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AD7877ACP-REEL-AD7877ACPZ-500RL7-AD7877ACPZ-REEL7
Touch Screen Digitizer with LCD Noise Reduction Feature
Touch Screen ControllerRev. A
FEATURES
4-wire touch screen interface
LCD noise reduction feature (STOPACQ pin)
Automatic conversion sequencer and timer
User-programmable conversion parameters
On-chip temperature sensor: −40°C to +85°C
On-chip 2.5 V reference
On-chip 8-bit DAC
3 auxiliary analog inputs
1 dedicated and 3 optional GPIOs
2 direct battery measurement channels (0.5 V to 5 V)
3 interrupt outputs
Touch-pressure measurement
Wake up on touch function
Specified throughput rate of 125 kSPS
Single supply, VCC of 2.7 V to 5.25 V
Separate VDRIVE level for serial interface
Shutdown mode: 1 µA maximum
32-lead LFCSP 5 mm x 5 mm package
APPLICATIONS
Personal digital assistants
Smart hand-held devices
Touch screen monitors
Point-of-sale terminals
Medical devices
Cell phones
Pagers
FUNCTIONAL BLOCK DIAGRAM
DIN
DCLK
DOUT
VDRIVE
DAV29ARNGAOUTAUX3/GPIO3AUX2/GPIO2AUX1/GPIO1VREFPENIRQALERTSTOPACQAGNDDGNDGPIO4BAT2BAT1Y–Y+X–X+
VCC03796-001
Figure 1.
GENERAL DESCRIPTION The AD7877 is a 12-bit successive approximation ADC with a
synchronous serial interface and low on resistance switches for
driving touch screens. The AD7877 operates from a single 2.7 V
to 5.25 V power supply (functional operation to 2.2V), and
features throughput rates of 125 kSPS. The AD7877 features
direct battery measurement on two inputs, temperature and
touch-pressure measurement.
The AD7877 also has an on-board reference of 2.5 V. When not
in use, it can be shut down to conserve power. An external
reference can also be applied and can be varied from 1 V to
+VCC, while the analog input range is from 0 V to VREF. The
device includes a shutdown mode, which reduces its current
consumption to less than 1 µA.
To reduce the effects of noise from LCDs, the acquisition phase
of the on-board ADC can be controlled via the STOPACQ pin.
User-programmable conversion controls include variable
acquisition time and first conversion delay. Up to 16 averages
can be taken per conversion. There is also an on-board DAC for
LCD backlight or contrast control. The AD7877 can run in
either slave or master mode, using a conversion sequencer and
timer. It is ideal for battery-powered systems such as personal
digital assistants with resistive touch screens and other portable
equipment.
The part is available in a 32-lead lead frame chip scale package
(LFCSP).
TABLE OF CONTENTS Specifications.....................................................................................3
Timing Specifications.......................................................................5
Absolute Maximum Ratings............................................................6
ESD Caution..................................................................................6
Pin Configuration and Function Descriptions.............................7
Terminology......................................................................................9
Typical Performance Characteristics...........................................10
Circuit Information........................................................................14
Touch Screen Principles............................................................14
Measuring Touch Screen Inputs...............................................15
Touch-Pressure Measurement..................................................16
STOPACQ Pin............................................................................16
Temperature Measurement.......................................................17
Battery Measurement.................................................................18
Auxiliary Inputs..........................................................................19
Limit Comparison......................................................................19
Control Registers............................................................................20
Control Register 1.......................................................................20
Control Register 2.......................................................................21
Sequencer Registers...................................................................22
Interrupts.....................................................................................24
Syncronizing the AD7877 to the Host CPU...........................25
8-Bit DAC........................................................................................26
Serial Interface................................................................................28
Writing Data...............................................................................28
Write Timing...............................................................................29
Reading Data...............................................................................29
VDRIVE Pin.....................................................................................29
General-Purpose I/O Pins.............................................................30
GPIO Configuration..................................................................30
Grounding and LayouT.................................................................32
PCB Design Guidelines for Chip Scale Packages...................32
Register Maps..................................................................................33
Detailed Register Descriptions.....................................................35
GPIO Registers...........................................................................41
Outline Dimensions.......................................................................43
Ordering Guide..........................................................................43
REVISION HISTORY
11/04—Changed from Rev. 0 to Rev. A Changes to Absolute Maximum Ratings ......................................6
Changes to Figure 4..........................................................................7
Changes to Table 4............................................................................7
Changes to Grounding and Layout section................................32
Changes to Figure 42......................................................................32
Changes to Ordering Guide..........................................................43
7/04—Revision 0: Initial Version SPECIFICATIONS VCC = 2.7 V to 3.6 V, VREF = 2.5 V internal or external, fDCLK = 2 MHz, TA = −40°C to +85°C, unless otherwise noted.
