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Texas Instruments Operation Schemes of Touch Screen Controllers Application notes
Application Report
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Operation Schemes of Touch Screen Controllers
Wendy Fang .................................................................................................. PAM - DAP Nyquist ADC
ABSTRACT
This application report describes the two operation schemes of touch-screen controller
(TSC) devices, command-based and register-based, to help designers to select and
use the TSC devices best for their systems or applications. Under the different
schemes, the typical TSC operation steps are listed, the analog and digital interfaces
are probed, and the system performances, including power consumption, are analyzed.
1
2
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5
6
7
8
9
Contents
Introduction .......................................................................................... 2
TSC System ......................................................................................... 3
Command-Based Scheme ........................................................................ 4
Advanced Command-Based Scheme............................................................ 8
Register-Based Scheme ......................................................................... 11
Register-Based Scheme With Batch-Delay.................................................... 13
Conclusion ......................................................................................... 15
References ......................................................................................... 15
Glossary ............................................................................................ 16
List of Figures
1
2
3
4
5
6
7
8
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10
11
12
Block Diagram of a 4-Wire Touch Screen System Using TI's TSC Device ................. 3
TSC2003 Analog Interface (Acquiring X, Y, Z1, Z2) ............................................ 5
TSC2003 Digital Interface (Host reads X Data, then Y, etc.) ................................. 5
Operating Sequence of Normal Command-Based TSC (for 2 touch data, X and Y) ...... 6
TSC2007 Analog Interface (Acquiring X, Y, Z1 and Z2) ....................................... 9
TSC2007 Digital Interface (Host reads X Data, then Y, etc.) ................................. 9
Operating Sequence of Advanced Command-Based TSC with Built-In 7-Data-Set
MAVF ............................................................................................... 10
TSC2004 Analog Interface Without Batch-Delay ............................................. 12
TSC2004 Digital Interface (X/Y/Z1/… Read)................................................... 12
Operating Sequence of Normal Register-Based TSC ........................................ 13
TSC2004 Analog Interface With Batch Delay ................................................. 14
Operating Sequence of Register-Based TSC With Batch Delay ........................... 15
List of Tables
1
2
3
4
5
6
7
TI Resistive Touch Screen Controllers Main Features ........................................ 3
Operating Sequence of Command-Based TSC Devices ...................................... 4
Maximum SSPS Rate of the TSC2003 I2C Digital Interface .................................. 7
Operating Sequence of TSC2007-Based Touch System ..................................... 8
Maximum SSPS Rate for the TSC2007 I2C Interface ........................................ 10
Operating Sequence of Register-Based TSC Devices ....................................... 11
Operating Sequence of Register-Based TSC with Batch Delay ............................ 14
I2C is a trademark of Koninklijke Philips Electronics N.V..
SPI is a trademark of Motorola, Inc..
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Introduction
1
Introduction
There are various resistive touch screen controller (TSC) devices on today's market. As a leading
provider, TI manufactures a family of high performance TSC devices, from the benchmark ADS7843,
ADS7845 and ADS7846, to the highly-popular TSC2003 and TSC2046, and further to the newly released
ultra low-power TSC2004, TSC2005 and TSC2007.
You may ask, "Which of these TSC devices best suits my application?" This application report answers
this question from a specific view: the operation schemes of the TSC devices.
Classified by operating scheme, there are two main types of resistive TSC devices: one uses
command-based technology, such as TI's ADS7846, TSC2003, or TSC2007; and another uses built-in
digital registers with fully software-programmable features and functions, and thus are called
register-based TSC devices, such as TI's TSC2004, or TSC2005.
The TSC's operation scheme greatly effects analog and digital interface design, touch-screen system
performance, system power consumption, and development costs.
Considering system performance, we can further classify TSC operation schemes into:
-- Normal Command-Based
{
Command-Based
Register-Based
{
{
(such as TSC2004/5/6)
(such as TSC2003, TSC2046)
-- Advanced Command-Based
(such as TSC2007)
-- Regsister-Based without Batch-Delay
-- Regsister-Based with Batch-Delay
This application report describes typical TSC-system operation under the different schemes, explores
system analog and digital interfaces, and analyzes system performance, including power consumption.
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TSC System
2
TSC System
As shown in Figure 1, a TSC device is used between a resistive touch screen (panel) and a host
processor. In a system where the touch panel is the analog sensor, the processor typically handles
multiple tasks/functions in an application such as a PDA.
