TSC2014 1.2V to 3.6V, 12-Bit, Nanopower, 4-Wire Touch Screen Controller with I2C Interface

7.3.4.1 Data Format

The TSC2014 output data are in Straight Binary format as shown in Figure 20. This figure shows the ideal output code for the given input voltage and does not include the effects of offset, gain, or noise.

1. Reference voltage at converter: +REF – (–REF). See Figure 19.

2. Input voltage at converter, after multiplexer: +IN – (–IN). See Figure 19.

Figure 20. Ideal Input Voltages and Output Codes

7.3.4.2 Reference

The TSC2014 uses an external voltage reference that applied to the VREF pin. It is possible to use V_DD as the reference voltage because the upper reference voltage range is the same as the supply voltage range.

7.3.4.3 Variable Resolution

The TSC2014 provides either 10-bit or 12-bit resolution for the A/D converter. Lower resolution is often practical for measuring slow changing signals such as touch pressure. Performing the conversions at lower resolution reduces the amount of time it takes for the A/D converter to complete its conversion process, which also lowers power consumption.

7.3.4.4 Conversion Clock and Conversion Time

The TSC2014 contains an internal clock (oscillator) that drives the internal state machines that perform the many functions of the part. This clock is divided down to provide a conversion clock for the A/D converter. The division ratio for this clock is set in the A/D Converter Control register (see Table 14). The ability to change the conversion clock rate allows the user to choose the optimal values for resolution, speed, and power dissipation. If the 4MHz (oscillator) clock is used directly as the A/D converter clock (when CL[1:0] = (0,0)), the A/D converter resolution is limited to 10 bits. Using higher resolutions at this speed does not result in more accurate conversions. 12-bit resolution requires that CL[1:0] is set to (0,1) or (1,0).

Regardless of the conversion clock speed, the internal clock runs nominally at 3.8MHz at a 3V supply (V_DD) and slows down to 3.6MHz at a 1.6V supply. The conversion time of the TSC2014 depends on several functions. While the conversion clock speed plays an important role in the time it takes for a conversion to complete, a certain number of internal clock cycles are needed for proper sampling of the signal. Moreover, additional times (such as the panel voltage stabilization time), can add significantly to the time it takes to perform a conversion. Conversion time can vary depending on the mode in which the TSC2014 is used. Throughout this data sheet, internal and conversion clock cycles are used to describe the amount of time that many functions take. These times must be taken into account when considering the total system design.

7.3.4.5 Touch Detect

PINTDAV can be programmed to generate an interrupt to the host. Figure 21 details an example for the Y-position measurement. While in the power-down mode, the Y– driver is on and connected to GND. The internal pen-touch signal depends on whether or not the X+ input is driven low. When the panel is touched, the X+ input is pulled to ground through the touch screen and the internal pen-touch output is set to low because of the detection on the current path through the panel to GND, which initiates an interrupt to the processor. During the measurement cycles for X- and Y-Position, the X+ input is disconnected, which eliminates any leakage current from the pull-up resistor to flow through the touch screen, thus causing no errors.

1. Untrimmed resistor; see the typical value in the

Figure 21. Example of a Pen-Touch Induced Interrupt via the PINTDAV Pin

In modes where the TSC2014 must detect whether or not the screen is still being touched (for example, when doing a pen-touch initiated X, Y, and Z conversion), the TSC2014 must reset the drivers so that the R_IRQ resistor is connected again. Because of the high value of this pull-up resistor, any capacitance on the touch screen inputs causes a long delay time, and may prevent the detection from occurring correctly. To prevent this possible delay, the TSC2014 has a circuit that allows any screen capacitance to be precharged, so that the pull-up resistor does not have to be the only source for the charging current. The time allowed for this precharge, as well as the time needed to sense if the screen is still touched, can be set in the configuration register.

This configuration underscores the need to use the minimum possible capacitor values on the touch screen inputs. These capacitors may be needed to reduce noise, but too large a value will increase the needed precharge and sense times, as well as the panel voltage stabilization time.