Table 1.
ADC
1 See the section. Terminology Difference between Temp0 and Temp1 measurement. No calibration necessary.
3 Temperature drift is −2.1 mV/°C. Sample tested @ 25°C to ensure compliance.
TIMING SPECIFICATIONS TA = TMIN to TMAX, unless otherwise noted; VCC = 2.7 V to 5.25 V, VREF = 2.5 V. Sample tested at 25°C to ensure compliance. All input signals
are specified with tR = tF = 5 ns (10% to 90% of VCC) and timed from a voltage level of 1.6 V.
Table 2. 1 Mark/space ratio for the DCLK input is 40/60 to 60/40. Measured with the load circuit of and defined as the time required for the output to cross 0.4 V or 2.0 V. Figure 3
Figure 3.3 t8 is derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of The measured number is then extrapolated
back to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, t8, quoted in the timing characteristics is the true bus relinquish
time of the part and is independent of the bus loading.
DCLK
DINDOUT
Figure 2. Detailed Timing Diagram
ABSOLUTE MAXIMUM RATINGS TA = 25°C, unless otherwise noted.
Table 3. Transient currents of up to 100 mA do not cause SCR latch-up.
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only and functional operation of the device at these or
any other conditions above those listed in the operational
sections of this specification is not implied. Exposure to
absolute maximum rating conditions for extended periods may
affect device reliability.
200µAIOL
200µAIOH
1.6VTO OUTPUTPINCL
50pF
Figure 3. Load Circuit for Digital Output Timing Specifications
ESD CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS BAT2
BAT1
VCC
DAV
ALERT
GPIO4
STOPACQ
DIN
PENIRQV
AOUTARNGV
DOUTDCLKNCY–X+Y+
AGNDDGND
NC = NO CONNECT
Figure 4. Pin Configuration
Table 4. Pin Function Descriptions
TERMINOLOGY
Integral Nonlinearity The maximum deviation from a straight line passing through
the endpoints of the ADC transfer function. The endpoints of
the transfer function are zero scale (a point 1 LSB below the
first code transition), and full scale (a point 1 LSB above the last
code transition).
Differential Nonlinearity The difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Offset Error The deviation of the first code transition (00…000) to
(00…001) from the ideal (AGND + 1 LSB).
Gain Error The deviation of the last code transition (111…110) to
(111…111) from the ideal (VREF − 1 LSB) after the offset error
has been adjusted out.
On Resistance A measure of the ohmic resistance between the drain and the
source of the switch drivers.
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, VCC = 2.7 V, VREF = 2.5 V, fSAMPLE = 125 kHz, fDCLK = 16 × fSAMPLE = 2 MHz, unless otherwise noted.
500–50–30–10030507090TEMPERATURE (°C)
CURRE
NT (
Figure 5. Supply Current vs. Temperature
2.02.32.62.93.23.53.84.14.44.75.0VCC (V)
CURRE
NT (
Figure 6. Supply Current vs. VCC
–50–30–101030507090TEMPERATURE (°C)
DELTA FROM 25
°C (LS
Figure 7. Change in ADC Gain vs. Temperature
–50–30–101030507090TEMPERATURE (°C)
CURRE
NT (nA)
Figure 8. Full Power-Down IDD vs. Temperature
–50–30–101030507090TEMPERATURE (°C)
DELTA FROM 25
°C (LS
Figure 9. Change in ADC Offset vs. Temperature
0.85001000150020002500300035004000CODE
INL (LSB)
Figure 10. ACD INL Plot
0.85001000150020002500300035004000CODE
DNL (LS
Figure 11. ADC DNL Plot
2.73.13.53.94.34.75.15.5VDD (V)
Figure 12. Switch On Resistance vs. VCC
(X+, Y+: VCC to Pin; X−, Y−: Pin to GND)
–40–20020406080TEMPERATURE (°C)
Figure 13. Switch On Resistance vs. Temperature
(X+, Y+: VCC to Pin; X−, Y−: Pin to GND)
–509070503010–10–30TEMPERATURE (°C)
NCE
CURRE
NT (
Figure 14. External Reference Current vs. Temperature
2.475–50–30–101030507090TEMPERATURE (°C)
(V
Figure 15. Internal VREF vs. Temperature
2.62.93.23.53.84.14.44.75.0VCC (V)
(V
Figure 16. Internal VREF vs. VCC
3045–50–30–101030507090TEMPERATURE (°C)
ADC CODE
(De
ima
Figure 17. ADC Code vs. Temperature (2.7 V Supply)
2.72.82.93.03.13.23.33.43.53.6VCC (V)
CODE
Figure 18. Temp1 vs. VCC
2.72.82.93.03.13.23.33.43.53.6VCC (V)
CODE
Figure 19. Temp0 vs. VCC
INTE
RNAL V
REF
(V
TURN-ON TIME (µs)406080100120–200
Figure 20. Internal VREF vs. Turn-On Time
–1010k20k30k
40kFREQUENCY
INP
T TONE
AMPLITUDE
(dB)
Figure 21. Typical FFT Plot for the Auxiliary Channels of the AD7877
at 90 kHz Sample Rate and 10 kHz Input Frequency