There are two interfaces in a TSC system:
• The analog interface, the connection between the TSC and the touch panel
• The digital interface, the bridge between the TSC and the host processor.
Analog Circuits
ADS7846,
TSC2003,
TSC2046,
TSC2004,
TSC2005,
TSC2007, or
…...
SPI or I2C
bus
Interrupt
Processor
{
{
Resistive
Touch
Screen
X+, X-, Y+, Y-
Digital Circuits
Digital Interface
Analog Interface
Figure 1. Block Diagram of a 4-Wire Touch Screen System Using TI's TSC Device
There are 4- (or 8-) wire and 5- (or 7-) wire resistive touch panels on the market. The most popular type is
the 4-wire, because it is the simplest, with lowest cost of hardware and software development.
On the digital interface side, a TSC is connected to a host processor through either a SPI™ or an I2C™
bus. Since it is the control center of an application/product, the processor usually connects with various
peripheral devices and performs multiple functions. As one of the processor's many peripheral devices, a
TSC is a digital-bus slave, communicating with the processor only when the TSC detects pressure on the
touch panel.
To select a resistive TSC device for your application, the first consideration is the TSC's interfaces. For
example, the analog interface may be connected to a panel with a 4- or 5-wire interface. The digital
interface can be SPI or I2C. The next most important consideration is the operation scheme of the TSC.
Table 1 provides a list for TI's TSC devices on the analog and digital interface types and operation
schemes.
Table 1. TI Resistive Touch Screen Controllers Main Features
TSC DEVICE
ANALOG
INTERFACE TYPE
DIGITAL INTERFACE
TYPE
ADS7843
4-wire
SPI
Normal Command-Based
ADS7845
5-wire
SPI
Normal Command-Based
ADS7846
4-wire
SPI
Normal Command-Based
TSC2003
4-wire
I2C
Normal Command-Based
TSC2046
4-wire
SPI
Normal Command-Based
TSC2004
4-wire
I2C
Register-Based with Batch-Delay
TSC2005
4-wire
SPI
Register-Based with Batch-Delay
TSC2006
4-wire
SPI
Register-Based with Batch-Delay
4-wire
I2C
Advanced Command-Based
TSC2007
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OPERATION SCHEME
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Command-Based Scheme
3
Command-Based Scheme
The command-based TSC device operates completely under the control or command of the host
processor. Usually, the command-based TSC device requests the full attention of the host during touch
pressure-ON. The TSC2003 (I2C digital bus) and the TSC2046 (SPI digital bus) are typical
command-based TSC devices.
3.1
Typical Operation
Under the command-based scheme, the typical operation sequence for the host to read the touch data is
listed in Table 2.
Table 2. Operating Sequence of Command-Based TSC Devices
Step
Condition
1
No pressure on the touch
panel
The TSC powers DOWN; no traffic is on either analog or digital interface; and host is
performing other tasks or idle.
Operation
2
Pressure on the panel
The TSC detects pressure on touch panel, and sends the host an interrupt (usually
denoted by PENIRQ).
3
The Host receives PENIRQ, disables/masks-out the interrupt, and starts the
corresponding interrupt service routine (ISR) .
4
The ISR sends a command to the TSC over the SPI or I2C bus, and requests the TSC
to read one of the touch coordinates, X for this example.
5
Upon receiving the command, the TSC turns ON its X driver to power up the touch
panel's X layer.
6
The TSC acquires X signal; converts the analog signal to digital data, and sends the
data bit-by-bit to the host, over the bus.
7
The Host receives an X data unit. If more than one data unit is needed (for
averaging/filtering purpose), Steps 4 - 6 are repeated multiple times. Or …
8
The ISR sends a command to the TSC by the SPI or I2C bus, and requests the TSC to
read another coordinate, Y.
9
The TSC receives the command, turns OFF the X and turns ON theY driver to power
up the touch panel's Y layer.
10
The TSC acquires the Y signal; converts the analog signal to digital data; and sends
the data bit-by-bit to the host over the bus.
11
The Host receives a Y data unit. If more than one data is needed for averaging/filtering
purpose, the Steps 8 - 10 will be repeated multiple times.
12
Upon receiving the complete set of touch data, the host enables the PENIRQ interrupt
and checks again if touch pressure is still on. If yes, it returns to Step 3 for the next set
of touch data.
