequentially by setting the CHAN(2:0) field to SEQ. The channels will be sampled in the
order defined by register I2CRegChanMsk from MSB to LSB.
If a “one” is written in a channel mask, the corresponding channel will be sampled, in the opposite case, it is ignored and
the next selected channel is chosen.
6.3. Noise Reduction
A noisy environment can decrease the performance of the controller. For example, an LCD display located just under the
touch screen can adds a lot of noise on the high impedance A/D converter inputs.
6.3.1. POWDLY
Adding a capacitor from the touch screen drivers to ground is a solution to minimize external noise. A low-pass filter
created by the capacitor may increase settling time. Therefore, use POWDLY to stretch the acquisition period. POWDLY
can be estimated by the following formula.
PowDly = 10 × Rtouch × Ctouch
Rtouch is the sum of the panel resistances plus any significant series input resistance, Rxtot + Rytot + Ri.
Ctouch is the sum of the touch panel capacitance plus any noise filtering and routing capacitances.
6.3.2. SETDLY
A second method of noise filtering uses an averaging filter as described in section [4.5] (Data processing). In this case, the
chip will sequence up to 7 conversions on each channel. The parameter SETDLY sets the settling time between the
consecutive conversions (shown in Figure 41).
In most applications, SETDLY can be set to 0. In some particular applications, where accuracy of 1LSB is required and
Ctouch is less than 100nF a specific value should be determined.
ACS Revision V2.15/October 2009
©2009 Semtech Corp.
Page 44
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