012345678910SOURCE/SINK CURRENT (mA)
DAC O/P LEVEL (V)
Figure 22. DAC Source and Sink Current Capability
Figure 23. DAC O/P Settling Time (Zero Scale to Half-Scale)
1000255075100125150175200225250INPUT CODE (Decimal)
DAC S
INK CURRE
NT (
Figure 24. DAC Sink Current vs. Input Code
–1012ERROR (%)
Figure 25. Typical Accuracy for Battery Channel (25°C)
CIRCUIT INFORMATION The AD7877 is a complete, 12-bit data acquisition system for
digitizing positional inputs from a touch screen in PDAs and
other devices. In addition, it can monitor two battery voltages,
ambient temperature, and three auxiliary analog voltages, with
high and low limit comparisons on three of the inputs, and has
up to four general-purpose logic I/O pins.
The core of the AD7877 is a high speed, low power, 12-bit
analog-to-digital converter (ADC) with input multiplexer,
on-chip track-and-hold, and on-chip clock. The results of
conversions are stored in 11 results registers, and the results
from one auxiliary input and two battery inputs can be
compared with high and low limits stored in limit registers to
generate an out-of-limit ALERT. The AD7877 also contains low
resistance analog switches to switch the X and Y excitation
voltages to the touch screen, a STOPACQ pin to control the
ADC acquisition period, 2.5 V reference, on-chip temperature
sensor, and 8-bit DAC to control LCD contrast. The high speed
SPI serial bus provides control of, and communication with, the
device.
Operating from a single supply from 2.2 V to 5 V, the AD7877
offers throughput rates of up to 125 kHz. The device is available
in a 5 mm by 5 mm 32-lead lead frame chip scale package.
The data acquisition system of the AD7877 has a number of
advanced features: Input channel sequenced automatically or selected by
the host STOPACQ feature to reduce noise from LCD Averaging of from 1 to 16 conversions for noise
reduction Programmable acquisition time Power management Programmable ADC power-up delay before first
conversion Choice of internal or external reference Conversion at preprogrammed intervals
TOUCH SCREEN PRINCIPLES A 4-wire touch screen consists of two flexible, transparent,
resistive-coated layers that are normally separated by a small air
gap. The X layer has conductive electrodes running down the
left and right edges, allowing the application of an excitation
voltage across the X layer from left to right.
03796-005
CONDUCTIVE ELECTRODE
PLASTIC FILM WITHTRANSPARENT, RESISTIVECOATING ON BOTTOM SIDETRANSPARENT, RESISTIVECOATING ON TOP SIDE
LCD SCREEN
CONDUCTIVE ELECTRODEFigure 26. Basic Construction of a Touch Screen
The Y layer has conductive electrodes running along the top
and bottom edges, allowing the application of an excitation
voltage down the layer from top to bottom.
Provided that the layers are of uniform resistivity, the voltage at
any point between the two electrodes is proportional to the
horizontal position for the X layer and the vertical position for
the Y layer.
When the screen is touched, the two layers make contact. If only
the X layer is excited, the voltage at the point of contact, and
therefore the horizontal position, can be sensed at one of the
Y layer electrodes. Similarly, if only the Y layer is excited, the
voltage, and therefore the vertical position, can be sensed at one
of the X electrodes. By switching alternately between X and
Y excitation and measuring the voltages, the X and Y coordi-
nates of the contact point can be found.
In addition to measuring the X and Y coordinates, it is also
possible to estimate the touch pressure by measuring the
contact resistance between the X and Y layers. The AD7877 is
designed to facilitate this measurement.
Figure 28 shows an equivalent circuit of the analog input
structure of the AD7877, showing the touch screen switches, the
main analog multiplexer, the ADC with analog and differential
reference inputs, and the dual 3-to-1 multiplexer that selects the
reference source for the ADC.
AUX3/GPIO4
BAT1
BAT2
AUX2/GPIO3
AUX1/GPIO2
REFINT/EXT03796-006
Figure 27. Analog Input Structure
The AD7877 can be set up to convert specific input channels or
to convert a sequence of channels automatically. The results of
the ADC conversions are stored in the results registers. See the
Serial Interface section for details.