13
Pressure removed from
the panel
Return to Step 1.
During the period of time when the touch system implements the above Steps 2 to 12 in Table 2, both
analog interface and digital interface are heavily involved, shown by Figure 2 and Figure 3. Figure 2
shows the analog interface timing, and Figure 3 is the digital (I2C) bus activity between TSC and host,
measured with a TSC2003EVM.
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Command-Based Scheme
In Figure 2), the intervals between samples are determined by the host's bandwidth; the faster the host's
speed is, the shorter the interval becomes. The TSC driver ON period is determined by the digital interface
clock rate; the faster the clock is, the shorter the ON time becomes.
Figure 2. TSC2003 Analog Interface (Acquiring X, Y, Z1, Z2)
Figure 3. TSC2003 Digital Interface (Host reads X Data, then Y, etc.)
Figure 3 shows a complete I2C read cycle for touch coordinate X. This takes 45 SCL clocks, plus one
START, one REPEATED START (or one STOP and one START), and one STOP. A complete I2C read
for a 12-bit touch data unit needs at least 48 SCL clocks, if using a REPEATED START between the
command write and data read, or 49 SCL clocks if using STOP and START between.
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Command-Based Scheme
Figure 4 displays the operation flow of a normal command-based touch system (with the TSC2003 for
example). The processing steps and interface activities have been detailed in Table 2 and Figure 2.
Host
Step 3 Step 4
Digital Interface
Figure 3
Bus No Activity
Step 11 Step 12 …
Step 7 Step 8
Figure 3
…
…
PENIRQ High
TSC
Step 2
Step 5
Step 6
Step 9 Step 10
…
Analog Interface
Figure 2 (Sample X)
No Activity
Figure 2 (Sample Y)
…
Touch Panel
Touch On
…
Step 1
Figure 4. Operating Sequence of Normal Command-Based TSC (for 2 touch data, X and Y)
Through the Operating Sequence shown in Figure 4, the host gets only 2 touch data units, an X and a Y
data unit.
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Command-Based Scheme
3.2
Digital Bus Limitations
To read the touch coordinates X Y, Z1, and Z2, a complete touch data set, the sequence in Figure 3 must
be repeated 4 times. Moreover, if the host needs multiple data units for each coordinate (to average or
filter out noise in data), the sequence must be repeated multiple times for each.
A command-based touch system has an obvious limitation on the number of sample sets per second
(SSPS) that can be acquired through the digital bus, due to the bus-traffic capacity. The bus limitation may
become a problem especially for I2C TSC devices. TI's TSC2003 device is used as an example for this
discussion.
As previously discussed, a complete I2C read for a 12-bit touch coordinate needs at least 48 SCL clocks.
(See Figure 3). Thus, the maximum SSPS rate of the TSC2003 is lower than:
f SCL
48
N
(1)
2
Where fSCL is the I C bus speed or SCL clock rate, and N is the number of data units in a set of
samples.
For example, N = 2 if a set of samples includes an X and a Y data; N = 4 if a set of samples includes an
X, a Y, a Z1 and a Z2; …… ; and N=28 if a set of samples has 7 [X, Y, Z1, and Z2], or 7×4 data units.
Table 3 shows the TSC2003 I2C bus capacity for transmitting at the maximum SSPS rate at three I2C
speed modes (using Equation 1).
Table 3. Maximum SSPS Rate of the TSC2003 I2C Digital Interface
SET OF SAMPLES
X AND Y
(N = 2)
X, Y AND Z
(N = 4)
3x (X, Y AND Z)
(N = 12)
7x (X, Y AND Z)
(N = 28)
Standard mode (SCL=100 kHz)
1,042
521
174
74
Fast mode (SCL=400 kHz)
4,167
2,083
694
298
High Speed mode (SCL=3.4 MHz)
35,417
17,708
5,903
2,530
When using a touch screen as human interface, one usually expects around 100 to 500 valid sets of
samples per second, in real world applications. Thus, the command-based touch system may not be able
to provide sufficient sets of valid touch data with a standard I2C digital interface.
For example, in Table 3, it can be seen that the host can read a maximum of 521 sets of complete touch
coordinates (X, Y, Z1 and Z2) with its standard I2C interface, adequate for many applications. However, if
there is noise in the data, which happens normally in practice, you need valid data, typically obtained by
using 7 (for example) raw data units to average or filter each coordinate. Under these conditions the host
can read less than 74 data sets per second.
