SER/DFR—The SER/DFR bit controls the reference mode,
either single-ended (high) or differential (low). The differential
mode is also referred to as the ratiometric conversion mode
and is preferred for X-Position, Y-Position, and Pressure-
Touch measurements for optimum performance. The refer-
ence is derived from the voltage at the switch drivers, which
is almost the same as the voltage to the touch screen. In this
case a reference voltage is not needed, as the reference
voltage to the ADC is the voltage across the touch screen. In
the single-ended mode, the converter reference voltage is
always the difference between the VREF and GND pins (see
Tables I and II, and Figures 2 through 5 for further informa-
tion).
sample-and-hold goes into the hold mode and the touch
panel drivers turn off (in single-ended mode). The next 12
clock cycles accomplish the actual analog-to-digital conver-
sion. If the conversion is ratiometric (SER/DFR = 0), the
drivers are on during the conversion and a 13th clock cycle
is needed for the last bit of the conversion result. Three more
clock cycles are needed to complete the last byte (DOUT will
be low), which are ignored by the converter.
Control Byte
The control byte (on DIN), as shown in Table III, provides the
start conversion, addressing, ADC resolution, configuration,
and power-down of the ADS7846. Figure 9 and Tables III
and IV give detailed information regarding the order and
description of these control bits within the control byte.
If X-Position, Y-Position, and Pressure-Touch are measured
in the single-ended mode, an external reference voltage is
needed. The ADS7846 should also be powered from the
external reference. Caution must be observed when using
the single-ended mode such that the input voltage to the
ADC does not exceed the internal reference voltage, espe-
cially if the supply voltage is greater than 2.7V.
Bit 7
Bit 0
(MSB)
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
(LSB)
S
A2
A1
A0
MODE SER/DFR PD1
PD0
TABLE III. Order of the Control Bits in the Control Byte.
NOTE: The differential mode can only be used for X-Position,
Y-Position, and Pressure-Touch measurements. All other
measurements require the single-ended mode.
BIT
NAME
DESCRIPTION
7
S
Start Bit. Control byte starts with first high bit on DIN.
A new control byte can start every 15th clock cycle
in 12-bit conversion mode or every 11th clock cycle
in 8-bit conversion mode (see Figure 12).
PD0 and PD1—Table V describes the power-down and the
internal reference voltage configurations. The internal refer-
ence voltage can be turned on or off independently of the
ADC. This can allow extra time for the internal reference
voltage to settle to the final value prior to making a conver-
sion. Make sure to also allow this extra wake-up time if the
internal reference is powered down. The ADC requires no
wake-up time and can be instantaneously used. Also note
that the status of the internal reference power-down is
latched into the part (internally) with BUSY going high.
Therefore, in order to turn the reference off, an additional
write to the ADS7846 is required after the channel is con-
verted.
6-4
A2-A0
MODE
Channel Select Bits. Along with the SER/DFR bit,
these bits control the setting of the multiplexer input,
touch driver switches, and reference inputs (see
Tables I and II).
3
2
12-Bit/8-Bit Conversion Select Bit. This bit controls
the number of bits for the next conversion: 12-bits
(low) or 8-bits (high).
SER/DFR Single-Ended/Differential Reference Select Bit. Along
with bits A2-A0, this bit controls the setting of the
multiplexer input, touch driver switches, and reference
inputs (see Tables I and I).
1-0
PD1-PD0 Power-Down Mode Select Bits. See Table V for
details.
TABLE IV. Descriptions of the Control Bits within the Control
Byte.
PD1 PD0 PENIRQ DESCRIPTION
0
0
Enabled Power-Down Between Conversions. When each
conversion is finished, the converter enters a
low-power mode. At the start of the next conver-
sion, the device instantly powers up to full power.
There is no need for additional delays to assure
full operation and the very first conversion is
valid. The Y– switch is on when in power-down.
Initiate START—The first bit, the S bit, must always be high
and initiates the start of the control byte. The ADS7846
ignores inputs on the DIN pin until the start bit is detected.
Addressing—The next three bits (A2, A1, and A0) select the
active input channel(s) of the input multiplexer (see Tables I,
II, and Figure 2), touch screen drivers, and the reference
inputs.
0
1
1
1
0
Disabled Reference is off and ADC is on.
Enabled Reference is on and ADC is off.
1
Disabled Device is always powered. Reference is on and
ADC is on.
MODE—The mode bit sets the resolution of the ADC. With
this bit low, the next conversion has 12 bits of resolution; with
this bit high, the next conversion has 8 bits of resolution.
TABLE V. Power-Down and Internal Reference Selection.
ADS7846
SBAS125H
13
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