When measuring the ancillary analog inputs (AUX1 to AUX3,
BAT1 and BAT2), the ADC uses the internal reference, or an
external reference applied to the VREF pin, and the measurement
is referred to GND.
MEASURING TOUCH SCREEN INPUTS When measuring the touch screen inputs, it is possible to
measure using the internal (or external) reference, or to use the
touch screen excitation voltage as the reference and perform a
ratiometric, differential measurement. The differential method
is the default and is selected by clearing the SER/DFR bit
(Bit 11) in Control Register 1. The single-ended method is
selected by setting this bit.
Single-Ended Method The single-ended method is illustrated for the Y position in
Figure 28. For the X position, the excitation voltage would be
applied to X+ and X− and the voltage measured at Y+.
VREF
VCC
Figure 28. Single-Ended Conversion of Touch Screen Inputs
The voltage seen at the input to the ADC in Figure 28 is
VIN = VCC ×
YTOTAL− (1)
The advantage of the single-ended method is that the touch
screen excitation voltage can be switched off once the signal has
been acquired. Because a screen can draw over 1 mA, this is a
significant consideration for a battery-powered system.
The disadvantages of the single-ended method are as follows:
• It can be used only if VCC is close to VREF. If VCC is greater than
VREF, some positions on the screen are outside the range of
the ADC. If VCC is less than VREF, the full range of the ADC is
not utilized.
• The ratio of VCC to VREF must be known. If VREF and/or VCC
vary relative to one another, this can introduce errors.
• Voltage drops across the switches can introduce errors. Touch
screens can have a total end-to-end resistance of from 200 Ω
to 900 Ω. Taking the lowest screen resistance of 200 Ω and a
typical switch resistance of 14 Ω, this could reduce the appar-
ent excitation voltage to 200/228 × 100 = 87% of its actual
value. In addition, the voltage drop across the low-side switch
adds to the ADC input voltage. This introduces an offset into
the input voltage, which means that it can never reach zero.
The single-ended method is adequate for applications in which
the input device is a fairly blunt and imprecise instrument such
as a finger.
Ratiometric Method The ratiometric method is illustrated in Figure 29. Here, the
negative input of the ADC reference is tied to Y− and the
positive input is connected to Y+, so the screen excitation
voltage provides the reference for the ADC. The input of the
ADC is connected to X+ to determine the Y position.
REF–
VCC
Figure 29. Ratiometric Conversion of Touch Screen Inputs
For greater accuracy, the ratiometric method has two significant
advantages:
• The reference to the ADC is provided from the actual voltage
across the screen, so voltage drops across the switches have
no effect.
• Because the measurement is ratiometric, it does not matter if
the voltage across the screen varies in the long term. However,
it must not change after the signal has been acquired.
The disadvantage of the ratiometric method is that the screen
must be powered up all the time, because it provides the
reference voltage for the ADC.
TOUCH-PRESSURE MEASUREMENT The pressure applied to the touch screen via a pen or finger can
also be measured with the AD7877 using some simple calcula-
tions. The contact resistance between the X and Y plates is
measured. This provides a good indication of the size of the
depressed area and, therefore, the applied pressure. The area of
the spot touched is proportional to the size of the object
touching it. The size of this resistance (RTOUCH) can be calculated
using two different methods.
First Method The first method requires the user to know the total resistance
of the X-plate tablet (RX). Three touch screen conversions are
required:
• Measurement of the X position, XPOSITION (Y+ input).
• Measurement of the Y− input with the excitation voltage
applied to Y+ and X− (Z1 measurement).
• Measurement of the X+ input with the excitation voltage
applied to Y+ and X− (Z2 measurement).
These three measurements are illustrated in Figure 30.
The AD7877 has two special ADC channel settings that
configure the X and Y switches for Z1 and Z2 measurement and
store the results in the Z1 and Z2 results registers. The Z1
measurement is ADC Channel 1010b, and the result is stored in
the register with Read Address 11010b. The Z2 measurement is
ADC Channel 0010b, and the result is stored in the register with
Read Address 10010b.
The touch resistance can then be calculated using the following
equation:
RTOUCH = (RXPlate) × (XPOSITION /4096 × [Z2/Z1) − 1] (2)
MEASUREZ1 POSITION
MEASUREZ2 POSITION
Figure 30. Three Measurements Required for Touch Pressure
Second Method The second method requires that the resistance of the X-plate
and Y-plate tablets be known. Three touch screen conversions
again are required, a measurement of the X Position (XPOSITION),
Y Position (YPOSITION), and Z1 position.