The SSPS rates in Table 3 are calculated with the assumption that:
1. The host has no delay for responding to the PENIRQ interrupt from the TSC.
2. The I2C bus is kept running without delay between two consecutive data sets.
Therefore, the practical SSPS rate can be expected to be lower than that shown in Table 3, because both
assumptions are practically impossible.
For a SPI TSC device, even though the digital bus speed may not become a problem to limit the touch
data's SSPS rate, but heavy bus traffic may still cause problems on the host, and/or consume more
power.
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Advanced Command-Based Scheme
3.3
Other Limitations
The host processor's bandwidth and resources may need to be concerned when using a normal
command-based (such as TSC2003) touch system, because:
• With a command-based touch system, the analog interface interval (for example, the interval between
sample X and Y in Figure 2) is determined by the host response speed and bandwidth.
• When there is noise in the touch system and the host processor must perform averaging or filtering
with multiple touch data units, the number of multiple samples is also limited by the host bandwidth and
resources (computational time and space).
The analog driver power-ON period in a command-based touch system may be also related to the
digital-bus clock rate (refer to the data sheet of TSC2046 for example). Slower digital-bus clocks typically
result in longer driver-ON time; longer driver-ON time obviously increases analog power consumption.
Under the high speed (HS) I2C mode, the bus has the capability to transmit much more data and thus the
digital bus traffic may not be a problem. The maximum data rate may be limited by the TSC ADC's sample
rate, which is another topic that is beyond the scope of this application note.
4
Advanced Command-Based Scheme
A new addition to TI's TSC family is the TSC2007. Even though this is a command-based device, it has a
built-in 7-data-unit median value and averaging filter (MAVF). The MAVF not only eliminate the need for
the host software routine to reduce touch-data noise, but also greatly reduces I2C bus traffic. (See item 9
in the References section.)
4.1
Typical Operation
With the advanced command-based touch system, such as using TSC2007, the operation sequence for
the host to read a set of touch coordinates is simplified, as shown in Table 4. Where the TSC2007 MAVF
is enabled to reduce noise and get the refined sets of touch data.
Table 4. Operating Sequence of TSC2007-Based Touch System
Step
Condition
1
No pressure on the touch
panel
TSC powers DOWN; no traffic is on either the analog or the digital interface; host is
performing other tasks or is idle.
2
Pressure on the panel
TSC detects pressure on touch panel, and sends the host an interrupt, PENIRQ.
3
Host receives PENIRQ, disables/Masks-out the interrupt, and starts the corresponding
interrupt service routine (ISR) .
4
ISR sends a command to TSC by I2C bus, and requests TSC to read one of the touch
coordinates X.
5
TSC receives the commands, turns ON the X driver, and powers up the touch panel's
X layer.
6
TSC acquires X data, 7 times; converts the 7 data, filters them (deletes 2 largest and 2
smallest, averages the middle 3 values) to get one low-noise data, and sends the data
bit-by-bit to host, through the bus.
7
Host receives the refined X data. (one, not 7 data units). The host repeats Steps 2 - 6
to get Y, Z1 and Z2 as needed.
8
Host enables PENIRQ interrupt and check again if touch is still on. If yes, go back to
Step 3 for the next set of touch data.
9
8
Operation
Pressure removed from
the panel
Otherwise, return to Step 1.
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Advanced Command-Based Scheme
During the period of time when the touch system implements the above Steps 2 to 8 in Table 4, the
analog interface traffic (Figure 5) seems similar to that of the normal command-based system (Figure 2)
but, in fact, TSC2007 samples each touch coordinate 7 times each time when the driver is powered ON.
Figure 5. TSC2007 Analog Interface (Acquiring X, Y, Z1 and Z2)
Figure 6. TSC2007 Digital Interface (Host reads X Data, then Y, etc.)
Thus, the touch-screen driving-power ON/OFF frequency is reduced, because for sampling multiple data,
it is powered ON only once no matter how many data units must be sampled for each touch coordinate.
The analog traffic is reduced, and the driver-ON time as whole gets shorter.
