TSC2100_16 Datasheet, PDF (57/77 Pages) Texas Instruments – PROGRAMMABLE TOUCH SCREEN CONTROLLER WITH INTEGRATED

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TSC2100
SLAS378− NOVEMBER 2003
The TSC2100 architecture offers no inherent rejection of noise or voltage variation in regards to using an external reference
input. This is of particular concern when the reference input is tied to the power supply. Any noise and ripple from the supply
appears directly in the digital results. While high frequency noise can be filtered out, voltage variation due to line frequency
(50 Hz or 60 Hz) can be difficult to remove.
The ground pins 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.
In the specific case of use with a resistive touch screen, care must be taken with the connection between the converter
and the touch screen. Since resistive touch screens have fairly low resistance, the interconnection must be as short and
robust as possible. Loose connections can be a source of error when the contact resistance changes with flexing or
vibrations.
As indicated previously, noise can be a major source of error in touch-screen applications (e.g., applications that require
a back-lit LCD panel). This EMI noise can be coupled through the LCD panel to the touch screen and cause flickering of
the converted ADC data. Several things can be done to reduce this error, such as utilizing a touch screen with a bottom-side
metal layer connected to ground. This couples the majority of noise to ground. Additionally, filtering capacitors, from Y+,
Y–, X+, and X– to ground, can also help. Note, however, that the use of these capacitors increases screen settling time
and requires longer panel voltage stabilization times, as well as increased precharge and sense times for the PINTDAV
circuitry of the TSC2100.
CONVERSION TIME CALAULATIONS FOR THE TSC2100
Touch Screen Conversion Initiated At Touch Detect
The time needed to get a converted X/Y coordinate for reading can be calculated by (not including the time needed to send
the command over the SPI bus):
ȧȱȲ ȧȳȴ ^{tcoordinate + 2}
ǒ Ǔ tPRE ) tSNS ) tPVS
125 ns
tOSC ) 2
NJ ƪǒ Ǔ NAVG NBITS ) 1
ƫ Nj ^{8 MHz}
ƒ_conv
)
n1
)
12
)1
tOSC ) 18 tOSC ) n2 tOSC ) n3 tOSC
where:
t_coordinate = time to convert X/Y coordinate
t_PVS = Panel voltage stabilization time
t_PRE = precharge time
t_SNS = sense time
N_AVG = number of averages; for no averaging, N_AVG = 1
N_BITS = number of bits of resolution
ƒ_conv = A/D converter clock frequency
t_OSC = Oscillator clock period
n₁ = 6 ; if ƒ_conv = 8 MHz
7 ; if ƒ_conv ≠ 8 MHz
n₂ = 4 ; if t_PVS = 0 µs
0 ; if t_{PVS â‰}  0 µs
n₃ = 0 ; if t_SNS = 32 µs
2 ; if t_{SNS â‰}  32 µs
NOTE^: The above formula is exactly valid only when the codec is powered down. Also, after touch detect, the formula holds true from second
conversion onwards.
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