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Touch Screen Controller

Data Sheet

AD7877

Rev. D

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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, V

of 2.7 V to 5.25 V

Separate V

DRIVE

level for serial interface

Shutdown mode: 1 µA maximum

32-lead, LFCSP, 5 mm × 5 mm package

25-ball,WLCSP, 2.5 mm × 2.8 mm package

Qualified for automotive applications

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

DRIVE

DAV

CONTROL LOGICAND SERIAL PORT

DAC

CONTROL

REGISTERS

GPIO

REGISTERS

ALERT STATUS/

MASK REGISTER

LIMIT

REGISTERS

LIMIT

COMPARATOR

RESULTS

REGISTERS

SEQUENCER

8-BIT

DAC

ARNG

AOUT

AUX3/GPIO3

AUX2/GPIO2

AUX1/GPIO1

REF

PEN INTERRUPT

AND WAKE-UP

ON TOUCH

PENIRQ

ALERT

LOGIC

ALERT

STOP

ACQ

LOGIC

STOPACQ

AGND

DGND

GPIO4

2.5V

REF

BUF

GPIO1-3

ADC DATA

12-BIT SUCCESSIVE

APPROXIMATION ADC

WITH TRACK-AND-HOLD

CLOCK

9 TO 1

INPUT

MUX

TEMPERATURE

SENSOR

BAT2

BATTERY

MONITOR

BAT1

BATTERY

MONITOR

Y–

Y+

X–

X+

REF–

REF+

DUAL 3-1

MUX

X–

Y–

GND

X+

Y+

REF

AD7877

03796-001

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 2.2 V), 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 varied from 1 V to +V

, with

an analog input range of 0 V to V

REF

. The device includes a

shutdown mode that 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 is 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 back-

light or contrast control. The AD7877 runs 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), and a 25-ball wafer level chip scale package (WLCSP).

Summary of content (45 pages)