Shorter driver-ON time saves significant power. For example, consider applying 3.0-V touch driver power
on a 400-Ω touch panel; at the instant of driver-ON, current is (3.0/400 =) 7.5 mA. Using the TSC2003
(Figure 2) as an example, the driver-ON time for a sample is about 100 µS. To get 100 SSPS of low-noise
data, you need to sample 4 (X, Y, Z1, Z2) × 7 (7 samples per coordinate) × 100 = 2800 times per second.
The average TSC2003 driver-ON current is then:
7.5 mA × 100 µS × 2800 = 2.1 mA
Considering another device, the TSC2007 (Figure 5) driver-ON time for reading a coordinate (7 samples)
is about 150µS. To get 100 SSPS of low-noise data, you need to sample 4 (X, Y, Z1, Z2) × 100 = 400
times per second. The average TSC2007 driver-ON current is then:
7.5 mA × 150 µS × 400 = 450 µA
This represents a saving of 80% in analog power.
At the digital-interface side, the data traffic (Figure 6) is also notably decreased by 6/7 or 86%, because
instead of multiple data sets, only one refined data set must be transmitted through the digital bus.
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Advanced Command-Based Scheme
Figure 7 displays the Operating Sequence of an advanced command-based touch system using a
TSC2007. The Operating Sequence and timing are the same as those shown in Table 4, Figure 5 and
Figure 6.
Host
Step 7
Step 3 Step 4
Digital Interface
Figure 6 (read a refined X)
Bus No Activity
…
…
…
PENIRQ High
TSC
Step 2
Analog Interface
Step 5
Step 6
Figure 5 (Sample 7 X)
No Activity
…
…
…
Touch On
Step 1
Figure 7. Operating Sequence of Advanced Command-Based TSC
with Built-In 7-Data-Set MAVF
Through the Operating Sequence shown in Figure 7, the TSC samples 7 data sets, processes them via
the MAVF, then sends the host a low-noise X-data sample. Compare this scenario with the normal
command-based scheme (Figure 4) where the TSC samples only one data set during each touch
driver-ON period.
4.2
Digital Bus Capacity
Table 5 provides the I2C bus capability for host/TSC communication, under different I2C modes and with
the same functions shown by Table 3.
Table 5. Maximum SSPS Rate for the TSC2007 I2C Interface
SET OF SAMPLES
X, and Y (7x2 Data)
X, Y and Z (7x4 Data)
Standard mode (SCL=100 kHz)
1,042
521
Fast mode (SCL=400 kHz)
4,167
2,083
High Speed mode (SCL=3.4 MHz)
35,417
17,708
Even though it is the same I2C interface, the TSC2007 sends only one data set through the I2C bus due to
the built-in MAVF filter that pre-processes 7 samples of each coordinate. Thus, the TSC2007 can handle
521 sets of low-noise touch data through the standard I2C bus, providing sufficient sets of data for human
interface applications. Another major advantage is that the host does not need to run the data
averaging/filtering routine, freeing resources and processing power for other tasks.
Unlike older command-based TSC devices such as the TSC2003, in systems based on advanced
command-based TSC devices such as the TSC2007, the I2C digital bus is not the SSPS bottleneck.
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Register-Based Scheme
5
Register-Based Scheme
The register-based TSC device has built-in computational data and configuration registers, usually with
pre-processing capabilities, and can operate either in host-controlled mode or TSC self-controlled mode.
The register-based TSC device is often more flexible in applications. the TSC2004 (I2C digital interface)
and TSC2005 (SPI digital interface) are typical register-based TSC devices.
5.1
Typical Operation
Table 6 shows the operating sequence of typical register-based TSC devices in TSC self-controlled mode.
Table 6. Operating Sequence of Register-Based TSC Devices
Step
Condition
Operation Sequence
1
System is powered up
The host initializes the TSC to set up touch mode and various timings based on application
and hardware circuit (more options than command-based scheme).
2
No pressure on the
touch panel
The TSC powers DOWN, no traffic is on either analog or digital interface, and the host is
performing other tasks or is idle.
3
Pressure on the panel
The TSC detects pressure on the touch panel, powers ON the Y driver, acquires the Y
signal, converts analog to digital, taking and processing multiple data if programmed to do
so, and saving the final data to the corresponding register. (The above tasks are
implemented by the TSC according to programmed timings.)
4
The TSC repeats Step 3 for X, Z1 and Z2 coordinates as needed.