7.3.4.6 Preprocessing

The TSC2014 offers an array of powerful preprocessing operations that reduce unnecessary traffic on the bus and reduce the host processor loading. This reduction is especially critical for the serial interface, where limited bandwidth is a tradeoff, keeping the connection lines to a minimum.

All data acquisition tasks are looking for specific data that meet certain criteria. Many of these tasks fall into a predefined range, while other tasks may be looking for a value in a noisy environment. If these data are all to be retrieved by host processor for processing, the limited bus bandwidth quickly saturates, along with the host processor processing capability. In any case, the host processor must always be reserved for more critical tasks, not for routine work.

The preprocessing unit consists of two main functions: the combined MAV filter (median value filter and averaging filter), followed by the zone detection.

7.3.4.6.1 Preprocessing—Median Value Filter and Averaging Value Filter

The first preprocessing function, a combined MAV filter, can be operated independently as a median value filter (MVF), an averaging value filter (AVF), and a combined filter (MAVF).

If the acquired signal source is noisy because of the digital switching circuit, it may be necessary to evaluate the data without noise. In this case, the median value filter (MVF) operation helps to discard the noise. The array of N converted results is first sorted. The return value is either the middle (median value) of an array of M converted results, or the average value of a window size of W of converted results:

• N = the total number of converted results used by the MAV filter
• M = the median value filter size programmed
• W = the averaging window size programmed

If M = 1, then N = W. A special case is W = 1, which means the MAVF is bypassed. Otherwise, if W > 1, only averaging is performed on these converted results. In either case, the return value is the averaged value of window size W of converted results. If M > 1 and W = 1, then N = M, meaning only the median value filter is operating. The return value is the middle position converted result from the array of M converted results. If M > 1 and W > 1, then N = M. In this case, W < M. The return value is the averaged value of middle portion W of converted results out of the array of M converted results. Since the value of W is an odd number in this case, the averaging value is calculated with the middle position converted result counted twice (so a total of W + 1 converted results are averaged).

Table 1. Median Value Filter Size Selection

M1	M0	MEDIAN VALUE FILTER M =	POSSIBLE AVERAGING WINDOW SIZE W =
0	0	1	1, 4, 8, 16
0	1	3	1
1	0	7	1, 3
1	1	15	1, 3, 7

Table 2. Averaging Value Filter Size Selection

		AVERAGING VALUE FILTER SIZE SELECTION W =
W1	W0	M = 1 (Averaging Only)	M > 1
0	0	1	1
0	1	4	3
1	0	8	7
1	1	16	Reserved

NOTE

The default setting for MAVF is MVF (median value filter with averaging bypassed) for any invalid configuration. For example, if (M1, M0, W1, W0) = (1,0,1,0), the MAVF performs as it was configured for (1,0,0,0), median filter only with filter size = 7 and no averaging. The only exception is M > 1 and (W1, W0) = (1,1). This setting is reserved and should not be used.

Table 3. Combined MAV Filter Setting

M	W	INTERPRETATION	N =	OUTPUT
= 1	= 1	Bypass both MAF and AVF	W	The converted result
= 1	> 1	Bypass MVF only	W	Average of W converted results
> 1	= 1	Bypass AVF only	M	Median of M converted results
> 1	> 1	M > W	M	Average of middle W of M converted results with the median counted twice

The MAV filter is available for all analog inputs including the touch screen inputs, temperature measurements TEMP1 and TEMP2, and the AUX measurement.

Figure 22. MAV Filter Operation (patent pending)

7.3.4.7 Zone Detection

The Zone Detection unit is capable of screening all processed data from the MAVF and retaining only the data of interest (data that fit the prerequisite). This unit can be programmed to send an alert if a predefined condition set by two threshold value registers is met. Three different zones may be set:

1. Above the upper limit (X ≥ Threshold High)
2. Between the two thresholds (Threshold Low < X < Threshold High)
3. Below the lower limit (X ≤ Threshold Low)

The AUX and temperatures TEMP1 and TEMP2 have separate threshold value registers that can be enabled or disabled. This function is not available to the touch screen inputs. Once the preset condition is met, the DAV output to the PINTDAV pin is pulled low and the corresponding DAV bit is set.