PAGE 1
Data Sheet Touch Screen Controller AD7877 FEATURES FUNCTIONAL BLOCK DIAGRAM VCC 7 X– 10 Y+ 13 Y– 11 AUX2/GPIO2 5 REF– AUX3/GPIO3 4 BAT1 3 9 TO 1 INPUT MUX BATTERY MONITOR IN SEQUENCER BAT2 2 REF+ CLOCK 12-BIT SUCCESSIVE APPROXIMATION ADC WITH TRACK-AND-HOLD STOP ACQ LOGIC 20 STOPACQ RESULTS REGISTERS BATTERY MONITOR LIMIT COMPARATOR LIMIT REGISTERS TEMPERATURE SENSOR 14 AGND 15 DGND VREF 31 ALERT STATUS/ MASK REGISTER 2.
PAGE 2
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PAGE 3
AD7877 Data Sheet TABLE OF CONTENTS Features .............................................................................................. 1 Control Registers ............................................................................ 20 Applications ....................................................................................... 1 Control Register 1 ...................................................................... 20 Functional Block Diagram .............................................
PAGE 4
Data Sheet AD7877 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.
PAGE 5
AD7877 Parameter DAC Resolution Integral Nonlinearity Differential Nonlinearity Voltage Mode Output Voltage Range Slew Rate Output Settling Time Capacitive Load Stability Output Impedance Short-Circuit Current Current Mode Output Current Range Output Impedance LOGIC INPUTS Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN 4 LOGIC OUTPUTS Output High Voltage, VOH Output Low Voltage, VOL Floating-State Leakage Current Floating-State Output Capacitance4 Output Coding CO
PAGE 6
Data Sheet AD7877 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.
PAGE 7
AD7877 Data Sheet ABSOLUTE MAXIMUM RATINGS TA = 25°C, unless otherwise noted. Table 3.
PAGE 8
Data Sheet AD7877 NC VREF AOUT ARNG VDRIVE DOUT DCLK NC PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS 32 31 30 29 28 27 26 25 24 NC 23 DAV BAT1 3 22 ALERT 21 GPIO4 20 STOPACQ AUX1/GPIO1 6 19 DIN VCC 7 18 CS 17 PENIRQ AD7877 AUX3/GPIO3 4 TOP VIEW (Not to Scale) AUX2/GPIO2 5 9 10 11 12 13 14 15 16 NC X– Y– X+ Y+ AGND DGND NC NC 8 NOTES 1. NC = NO CONNECT 2. THE EXPOSED PAD IS NOT CONNECTED INTERNALLY.
PAGE 9
AD7877 Data Sheet Pin No. 28 Mnemonic VDRIVE 29 30 31 ARNG AOUT VREF Description Logic Power Supply Input. The voltage supplied at this pin determines the operating voltage for the serial interface of the AD7877. When the DAC is in current output mode, a resistor from ARNG to GND sets the output range. Analog Output Voltage or Current from DAC. Reference Output for the AD7877. The internal 2.5 V reference is available on this pin for use external to the device.
PAGE 10
Data Sheet AD7877 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. 200 800 ADC, REF, AND DAC 180 160 CURRENT (nA) CURRENT (µA) 700 ADC AND REF 600 140 120 –30 –10 0 30 TEMPERATURE (°C) 50 70 80 –50 90 Figure 6. Supply Current vs. Temperature 03796-032 500 –50 03796-030 100 –30 –10 10 30 TEMPERATURE (°C) 50 70 90 Figure 9. Full Power-Down IDD vs. Temperature 1000 0.6 0.5 900 0.
PAGE 11
AD7877 Data Sheet 16 1.0 0.8 14 REFERENCE CURRENT (µA) 0.6 0.2 0 –0.2 –0.4 –0.8 –1.0 0 500 1000 1500 2000 2500 CODE 3000 3500 10 8 6 4 2 03796-045 –0.6 12 0 –50 4000 Figure 12. ADC DNL Plot 03796-046 DNL (LSB) 0.4 –30 –10 10 30 TEMPERATURE (°C) 50 70 90 Figure 15. External Reference Current vs. Temperature 22 2.520 2.515 20 2.510 18 2.505 VREF (V) RON (Ω) X– TO GND 16 Y– TO GND 14 Y+ TO VCC 2.500 2.495 2.490 12 03796-048 X+ TO VCC 8 2.7 3.1 3.5 3.9 4.3 4.7 5.
PAGE 12
Data Sheet AD7877 3145 6 3135 3115 INTERNAL VREF (V) ADC CODE (Decimal) 3125 3105 3095 3085 3075 NO CAP 0.711µs SETTLING TIME 2.5 100nF CAP 54.64µs SETTLING TIME 3055 3045 –50 –30 –10 10 30 TEMPERATURE (°C) 50 70 0 –20 90 1182 –10 INPUT TONE AMPLITUDE (dB) 10 1180 1179 1178 3.1 3.2 VCC (V) 3.3 60 80 100 120 3.4 3.5 SNR 70.25dB THD 78.11dB –30 –50 –70 –90 –110 –130 03796-042 1177 3.0 40 03796-035 TEMP1 CODE 1181 2.9 20 Figure 21. Internal VREF vs.
PAGE 13
AD7877 Data Sheet 03796-037 VDD = 3V TEMPERATURE = 25°C 1 CH1 200mV CH2 100mV M2.00µs CH1 03796-050 ∆: 144mV @: 1.296V –2 780mV 600 400 DAC SINK CURRENT 300 200 03796-038 DAC SINK CURRENT (µA) 500 NOTE: MAXIMUM DAC SINK CURRENT IS SET ACCORDING TO THE EQUATION: IMAX = VCC /(RRNG × 6) 0 0 25 50 75 100 125 150 175 200 225 0 ERROR (%) 1 Figure 26. Typical Accuracy for Battery Channel (25°C) Figure 24.
PAGE 14
Data Sheet AD7877 TERMINOLOGY Integral Nonlinearity (INL) 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 at 1 LSB below the first code transition, and full scale at 1 LSB above the last code transition. Differential Nonlinearity (DNL) The difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC.
PAGE 15
AD7877 Data Sheet CIRCUIT INFORMATION 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.
PAGE 16