5
The TSC sends a data-ready or DAV interrupt to host, and checks to see if the pressure is
still on. If yes, back to Step 3 for the next set of touch data.
6
The Host receives the DAV interrupt, disables/Masks-out the interrupt, and starts the
corresponding interrupt service routine (ISR). The ISR reads back the refined data set from
the TSC data registers.
7
Pressure removed from
the panel
Return to Step 2.
The register-based TSC device can greatly reduce the traffic to the host, since the TSC performs all of the
tasks automatically, needing the host's attention only at the end of its processing. Thus, the register-based
TSC reduces digital bus traffic, reduces digital power consumption, and also reduces the host's overhead.
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Register-Based Scheme
Figure 8. TSC2004 Analog Interface Without Batch-Delay
Figure 9. TSC2004 Digital Interface (X/Y/Z1/… Read)
During the period of time when the touch system implements Steps 3-6 in Table 6 and at normal (without
batch-delay) mode, TSC2004's analog interface traffic is shown in Figure 8, and digital bus is shown in
Figure 9.
Similar to the analog interface of the advanced command-based scheme (Figure 5), the register-based
TSC samples multiple touch data sets and automatically performs averaging/filtering on each touch
coordinate according to the programmed parameters. Moreover, register-based TSC devices usually
provide more choices or options in the filtering feature.
The interval between two coordinates, such as between Y and X, can be set by programming the TSC
touch timings, including panel voltage stabilization (or PVS) time, pre-charge (or PR) time, and sense (or
SN) time.
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Register-Based Scheme With Batch-Delay
Figure 10 displays the Operating Sequence of a normal register-based touch system (such as TSC2004
or TSC2005). The operating sequence interface activity are shown in Table 6, Figure 8, and Figure 9.
Host
Step 6
Step 1
Digital Interface
Step 6
Figure 9
Figure 9
Bus No Activity
…
…
DAV High
TSC
Step 3/4(XYZ) 5
Analog Interface
No Activity
…
Step 3/4(XYZ) 5
Step 3/4(XYZ) 5
Driving Y/X/Z1/Z2
(refer to Figure 8
…
…
Touch Panel
…
Touch On
Step 2
Figure 10. Operating Sequence of Normal Register-Based TSC
In Figure 10 within steps 3 and 4, N (= 4×W, with W being the filter's window width) data units are
acquired / sampled / processed, while only 4 refined data units (X, Y, Z1 and Z2) are sent to the host in
Step 6.
6
Register-Based Scheme With Batch-Delay
While pressure remains on the touch panel, the normal register-based TSC devices above respond to the
touch continuously, and both analog and digital buses become very busy. Many sets of refined touch data
can be generated from the system, greatly exceeding real-world application requirements. The
disadvantage is that much more power is unnecessarily consumed to generate extra sets of data that can
never be used.
There are three ways to slow down the register-based touch system and avoid the extra data and extra
power consumption:
1. Use a host-processor timer to control the interval between sampling two sets of touch data, with the
TSC in host-controlled mode. This is similar to the command-based scheme, with more host
computational overhead.
2. Initialize the TSC with longer touch timings to add delays between coordinates, (See References, items
6 - 8 for more details).
3. Utilize the batch delay feature of the TSC, if available.
Batch delay is a feature that allows the TSC2004/5/6 to control the time interval between two sets of touch
screen data. With batch-delay, the TSC drives and samples the touch data at fixed intervals (the
batch-delay) while touch pressure is on the panel.
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Register-Based Scheme With Batch-Delay
6.1
Typical Operation
Table 7 shows the operating sequence of a typical register-based TSC with batch delay.
Table 7. Operating Sequence of Register-Based TSC with Batch Delay
Step
Condition
Operation Sequence
1
System is powered up
The Host initializes the TSC to set up the TSC self-controlled and batch modes, and also
various modes and timings based on the application and hardware.
2
No pressure on the touch
panel
The TSC powers DOWN, no traffic is on either analog or digital interface, and the host is
performing other tasks or is idle.
3
Pressure on the panel
The TSC detects pressure on the touch panel, powers ON the Y driver, acquires the Y signal,
converts analog to digital, taking and processing multiple data if programmed to do so, and
saving the final data to the corresponding register. (The above tasks are implemented by the
TSC according to programmed timings.)
4
The TSC repeats Step 3 for X, Z1 and Z2 coordinates as needed.