7.4.1.1 I²C Fast or Standard Mode (F/S Mode)

In I²C Fast or Standard (F/S) mode, serial data transfer must meet the timing shown in the Timing Information section.

In the serial transfer format of F/S mode, the master signals the beginning of a transmission to a slave with a START condition (S), which is a high-to-low transition on the SDA input while SCL is high. When the master has finished communicating with the slave, the master issues a STOP condition (P), which is a low-to-high transition on SDA while SCL is high, as shown in Figure 23. The bus is free for another transmission after a stop condition has occurred. Figure 23 shows the complete F/S mode transfer on the I²C, two-wire serial interface. The address byte, control byte, and data byte are transmitted between the START and STOP conditions. The SDA state is only allowed to change while SCL is low, except for the START and STOP conditions. Data are transmitted in 8-bit words. Nine clock cycles are required to transfer the data into or out of the device (8-bit word plus acknowledge bit).

Figure 23. Complete Fast- or Standard-Mode Transfer

7.4.1.2 I²C High-Speed Mode (Hs Mode)

Serial data transfer format in High-Speed (Hs) mode meets the Fast or Standard (F/S) mode I²C bus specification. Hs mode can only commence after the following conditions (all of which are in F/S mode) exist:

1. START condition (S)
2. 8-bit master code (00001xxx)
3. not-acknowledge bit (N)

Figure 24 shows this sequence in more detail. Hs-mode master codes are reserved 8-bit codes used only for triggering Hs mode, and are not to be used for slave addressing or any other purpose. The master code indicates to other devices that an Hs-mode transfer is about to begin and the connected devices must meet the Hs mode specification. Because no device is allowed to acknowledge the master code, the master code is followed by a not-acknowledge bit (N).

After the not-acknowledge bit (N) and SCL have been pulled up to a HIGH level, the master switches to Hs-mode and enables (at time t_H; shown in Figure 24) the current-source pull-up circuit for SCL. Because other devices can delay the serial transfer before t_H by stretching the LOW period of SCL, the master enables its current-source pull-up circuit when all devices have released SCL, and SCL has reached a HIGH level, thus speeding up the last part of the rise time of the SCL.

The master then sends a repeated START condition (Sr) followed by a 7-bit slave address with a R/W bit address, and receives an acknowledge bit (A) from the selected slave. After a repeated START (Sr) condition and after each acknowledge bit (A) or not-acknowledge bit (N), the master disables its current-source pull-up circuit. This disabling enables other devices to delay the serial transfer by stretching the LOW period of SCL. The master re-enables its current-source pull-up circuit again when all devices have released, and SCL reaches a HIGH level, which speeds up the last part of the SCL signal rise time.

Data transfer continues in Hs mode after the next repeated START (Sr), and only switches back to F/S mode after a STOP condition (P). To reduce the overhead of the master code, it is possible that a master links a number of Hs mode transfers, separated by repeated START conditions (Sr).

Figure 24. Complete High-Speed Mode Transfer

7.4.2.1 Conversion Controlled by TSC2014 Initiated by TSC2014 (TSMode 1)

In TSMode 1, before a pen touch can be detected, the TSC2014 must be programmed with PSM = 1 and one of two scan modes:

1. X-Y-Z Scan (converter function select bits C[3:0] = Control Byte 1 D[6:3] = 0000); or
2. X-Y Scan (converter function select bits C[3:0] = Control Byte 1 D[6:3] = 0001).

See Table 9 for more information on the converter function select bits.

When the touch panel is touched, and the internal pen-touch signal activates, the PINTDAV output is lowered if it is programmed as PENIRQ. The TSC2014 then executes the preprogrammed scan function without a host intervention.

At the same time, the TSC2014 starts up its internal clock. It then turns on the Y-drivers, and after a programmed panel voltage stabilization time, powers up the A/D converter and converts the Y coordinate. If preprocessing is selected, several conversions may take place. When data preprocessing is complete, the Y coordinate result is stored in a temporary register.

If the screen is still touched at this time, the X-drivers are enabled, and the process repeats, but measures the X coordinate instead, and stores the result in a temporary register.