The following equation also calculates the touch resistance:
RTOUCH = RXPlate × (XPOSITION /4096) × [(4096/Z1) − 1]
− RYPlate × [1 − (YPOSITION /4096)] (3)
STOPACQ PIN As explained previously, touch screens are composed of two
resistive layers, normally placed over an LCD screen. Because
these layers are in close proximity to the LCD screen, noise can
be coupled from the screen onto these resistive layers, causing
errors in the touch screen positional measurements.
For example, a jitter might be noticeable in the cursor on-
screen. In most LCD touch screen systems, a signal, such as an
LCD invert signal or other control signal, is present, and noise is
usually coupled onto the touch screen during this signal’s active
period, as shown in Figure 31.
LCD SIGNAL
TOUCH SCREENSIGNALNOISYPERIOD
It is only during the sample or acquisition phase of the
AD7877’s ADC operation that noise from the LCD screen has
an effect on the ADC’s measurements. During the hold or
conversion phase, the noise has no effect, because the voltage at
the input of the ADC has already been acquired. Therefore, to
minimize the effect of noise on the touch screen measurements,
the ADC acquisition phase should be halted.
The LCD control signal should be applied to the STOPACQ pin.
To ensure that acquisition never takes place during the noisy
period when the LCD signal is active, the AD7877 monitors this
signal. No acquisitions take place when the control signal is
active. Any acquisition that is in progress when the signal
becomes active is aborted and restarts when the signal becomes
inactive again.
To accommodate signals of different polarities on the
STOPACQ pin, a user-programmable register bit is used to
indicate whether the signal is active high or low. The POL bit is
Bit 3 in Control Register 2, Address 02h. Setting POL to 1
indicates that the signal on STOPACQ is active high; setting
POL to 0 indicates that it is active low. POL defaults to 0 on
power-up. To disable monitoring of STOPACQ, the pin should
be tied low if POL = 1, or tied high if POL = 0. Under no
circumstances should the pin be left floating.
The signal on STOPACQ has no effect while the ADC is in
conversion mode, or during the first conversion delay time. (See
the Control Registers section for details on first conversion
delay.)
When enabled, the STOPACQ monitoring function is imple-
mented on all input channels to the ADC: AUX1, AUX2, BAT1,
BAT2, TEMP1, and TEMP2, as well as on the touch screen input
channels.
TEMPERATURE MEASUREMENT Two temperature measurement options are available on the
AD7877: the single conversion method and the differential
conversion method. The single conversion method requires
only a single measurement on ADC Channel 1000b. Differential
conversion requires two measurements, one on ADC Channel
1000b and a second on ADC Channel 1001b. The results are
stored in the results registers with Addresses 11000b (TEMP1)
and 11001b (TEMP2). The AD7877 does not provide an explicit
output of the temperature reading. Some external calculations
must be performed by the system. Both methods are based on
an on-chip diode measurement.
Single Conversion Method The single conversion method makes use of the fact that the
temperature coefficient of a silicon diode is approximately
−2.1 mV/°C. However, this small change is superimposed on the
diode forward voltage, which can have a wide tolerance. It is,
change in forward voltage with temperature can be measured.
This method provides a resolution of approximately 0.3°C and a
predicted accuracy of ±2.5°C.
The temperature limit comparison is performed on the result in
the TEMP1 results register, which is simply the measurement of
the diode forward voltage. The values programmed into the
high and low limits should be referenced to the calibrated diode
forward voltage to make accurate limit comparisons. An
example is shown in the Limit Comparison section.
Differential Conversion Method The differential conversion method is a 2-point measurement.
The first measurement is performed with a fixed bias current
into a diode (when the TEMP1 channel is selected), and the
second measurement is performed with a fixed multiple of the
bias current into the same diode (when the TEMP2 channel is
selected). The voltage difference in the diode readings is
proportional to absolute temperature and is given by the
following formula:
∆VBE = (KT/q) × (1n N) (4)
where:
VBE represents the diode voltage.
N is the bias current multiple (typical value for AD7877 =120).
k is Boltzmann’s constant.
q is the electron charge.
This method provides a resolution of approximately 1.6°C, and
a guaranteed accuracy of ±4°C without calibration. Determina-
tion of the N value on a part-by-part basis improves accuracy.
Assuming a current multiple of 120, which is a typical value for
the AD7877, taking Boltzmann’s constant, k = 1.38054 × −23 electrons V/°K, the electron charge q = 1.602189 × 10−19,
then T, the ambient temperature in Kelvin, would be calculated
as follows:
∆VBE = (KT/q) × (1n N)
T°K = (∆VBE × q)/(k × 1n N)
= ∆VBE × 1.602189 × 10−19)/(1.38054 × 10−23 × 4.65)
T°C = 2.49 × 103 × ∆VBE − 273
∆VBE is calculated from the difference in readings from the first
conversion and second conversion. The user must perform the
calculations to get ∆VBE, and then calculate the temperature
value in degrees.
