參數(shù)資料
型號(hào)： TSC2008TRGVRQ1
廠商： TEXAS INSTRUMENTS INC
元件分類(lèi)： 消費(fèi)家電
英文描述： SPECIALTY CONSUMER CIRCUIT, PQCC16
封裝： 4 X 4 MM, PLASTIC, VQFN-16
文件頁(yè)數(shù)： 10/35頁(yè)
文件大小： 418K
代理商： TSC2008TRGVRQ1
SBAS552
– JUNE 2011
Touch Screen Settling
In some applications, external capacitors may be required across the touch screen to filter noise picked up by the
touch screen (that is, noise generated by the LCD panel or backlight circuitry). These capacitors provide a
low-pass filter to reduce the noise, but they also cause a settling time requirement when the panel is touched.
The settling time typically shows up as a gain error. The problem is that the input and/or reference has not
settled to its final steady-state value before the A/D converter samples the input(s) and provides the digital
output. Additionally, the reference voltage may continue to change during the measurement cycle.
There are two ways to resolve this issue. Option 1 is to stop or slow down the TSC2008-Q1 SCLK for the
required touch screen settling time. This option allows the input and reference to have stable values for the
Acquire period (three clock cycles of the TSC2008-Q1; see Figure 31). This option works for both the
single-ended and the differential modes. Option 2 is to operate the TSC2008-Q1 in the differential mode only for
the touch screen measurements and command the TSC2008-Q1 to remain on (touch screen drivers ON) and not
go into power-down (PD0 = 1). Several conversions are made, depending on the settling time required and the
TSC2008-Q1 data rate. Once the required number of conversions have been made, the processor commands
the TSC2008-Q1 to go into its power-down state on the last measurement. This process is required for
X-Position, Y-Position, and Z-Position measurements.
Touch Detect
The PENIRQ can be used as an interrupt to the host. RIRQ is an internal pull-up resistor with a programmable
value of either 50k
(default) or 90k (which allows the total resistance from X+ to Y– to be as high as 30k).
Write command '1010' (setup command) followed by data '1xx0' sets the pull-up resistor to 90k
. NOTE: The
first three bits must be '0's and the select bit is the last bit. To change the pull-up resistor back to 50k
, issue
write command '1010' followed by data '0xx0'.
An example for the Y-position measurement is detailed in Figure 28. The PENIRQ output is pulled high by an
internal pull-up resistor. While in power-down mode with PD0 = 0, the Y
– driver is on and connected to GND,
and the PENIRQ output is connected to the X+ input. When the panel is touched, the X+ input is pulled to ground
through the touch screen, and PENIRQ output goes low because of the current path through the panel to GND,
initiating an interrupt to the processor. During the measurement cycle for X-, Y-, and Z-Position, the X+ input is
disconnected from the PENIRQ pull-down transistor to eliminate any pull-up resistor leakage current from flowing
through the touch screen, thus causing no errors.
If the last command byte written to the TSC2008-Q1 contains PD0 = 1, the pen-interrupt output function is
disabled and cannot detect when the panel is touched. In order to re-enable the pen-interrupt output function
under these circumstances, a command byte must be written to the TSC2008-Q1 with PD0 = 0.
If the last command byte contains PD0 = 0, then the pen-interrupt function is enabled at the end of a conversion.
The end of conversion (EOC) occurs on the rising edge of PENIRQ.
In both cases previously listed, it is recommended that whenever the host writes to the TSC2008-Q1, the master
processor masks the interrupt associated to PENIRQ. This masking prevents false triggering of interrupts when
the PENIRQ line is disabled in the cases previously listed.
18
Copyright
2011, Texas Instruments Incorporated
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