Data Sheet AD7877 VCC The voltage seen at the input to the ADC in Figure 29 is VIN = VCC × X+ X– REF INT/EXT Y+ Y– X– Y– GND X+ Y+ VREF DUAL 3-1 MUX 9 TO 1 I/P MUX AUX1/GPIO2 AUX2/GPIO3 AUX3/GPIO4 REF– IN+ BAT1 REF+ 12-BIT SUCCESSIVE APPROXIMATION ADC WITH TRACK-AND-HOLD 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.
PAGE 17
AD7877 Data Sheet MEASURE X POSITION For greater accuracy, the ratiometric method has two significant advantages: TOUCH RESISTANCE The reference to the ADC is provided from the actual voltage across the screen; therefore, when the voltage drops across the switches, it has no effect. Y– Y+ X+ MEASURE Z1 POSITION 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.
PAGE 18
Data Sheet AD7877 The LCD control signal should be applied to the STOPACQ pin. To ensure that acquisition never occurs 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.
PAGE 19
AD7877 Data Sheet Temperature Calculations BATTERY MEASUREMENT If an explicit temperature reading in °C is required, then this is calculated as follows for the single measurement method: The AD7877 can monitor battery voltages from 0.5 V to 5 V on two inputs, BAT1 and BAT2. Figure 34 shows a block diagram of a battery voltage monitored through the BAT1 pin.
PAGE 20
Data Sheet AD7877 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 Register 10011b, Register 10100b, and Register 10101b. These pins can also be reconfigured as general-purpose logic inputs/outputs, as described in the GPIO Configuration section.
PAGE 21
AD7877 Data Sheet 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 0x00.
PAGE 22
Data Sheet AD7877 Table 6.
PAGE 23
AD7877 Data Sheet STOPACQ Polarity (Control Register 2, Bit[3]) Acquisition Time (Control Register 2, Bits[9:8]) 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 high period of the signal, 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.
PAGE 24
Data Sheet AD7877 HOST PROGRAMS AD7877 IN MODE 10 HOST PROGRAMS AD7877 IN MODE 01 VALID SEQUENCE 0? NO IS FCD REQUIRED? GO TO MODE 00 NO YES SELECT NEXT CHANNEL YES START FCD TIMER IS FCD REQUIRED? IS FCD FINISHED? YES NO START FCD TIMER YES YES NO IS FCD FINISHED? IS STOPACQ SIGNAL ACTIVE? NO YES NO IS STOPACQ SIGNAL ACTIVE? YES START ACQUISITION TIMER NO START ACQUISITION TIMER IS STOPACQ SIGNAL ACTIVE? YES YES IS STOPACQ SIGNAL ACTIVE? NO NO NO IS ACQUISITION TIME FINISHED
PAGE 25
AD7877 Data Sheet HOST PROGRAMS AD7877 IN MODE 11 GO TO MODE 00 INTERRUPTS Data Available Output (DAV) VALID SEQUENCE 1? NO The data available output (DAV) indicates that new ADC data is available in the results registers. While the ADC is idle or is converting, DAV is high. Once the ADC has finished converting and new data has been written to the results registers, DAV goes low. Taking DAV low to read the registers resets DAV to a high condition.
PAGE 26
Data Sheet AD7877 NOT SCREEN TOUCHED t2 = 512 µs with timer set to 1024 (TMR bits = 01b) tWRITE = tREAD = 16 CLK period × tDCLK = 800 ns PENIRQ DETECTS TOUCH PENIRQ t3 = maximum time taken to write read address and read 11 registers = 800 ns (write) + [800 ns (read) × 11] = 9.6 µs. ADC STATUS t4MAX = t2 − t3 = 512 µs − 9.6 µs = 502.4 µs Pen Interrupt (PENIRQ) Y+ VCC VCC 50kΩ PENIRQ X+ X– TOUCH SCREEN Y– 03796-021 PENIRQ ENABLE Figure 43.
PAGE 27
AD7877 Data Sheet 8-BIT DAC In current mode, it is quite easy to calculate the resistor values to give the required adjustment range in VOUT using the following steps: The AD7877 features an on-chip 8-bit DAC for LCD contrast control. The DAC can be configured for voltage output by clearing Bit 2 of the DAC register (Address 1110b), or for current output by setting this bit. The output voltage range can be set to 0 to VCC/2 by clearing Bit 0 of the DAC register, or to 0 to VCC by setting this bit.
PAGE 28
Data Sheet AD7877 When the DAC output voltage is zero, it sinks the maximum current through R1. The feedback current and, therefore, VOUT are at their maximum. As the DAC output voltage increases, the sink current and, thus, the feedback current decrease, and VOUT falls. If the DAC output exceeds VREF, it starts to source current, and VOUT has to further decrease to compensate. When the DAC output is at full scale, VOUT is at its minimum.
PAGE 29
AD7877 Data Sheet SERIAL INTERFACE Register Address 1111b is not a physical register, but enables an extended writing mode that allows writing to the GPIO configuration registers. When the register address is 1111b, the next four bits of the data-word are the address of a GPIO configuration register and the eight LSBs are the GPIO configuration data. For details on the configuration of the GPIO pins, see the General-Purpose I/O Pins section.
PAGE 30