If only X and Y coordinates are to be measured, then the conversion process is complete. A set of X and Y coordinates are stored in the X and Y registers. Figure 25 shows a flowchart for this process. The time it takes to go through this process depends upon the selected resolution, internal conversion clock rate, panel voltage stabilization time, precharge and sense times, and whether preprocessing is selected. The time needed to get a complete X and Y coordinate (sample set) reading can be calculated by:

Equation 5.

Where:

t_COORDINATE = time to complete X/Y coordinate reading.

t_PVS = panel voltage stabilization time, as given in Table 15.

t_PRE = precharge time, as given in Table 16.

t_SNS = sense time, as given in Table 17.

N = number of measurements for MAV filter input, as given in Table 3 as N.

(For no MAV: M1-0[1:0] = '00', W1-0[1:0] = '00', N = 1.)

B = number of bits of resolution.

f_OSC = TSC onboard OSC clock frequency. See for supply frequency (V_DD).

f_ADC = A/D converter clock frequency, as given in Table 14.

OH1 = overhead time #1 = 2.5 internal clock cycles.

OH_DLY1 = total overhead time for t_PVS, t_PRE, and t_SNS = 10 internal clock cycles.

OH_CONV = total overhead time for A/D conversion = 3 internal clock cycles.

L_PPRO = preprocessor preprocessing time as given in Table 4.

Figure 25. Example of an X and Y Coordinate Touch Screen Scan Using TSMode 1

If the pressure of the touch is also to be measured, the process continues in the same way, but measuring the Z₁ and Z₂ values instead, and storing the results in temporary registers. Once the complete sample set of data (X, Y, Z₁, and Z₂) are available, they are loaded in the X, Y, Z₁, and Z₂ registers. This process is illustrated in Figure 26. As before, this process time depends upon the settings described above. The time for a complete X, Y, Z₁, and Z₂ coordinate reading is given by:

Equation 6.

Where:

OH2 = overhead time #2 = 3.5 internal clock cycles.

Figure 26. Example of an X, Y, and Z Coordinate Touch Screen Scan Using TSMode 1

7.4.2.2 Conversion Controlled by TSC2014 Initiated by Host (TSMode 2)

In TSMode 2, the TSC2014 detects when the touch panel is touched and causes the internal Pen-Touch signal to activate, which lowers the PINTDAV output if it is programmed as PENIRQ. The host recognizes the interrupt request, and then writes to the A/D Converter Control register to select one of the two touch screen scan functions:

1. X-Y-Z Scan (converter function select bits C[3:0] = Control Byte 1 D[6:3] = 0000); or
2. X-Y Scan (converter function select bits C[3:0] = Control Byte 1 D[6:3] = 0001).

See Table 9 for more information on the converter function select bits.

The conversion process then proceeds as shown in Figure 27; see the previous sections for more details.

The main difference between this mode and the previous mode is that the host, not the TSC2014, decides when the touch screen scan begins.

The time needed to convert both X and Y coordinates under host control (not including the time needed to send the command over the I²C bus) is given by:

Equation 7.

Figure 27. Example of an X and Y Coordinate Touch Screen Scan Using TSMode 2

7.4.2.3 Conversion Controlled by Host (TSMode 3)

In TSMode 3, the TSC2014 detects when the touch panel is touched and causes the internal Pen-Touch signal to be active, which lowers the PINTDAV output if it is programmed as PENIRQ. The host recognizes the interrupt request. Instead of starting a sequence in the TSC2014, which then reads each coordinate in turn, the host must now control all aspects of the conversion. Generally, upon receiving the interrupt request, the host turns on the X drivers.

After waiting for the settling time, the host then addresses the TSC2014 again, this time requesting an X coordinate conversion.

The process is then repeated for the Y and Z coordinates. The processes are outlined in Figure 28 and Table 5. Figure 28 shows two consecutive scans on X and Y. Figure 29 shows a single Z scan.

The time needed to convert any single coordinate X or Y under host control (not including the time needed to send the command over the I²C bus) is given by:

Equation 8.

Where:

OH_DLY2 = total overhead time for t_PRE and t_SNS = 6 internal clock cycles.