Figure 32 shows a block diagram of the temperature
measurement circuit.
TEMP1TEMP2
VBE
105× I
Figure 32. Block Diagram of Temperature Measurement Circuit
Temperature Calculations If an explicit temperature reading in °C is required, then this
can be calculated as follows for the single measurement
method:
1. Calculate the scale factor of the ADC in degrees per LSB:
Degrees per LSB = ADC LSB size/−2.1 mV =
VREF/4096)/−2.1 mV
2. Save the ADC output DCAL at the calibration temperature
TCAL.
3. Take ADC reading DAMB at temperature to be measured
TAMB.
4. Calculate the difference in degrees between TCAL and TAMB
using
∆T = (DAMB − DCAL) × degrees per LSB
5. Add ∆T to TCAL.
Example: The internal 2.5 V reference is used.
1. Degrees per LSB = (2.5/4096)/−2.1 × 10−3 = −0.291.
2. The ADC output is 983 decimal at 25°C, equivalent to a
diode forward voltage of 0.6 V.
3. The ADC output at TAMB is 880.
4. ∆T = (880 − 983) × −0.291 = 30°.
5. TAMB = 25 + 30 = 55°C.
To calculate the temperature explicitly using the differential
method:
1. Calculate the LSB size of the ADC in V:
LSB = VREF/4096
2. Subtract TEMP1 from TEMP2 and multiply by LSB size to
get ∆VBE.
Example: The internal 2.5 V reference is used.
1. LSB size = 2.5 V/4096 = 6.1 × 10−4 V (610 µV).
2. TEMP1 = 880 and TEMP2 = 1103:
∆VBE = (1103 − 880) × 6.1× 10−4 = 0.136 V
3. T = 0.136 × 2490 − 273 = 65°C.
BATTERY MEASUREMENT The AD7877 can monitor battery voltages from 0.5 V to 5 V on
two inputs, BAT1 and BAT2. Figure 33 shows a block diagram
of a battery voltage monitored through the BAT1 pin. The
voltage to the VCC pin of the AD7877 is maintained at the
desired supply voltage via the dc/dc regulator while the input to
the regulator is monitored. This voltage on BAT1 is divided
down by 2 internally, so that a 5 V battery voltage is presented to
the ADC as 2.5 V. To conserve power, the divider circuit is on
only during the sampling of a voltage on BAT1. The BAT2 input
circuitry is identical.
The BAT1 input is ADC Channel 0110b and the result is stored
in Register 10110b. The BAT2 input is ADC Channel 0111b and
the result is stored in Register 10111b.
03796-012
BATTERY0.5V TO 5VFigure 33. Block Diagram of Battery Measurement Circuit
Figure 33 shows the ADC using the internal reference of 2.5 V.
If a different reference voltage is used, then the maximum
battery voltage that the AD7877 can measure changes. The
maximum voltage measurable is VREF × 2, because this voltage
gives a full-scale output from the ADC. If a smaller reference is
used, such as 2 V, then the maximum battery voltage measurable
is 4 V. If a larger reference is used, such as 3.5 V, then the
maximum battery voltage measurable is 7 V. The internal
reference is particularly suited for use when measuring Li-Ion
batteries, where the minimum voltage is about 2.7 V and the
maximum is about 4.2 V. A proper choice of external reference
ensures that other voltage ranges can be accommodated.
AUXILIARY INPUTS The AD7877 has three auxiliary analog inputs, AUX1 to AUX3.
These channels have a full-scale input range from 0 V to VREF.
The ADC channel addresses for AUX1 to AUX3 are 0011b,
0100b, and 0101b, and the results are stored in Registers 10011b,
10100b, and 10101b. These pins can also be reconfigured as
general-purpose logic inputs/outputs, as described in the GPIO
Configuration section.
LIMIT COMPARISON The AUX1 measurement, the two battery measurements, and
the TEMP1 measurement can all be compared with high and
low limits, and an out-of-limit result made to generate an alarm
output at the ALERT pin. The limits are stored in registers with
addresses from 00100b to 01011b. After a measurement from
any one of the four channels is converted, it is compared with
the corresponding high and low limits. An out-of-limit result
sets one of the status bits in the alert status/enable register. For
details on these and other registers, see the Register Maps and
Detailed Register Descriptions sections. For details on writing
and reading data, see the Serial Interface section.
