SX8652ICSTRT Semtech, SX8652ICSTRT Datasheet - Page 28

• Manufacturer Part Number: SX8652ICSTRT
• Description: 4/5-wire Resistive Touch Screen Controller (SPI)
• Manufacturer: Semtech

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:
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.
8.3.2. SETDLY
A second method of noise filtering uses an averaging filter as described in section [6] (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.
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.
8.3.3. AUX Input
An alternate conversion trigger method can be used with 4-wire touchscreen if the host system provides additional digital
signals that indicate noisy or noise-free periods. The SX8652 can be set up to start conversions triggered by the AUX pin.
A rising edge, a falling edge or both can trigger the conversion. To enter this mode, AUXACQ must be set to a different
value than '00' as defined in Table 13. The AUX edge will first trigger the bias delay (POWDLY). Following the programmed
delay, the channel acquisition takes place.
8.4. Interrupt Generation
An interrupt (NIRQ=0) will be generated:
The NIRQ will be released and pulled high(NIRQ=1) by the external pull-up resistor:
An active NIRQ (low) needs to be cleared before any new conversions will occur.
8.5. Coordinate Throughput Rate
The coordinate throughput rate depends on the following factors:
The coordinate rate is the frequency to get the X, Y, Z1 and Z2 coordinate:
8.5.1. SPI Communication Time
The minimum time to read the channel data in PENTRIG mode is:
The highest throughput will be obtained with a SPI frequency of 5MHz when the host read the channel data as quickly as
possible after the NIRQ falling edge.
Revision V1.6/July 2010
©2010 Semtech Corp.
ADVANCED COMMUNICATIONS & SENSING
During the power-up phase or after a reset
After completion of a conversion in MANUAL, PENTRIG or AUTO mode. CONVIRQ (bit [7] of RegStat) will be set at the
same time.
After a touch on the panel is detected in PENDET mode. PENIRQ (bit [6] of RegStat) will be set at the same time.
When the power-up phase is finished
When the host read all channels data that were previously converted by the SX8652 in MANUAL, PENTRIG or AUTO
mode. CONVIRQ will be cleared at the same time.
When the host read the status register in PENDET mode. PENIRQ, will be cleared at the same time.
The SPI communication time: T
The conversion time: T
conv
com
PowDly
=
15kV ESD Low Power 4-Wire / 5-Wire Resistive
10 Rtouch
Page 28
Touchscreen Controller with SPI Interface
T
com
=
Ctouch
CoordRate
8
+
16
=
N
-------------------------------
T
chan
com
+
1
T
conv
T
SPI
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SX8652