Figure 28. Example of X and Y Coordinate Touch Screen Scan Using TSMode 3

The time needed to convert any Z₁ and Z₂ coordinate under host control (not including the time needed to send the command over the I²C bus) is given by:

Equation 9.

Figure 29. Example of Z₁ and Z₂ Coordinate Touch Screen Scan
(without Panel Stabilization Time) Using TSMode 3

If the drivers are not turned on before the touch screen scan mode is programmed, the panel stabilization time should be included. In this case, the time needed to convert any single X or Y under host control (not including the time needed to send the command over the I²C bus) is given by:

Equation 10.

Figure 30. Example of a Z₁ and Z₂ Coordinate Touch Screen Scan
(with Panel Stabilization Time) Using TSMode 3

The time needed to convert any single coordinate (either X or Y) under host control (not including the time needed to send the command over the I²C bus) is given by:

Equation 11.

Figure 31. Example of a Single X Coordinate Touch Screen Scan
(with Panel Stabilization Time) using TSMode 3

7.5.1.1 Address Byte

The TSC2014 has a 7-bit slave address word. The first six bits (MSBs) of the slave address are factory-preset to comply with the I²C standard for A/D converters and are always set at '100100'. The logic state of the address input pin determines the LSB of the device address to activate communication. Therefore, a maximum of two devices with the same preset code can be connected on the same bus at one time.

The AD0 address input is only read during a power-up of the device, and should be connected to supply (VDD/REF) or ground (GND). The slave address is latched into the TSC2014 on the falling edge of SCL after the read/write bit has been received by the slave.

The last bit of the address byte (R/W) defines the operation to be performed. When set to a '1', a read operation is selected; when set to a ‘0’, a write operation is selected. Following the START condition, the TSC2014 monitors the SDA bus, checking the device type identifier being transmitted. Upon receiving the '10010' code, the appropriate device select bits, and the R/W bit, the slave device outputs an acknowledge signal on the SDA line.

7.5.1.1.1 Bit D0: R/W

1: I²C master read from TSC (I²C read addressing).

0: I²C master write to TSC (I²C write addressing).

Figure 32. I²C Bus Addressing (Slave Address Byte Format)

Table 6. Control Byte 0 Bit Register Description (D7 = 0)

Table 7. Internal Register Map

Table 8. Control Byte 1 Bit Register Description (D7 = 1)

Table 9. Converter Function Select

7.6.3.1 Touch Screen Scan Function for XYZ or XY

7.6.3.2 Touch Screen Sensor Connection Tests for X-Axis and Y-Axis

Range of resistances of different touch screen panels can be selected by setting the TBM bits in CFR1; see Table 18. Once the resistance of the sensor panel is selected, two continuity tests are run separately for the X-axis and Y-axis. The unit under test must pass both connection tests to ensure that a proper connection is secured.

7.6.3.3 Touch Sensor Short-Circuit Test

If the TBM bits of CFR1 detailed in Table 18 are all set to '1', a short-circuit in the touch sensor can be detected.

7.6.4.1 Configuration Register 0

7.6.4.2 Configuration Register 1

Configuration register 1 (CFR1) defines the connection test-bit modes configuration and batch delay selection.

7.6.5.1 PINTS1 (default 0)

This bit controls the output format of the PINTDAV pin. When this bit is set to '0', the output format is shown as the AND-form of internal signals of PENIRQ and DAV). When this bit is set to '1', PINTDAV outputs PENIRQ only.

7.6.5.2 PINTS0 (default 0)

This bit selects what is output on the PINTDAV pin. If this bit set to '0', the output format of PINTDAV depends on the selection made on the PINTS1 bit. If this bit set to '1', the internal signal of DAV is output on PINTDAV.

7.6.5.3 M1, M0, W1, W0 (default 0000)

Preprocessing MAV filter control. Note that when the MAV filter is processing data, the STS bit and the corresponding DAV bits in the status register indicate that the converter is busy until all conversions necessary for the preprocessing are complete. The default state for these bits is 0000, which bypasses the preprocessor. These bits are the same whether reading or writing.