As mentioned previously, the temperature comparison is made
using the result of the TEMP1 measurement, which is the diode
forward voltage. Because the temperature coefficient of the
diode is known but the actual forward voltage can have a wide
tolerance, it is not possible to program the high and low limit
registers with predetermined values.
Instead, it is necessary to calibrate the temperature measure-
ment, calculate the TEMP1 readings at the high and low limit
temperatures, and then program those values into the limit
registers, as follows:
1. Calculate LSB per degree = −2.1 mV/(VREF/4096).
2. Save the calibration reading DCAL at calibration temperature
TCAL.
3. Subtract TCAL from limit temperatures THIGH and TLOW to get
the difference in degrees between the limit temperatures
and the calibration temperature.
4. Multiply this value by LSB per degree to get the value in
LSBs.
5. Add these values to the digital value at the calibration
temperature to get the digital high and low limit values.
Example: The internal 2.5 V reference is used.
1. THIGH = +65°C and TLOW = −10°C.
2. LSB per degree = −2.1 × 10−3/(2.5/4096) = −3.44.
3. DCAL = 983 decimal at 25°C.
4. DHIGH = (65 − 25) × −3.44 + 983 = 845.
5. DLOW = (−10 − 25) × −3.44 + 983 = 1103.
CONTROL REGISTERS Control Register 1 contains the ADC channel address, the
SER/DFR bit (to choose single or differential methods of touch
screen measurement), the register read address, and the ADC
mode bits. Control Register 1 should always be the last register
to be programmed prior to starting conversions. Its power-on
default value is 00h. To change any parameter after conversion
has begun, the part should first be put into mode 00, the
changes made, and then Control Register 1 reprogrammed,
ensuring that it is always the last register to be programmed
before conversions begin.
03796-013
Figure 34. Control Register 1
Control Register 2 sets the timer, reference, polarity, first
conversion delay, averaging, and acquisition time. Its power-on
default value is 00h. See the Detailed Register Descriptions
section for more information on the control registers.
03796-014
0Figure 35. Control Register 2
CONTROL REGISTER 1
ADC Mode (Control Register 1 Bits <1:0>) These bits select the operating mode of the ADC. The AD7877
has three operating modes. These are selected by writing to the
mode bits in Control Register 1. If the mode bits are 00, no
conversion is performed.
Table 5. Control Register 1 Mode Selection If the mode bits are 01, a single conversion is performed on the
channel selected by writing to the channel bits of Control
Register 1 (Bits 7 to 10). At the end of the conversion, if the
TMR bits in Control Register 2 are set to 00, the mode bits
revert to 00 and the ADC returns to no convert mode until a
new conversion is initiated by the host. Setting the TMR bits to
a value other than 00 causes the conversion to be repeated, as
described in the Timer (Control Register 2 Bits <1:0>) section.
The flowchart in Figure 37 shows how the AD7877 operates in
mode 01.
The AD7877 can also be programmed to convert a sequence of
selected channels automatically. The two modes for this type of
conversion are slave mode and master mode.
For slave mode operation, the channels to be digitized are
selected by setting the corresponding bits in Sequencer
Register 0. Conversion is initiated by writing 10b to the mode
bits of Control Register 1. The ADC then digitizes the selected
channels and stores the results in the corresponding results
registers. At the end of the conversion, if the TMR bits in
Control Register 2 are set to 00, the mode bits revert to 00 and
the ADC returns to no convert mode until a new conversion is
initiated by the host. Setting the TMR bits to a code other than
00 causes the conversion sequence to be repeated. The flowchart
in Figure 38 shows how the AD7877 operates in mode 10.
For master mode operation, the channels to be digitized are
written to Sequencer Register 1. Master mode is then selected
by writing 11 to the mode bits in Control Register 1. In this
mode, the wake-up on touch feature is active, so conversion
does not begin immediately. The AD7877 waits until the screen
is touched before beginning the sequence of conversions. The
ADC then digitizes the selected channels, and the results are
written to the results registers. The AD7877 waits for the screen
to be touched again, or for a timer event if the screen remains
touched, before beginning another sequence of conversions.
Figure 39 is a flowchart, showing how the AD7877 operates in
mode 11.
ADC Channel (Control Register 1 Bits <10:7>) The ADC channel is selected by Bits 10:7 of Control Register 1
(CHADD3 to CHADD0). In addition, the SER/DFR bit, Bit 11,
selects between single-ended and differential conversion. A
complete list of channel addresses is given in Table 6.
For mode 0 (single-channel) conversion, the channel is selected
by writing the appropriate CHADD3 to CHADD0 code to
Control Register 1.
For sequential channel conversion, channels to be converted are
selected by setting bits corresponding to the channel number in
Sequencer Register 1 for slave mode sequencing or Sequencer
Register 2 for master mode sequencing.