Data Sheet AD7877 WRITE TIMING No serial interface operations can take place when CS is high. To write to the AD7877, CS must be taken low. To write to the device, a burst of 16 clock pulses is input to DCLK while the write data is input to DIN. Data is clocked in on the rising edge of DCLK. If multiple write operations are to be performed, CS must be taken high after the end of each write operation before another write operation can be performed by taking CS low again.
PAGE 31
AD7877 Data Sheet GENERAL-PURPOSE I/O PINS The AD7877 has one dedicated general-purpose logic input/ output pin (GPIO4), and any or all of the three auxiliary analog inputs can also be reconfigured as GPIOs. Associated with the GPIOs are two 8-bit control registers and one 8-bit data register that are accessed using the extended write mode. As described in the Reading Data section, GPIO registers are written to using the extended writing mode.
PAGE 32
Data Sheet AD7877 If the ALERT source is a GPIO, then masking out the interrupt by clearing the corresponding ALEN bit to 0 or removing the source of the interrupt on the GPIO pin causes ALERT to go high again. If the ALERT source is an out-of-limit measurement, writing a 0 to the corresponding status bit in the alert status/enable register causes ALERT to go high. However, the status bit is set to 1 again on the next measurement cycle, if the measurement remains out of limit.
PAGE 33
AD7877 Data Sheet GROUNDING AND LAYOUT The bottom of the chip scale package has a central thermal pad. The thermal pad on the printed circuit board should be at least as large as this exposed pad. On the printed circuit board, provide a clearance of at least 0.25 mm between the thermal pad and the inner edges of the pad pattern to ensure that shorting is avoided. It is recommended that the ground pins, AGND and DGND, be shorted together as close as possible to the device itself on the user’s PCB.
PAGE 34
Data Sheet AD7877 REGISTER MAPS Table 13.
PAGE 35
AD7877 Data Sheet Table 15.
PAGE 36
Data Sheet AD7877 DETAILED REGISTER DESCRIPTIONS Register Name: Control Register 1 Write Address: 0001 Read Address: 00001 Default Value: 0x000 Type: Read/write Table 16.
PAGE 37
AD7877 Data Sheet Register Name: Control Register 2 Write Address: 0010 Read Address: 00010 Default Value: 0x000 Type: Read/write Table 17. Bit 0 1 Name TMR0 TMR1 Read/Write R/W R/W 2 REF R/W 3 POL R/W 4 5 FCD0 FCD1 R/W R/W 6 7 PM0 PM1 R/W R/W 8 9 ACQ0 ACQ1 R/W R/W 10 11 AVG0 AVG1 R/W R/W Description LSB of conversion interval timer MSB of conversion interval timer 00 = convert only once 01 = every 1024 clock periods (512 µs) 10 = every 2048 clock periods (1.
PAGE 38
Data Sheet AD7877 Register Name: Alert Status/Enable Register Write Address: 0011 Read Address: 00011 Default Value: 0x000 Type: Read/write Table 18.
PAGE 39
AD7877 Data Sheet Register Name: BAT2 Low Limit Register Name: TEMP1 High Limit Write Address: 1001 Write Address: 1011 Read Address: 01001 Read Address: 01011 Default Value: 0x000 Default Value: 0x000 Type: Read/write Type: Read/write This register contains the 12-bit low limit for Battery Monitoring Input 2. This register contains the 12-bit high limit for temperature measurement.
PAGE 40
Data Sheet AD7877 Register Name: Sequencer Register 0 Write Address: 1100 Read Address: 01100 Default Value: 0x000 Type: Read/write Table 19.
PAGE 41
AD7877 Data Sheet Register Name: DAC Register Write Address: 1110 Read Address: 01110 Default Value: 0x008 Type: Read/write Table 21.
PAGE 42
Data Sheet AD7877 Register Name: AUX2 Register Name: TEMP1 Write Address: N/A Write Address: N/A Read Address: 10100 Read Address: 11000 Default Value: 0x000 Default Value: 0x000 Type: Read only Type: Read only This register contains the 12-bit result of the measurement at Auxiliary Input 2. This register contains the 12-bit result of a temperature measurement using single-ended conversion.
PAGE 43
AD7877 Data Sheet GPIO REGISTERS GPIO registers are written to using an extended 8-bit address. The first four bits of the data-word are always 1111b to access the extended writing map. The next four bits are the register address. This leaves 8 bits for the GPIO data. GPIO registers are read like all other registers, by writing a 5-bit address to Control Register 1, then reading DOUT. See the GPIO Configuration section for information on configuring the GPIOs.
PAGE 44
Data Sheet AD7877 Register Name: GPIO Control Register 2 Write Address: [1111] 0001 Read Address: 11100 Default Value: 0x000 Type: Read/write Table 23.
PAGE 45
AD7877 Data Sheet OUTLINE DIMENSIONS 5.00 BSC SQ 0.60 MAX 0.60 MAX 25 1 0.50 BSC 3.25 3.10 SQ 2.95 EXPOSED PAD 17 1.00 0.85 0.80 12° MAX 0.80 MAX 0.65 TYP 9 0.25 MIN BOTTOM VIEW 3.50 REF 0.05 MAX 0.02 NOM COPLANARITY 0.08 0.20 REF 0.30 0.25 0.18 SEATING PLANE 8 16 0.50 0.40 0.30 TOP VIEW FOR PROPER CONNECTION OF THE EXPOSED PAD, REFER TO THE PIN CONFIGURATION AND FUNCTION DESCRIPTIONS SECTION OF THIS DATA SHEET. 05-25-2011-A 4.