7.6.5.4 TZ1 and TZ0, or AZ1 and AZ0 (default 00)

Zone detection bit definition (for TEMP or AUX measurements). TZ1 and TZ0 are for the TEMP measurement. AZ1 and AZ0 are for the AUX measurement. The action taken in zone detection is to store the processed data in the corresponding data registers and to update the corresponding DAV bits in status register. If the processed data do not meet the selected criteria, these data are ignored and the corresponding DAV bits are not updated. When zone detection is disabled, the processed data are simply stored in the corresponding data registers and the corresponding DAV bits are updated without any comparison of criteria. Note that the converted samples are always processed according to the setting of the MAVE bits for AUX/TEMP before zone detection takes effect. See Table 24 for thresholds.

7.6.5.5 MAVE (default is 00000)

MAV filter function enable bit. When the corresponding bit is set to '1', the MAV filter setup is applied to the corresponding measurement.

7.6.6.1 CFN15-CFN13

Touch screen drivers status. These bits represent the current status of the touch screen drivers that are turned on. CFN13 is set to '1' if both X+ and X- drivers are turned on. CFN14 is set to '1' if both Y+ and Y- drivers are turned on. CFN15 is set to '1' if Y+ and X- drivers are turned on. Otherwise, these bits are set to '0'. These bits are reset to 0h whenever the converter function is either complete, stopped by the STS bit, or reset (by a hardware reset from the RESET pin or a software reset from SWRST bit in Control Byte 1).

7.6.6.2 CFN12-CFN0

Converter function select status. These bits represent the converter function currently running, which is set in bits C3-C0 of Control Byte 1. When the CFNx bit shows '1', where x is the decimal value of converter function select bits C3-C0, it indicates that the converter function that is set in bits C3-C0 is running. For example, when CFN2 shows '1', it indicates the converter function set in bits C3-C0 ('0010') is running. The CFNx bits are reset to 0000h whenever the converter function is complete, stopped by STS bit, or reset (by the hardware reset from the RESET pin or the software reset from SWRST bit in Control Byte 1). However, if the TSC-initiated scan function mode is issued (by setting the PSM bit in the CFR0 register to '1'), the CFN0 or CFN1 bit will not be reset when the corresponding converter function is complete because there is no pen touch. This event allows the TSC2014 to immediately initiate the scan process (corresponding to CFN0 or CFN1 set to '1') when the next pen touch is detected.

7.6.6.3 DAV Bits

Data available bits. These seven bits mirror the operation of the internal signals of DAV. When any processed data are stored in data registers, the corresponding DAV bit is set to '1'. It stays at '1' until the register(s) updated to the processed data have been read out by the host.

7.6.6.4 RESET Flag

See Table 23 for the interpretation of the RESET flag bits.

7.6.6.5 X CON

This bit is '1' if the X axis of the touch screen panel is properly connected to the X drivers. This bit is the connection test result.

7.6.6.6 Y CON

This bit is '1' if the Y axis of the touch screen panel is properly connected to the Y drivers. This bit is the connection test result.

7.6.6.7 Y SHR

This bit is '1' if there is no short-circuit tested at the Y axis of the touch screen panel. This bit is the short-circuit test result.

7.6.6.8 PDST

Power down status. This bit reflects the setting of the PND0 bit in Control Byte 0. When this bit shows '0', it indicates A/D converter bias circuitry is still powered on after each conversion and before the next sampling; otherwise, it indicates A/D converter bias circuitry is powered down after each conversion and before the next sampling. However, it is powered down between conversion sets. Because this status bit is synchronized with the internal clock, it does not reflect the setting of the PND0 bit until a pen touch is detected or a converter function is running.

7.6.6.9 ID[1:0]

Device ID bits: These bits represent the version ID of TSC2014. This version defaults to '00'.

7.6.7.1 X, Y, Z1, Z2, AUX, TEMP1 and TEMP2 Registers

The results of all A/D conversions are placed in the appropriate data registers, as described in Table 7. The data format of the result word (R) of these registers is right-justified, as shown in Figure 44.

The data registers of the TSC2014 hold data results from conversions. All of these registers default to 0000h upon reset.

The TSC2014 has several 16-bit registers that allow control of the device, as well as providing a location to store results from the TSC2014 until read out by the host microprocessor. Table 24 shows the memory map.