For both single-channel and sequential conversion, normal
(single-ended) conversion is selected by clearing the SER/DFR
bit in Control Register 1. Ratiometric (differential) conversion is
selected by setting the SER/DFR bit.
Table 6. Codes for Selecting Input Channel and Normal or Ratiometric Conversion
CONTROL REGISTER 2
Timer (Control Register 2 Bits <1:0>) The TMR bits in Control Register 2 enable the ADC to
repeatedly perform a conversion or conversion sequence either
once only or at intervals of 512 µs, 1.024 ms, or 8.19 ms. In slave
mode, the timer starts as soon as the conversion sequence is
finished. In master mode, the timer starts at the end of a conver-
sion sequence only if the screen remains touched. If the touch is
released at any stage, then the timer stops and, the next time the
screen is touched, a conversion sequence begins immediately.
Table 7. Control Register 2 Timer Selection
Int/Ext Reference (Control Register 2 Bit <2>) If the REF bit in Control Register 2 is 0 (default value), the
internal reference is selected. If any connection is made to VREF
while the internal reference is selected (for example, to supply a
reference to other circuits), it should be buffered. An external
power supply should not be connected to this pin while REF is
equal to 0, because it might overdrive the internal reference.
Note also that, because the internal reference is 2.5 V, it operates
only with supply voltages down to 2.7 V. Below this value an
external reference should be used.
If the REF bit is 1, the VREF pin becomes an input and the
internal reference is powered down. This overrides any setting
of the PM bits with regard to the reference. An external
reference can then be applied to the REF pin.
STOPACQ Polarity (Control Register 2 Bit <3>) This bit should be set according to the polarity of the signal
applied to the STOPACQ pin. If that signal is active high, that is,
no acquisitions should occur during the signal’s high period,
then the POL bit should be set to 1. If the signal is active low,
then the POL bit should be 0. The default value for POL is 0.
First Conversion Delay (Control Register 2 Bits <5:4> ) The first conversion delay (FCD) bits in Control Register 2
program a delay of 500 ns (default), 128 µs, 1.024 ms, or 8.19 ms
before the first conversion, to allow the ADC time to power up.
This delay also occurs before conversion of the X and Y
coordinate channels, to allow extra time for screen settling, and
after the last conversion in a sequence, to precharge PENIRQ. If
the signal on the STOPACQ pin is being monitored and goes
active during the FCD, it is ignored until after the FCD period.
Table 8. First Conversion Delay Selection
Power Management (Control Register 2 Bits <7:6>) The power management (PM) bits in Control Register 2 allow
the power management features of the ADC to be programmed.
If the PM bits are 00, the ADC is powered down permanently.
This overrides any setting of the mode bits in Control
Register 1. If the PM bits are 01, the ADC and the reference
both power down when the ADC is not converting. If the PM
bits are 10, the ADC and reference are powered up continuously.
If the PM bits are 11, the ADC, but not the reference, powers
down when the ADC is not converting.
Table 9. Power Management Selection
Acquisition Time (Control Register 2 Bits <9:8>) The ACQ bits in Control Register 2 allow the selection of
acquisition times for the ADC of 2 µs (default), 4 µs, 8 µs, or
16 µs. The user can program the ADC with an acquisition time
suitable for the type of signal being sampled. For example,
signals with large RC time constants might require longer
acquisition times.
Table 10. Acquisition Time Selection
Averaging (Control Register 2 Bits <11:10>) Signals from touch screens can be extremely noisy. The AVG
bits in Control Register 2 allow multiple conversions to be
performed on each input channel and averaged to reduce noise.
A single conversion can be selected (no averaging), which is the
default, or 4, 8, or 16 conversions can be averaged. Only the final
averaged result is written into the results register.
Table 11. Averaging Selection
SEQUENCER REGISTERS There are two sequencer registers on the AD7877. Sequencer
Register 0 controls the measurements performed during a slave
mode sequence. Sequencer Register 1 controls the measure-
ments performed during a master mode sequence.
To include a measurement in a slave mode or master mode
sequence, the relevant bit must be set in Sequencer Register 0 or
Sequencer Register 1. Setting Bit 11 includes a measurement on
ADC Channel 0 in the sequence, which is the Y positional
measurement. Setting Bit 10 includes a measurement on ADC
Channel 1 (X+ measurement), and so on, through Bit 1 for
Channel 10. Figure 36 illustrates the correspondence between
the bits in the sequencer registers and the various measure-
ments. Bit 0 in both sequencer registers is not used. See also the
Detailed Register Descriptions section.
03796-015
Figure 36. Sequencer Register
