AT91SAM9G45PRE Datasheet, PDF (939/1218 Pages) ATMEL Corporation – DDR2 Controller 4-bank DDR2/LPDDR, SDRAM/LPSDR

AT91SAM9G45
40.6.4
Startup Time
The Touch Screen ADC has a minimal Startup Time when it exits the Sleep Mode. As the ADC
Clock depends on the application, the user has to program the field STARTUP in the “TSADCC
Mode Register”, which defines how many ADC Clock cycles to wait before performing the first
conversion of the sequence.
The field STARTUP can define a Startup Time between 8 and 1024 ADC Clock cycles by steps
of 8.
The user must assure that ADC Startup Time given in the section “Electrical Characteristics” is
covered by this wait time.
40.6.5
Sample and Hold Time
In the same way, a minimal Sample and Hold Time is necessary for the TSADCC to guarantee
the best converted final value between selection of two channels. This time depends on the input
impedance of the analog input, but also on the output impedance of the driver providing the sig-
nal to the analog input, as there is no input buffer amplifier.
The Sample and Hold time has to be programmed through the bitfields SHTIM in the “TSADCC
Mode Register” and TSSHTIM in the “TSADCC Touch Screen Register”.
The field SHTIM defines the number of ADC Clock cycles for an analog input, while the field
TSSHTIM defines the number of ADC Clock cycles for a Touch Screen input.
These both fields can define a Sample and Hold time between 1 and 16 ADC Clock cycles.
The field TSSHTIM defines also the time the power switches of the Touch Screen are closed
when the TSADCC performs a conversion for the Touch Screen.
40.7 Touch Screen
40.7.1
Resistive Touch Screen Principles
A resistive touch screen is based on two resistive films, each one being fitted with a pair of elec-
trodes, placed at the top and bottom on one film, and on the right and left on the other. Between
the two, there is a layer that acts as an insulator, but also enables contact when you press the
screen. This is illustrated in Figure 40-2.
The TSADC controller has the ability to perform without external components:
• Position Measurement
• Pressure Measurement
• Pen Detection
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