5
The TSC sends a data-ready or DAV interrupt to host and waits until the batch delay time is
reached, then goes back to Step 3 for the next set of touch data.
6
The host receives the DAV interrupt, disables/Masks-out the interrupt, and starts the
corresponding interrupt service routine (ISR) .The ISR reads back the refined data set from
the TSC data registers.
7
Pressure removed from
the panel
Return to Step 2.
With the batch-delay, the TSC enters the batch-wait mode after the first processed sample set is
completed and until the end of the batch delay time, even though the pen touch is still detected during the
whole time. Therefore, the unneeded panel driver-ON is eliminated, the analog/digital interface traffic is
reduced, and power is saved.
Comparing the analog interface (Figure 11) with batch delay to that without batch delay (Figure 8), you
can see that the TSC without batch delay keeps driving, sampling, and converting Y, X, Z1 and Z2 data at
a rate of 1/(274µS) = 3650 sets per seconds (Figure 8) but the set sampling interval is controllable and the
analog interface traffic is greatly reduced with batch delay Figure 11.
Figure 11. TSC2004 Analog Interface With Batch Delay
TSC2004 and TSC2005/6 batch-delay can be programmed to 1 ms, 2 ms, 4 ms, 10 ms, 20 ms, 40 ms or
100 ms, corresponding to 1000, 500, 250, 100, 50, 25 or 10 SSPS (Sample Sets Per Second). See
References, items 6–8 for more information.
During the delay time in batch-wait mode, the touch-panel driver is powered OFF, the TSC's ADC is
powered OFF, and no traffic is on the analog bus. The TSC can have very good performance, and with far
fewer interrupts to the host, and with very low power consumption.
14
Operation Schemes of Touch Screen Controllers
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Conclusion
Figure 12 shows the Operating Sequence of the register-based TSC with batch delay. The steps are
described in Table 7. The analog interface activity is shown in Figure 11 and the digital interface is the
same as that in Figure 9 (TSC2004 example).
Host
…
Step 6
Step 1
Digital Interface
Figure 9
Bus No Activity
DAV High
TSC
Step 3/4(XYZ) 5
Analog Interface
No Activity
Touch Panel
…
…
…
…
…
Step 3/4(XYZ)
5
…
Figure 11
…
…
…
Touch On
Step 2
Figure 12. Operating Sequence of Register-Based TSC With Batch Delay
7
Conclusion
This application report discusses the two operation schemes of touch screen controller (TSC) devices,
command-based and register-based. The TSC's operation scheme greatly affects the analog and digital
interface activities, the touch screen system performance, the system power consumption, and the
application's hardware and software development costs.
Further, the command-based TSC scheme is further categorized to show the advantages of the advanced
command-based TSC scheme and its improvement on reducing both analog and digital interface traffic,
reducing data noise, and reducing host processing overhead.
The register-based TSC with batch delay, as a new feature, presents the combined advantages of the
advanced command-based and the normal register-based TSC schemes, and can be configured optimally
for various applications.
8
References
1.
2.
3.
4.
5.
6.
ADS7843 – Touch Screen Controller (SBAS090)
ADS7845 – Touch Screen Controller (SBAS104)
ADS7846 – Touch Screen Controller (SBAS125)
TSC2003 – I2C Touch Screen Controller (SBAS162)
TSC2046 – Low Voltage IO Touch Screen Controller (SBAS265)
TSC2004 – 1.2V to 3.6V, 12-Bit, Nanopower, 4-wire Touch Screen Controller with I2C Interface
(SBAS408)
7. TSC2005 – 1.6V to 3.6V, 12-Bit, Nanopower, 4-wire Touch Screen Controller with SPI Interface
(SBAS379)
8. TSC2006 – Nanopower, 4-wire Touch Screen Controller with SPI™ Interface (SBAS415)
9. TSC2007 – 1.2V to 3.6V, 12-Bit, Nanopower, 4-Wire Micro Touch Screen Controller with I2C Interface
(SBAS405)
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Operation Schemes of Touch Screen Controllers
15
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Glossary
9
Glossary
Data unit - group of bits treated as a unit, which can be 8 bits, 10 bits, or 12 bits in typical TSC devices.
Sample set - complete set of coordinates treated as a set. A sample set may include X, Y, Z1, and Z2. A
sample set contains multiple data units.
SSPS - sample sets per second.
MAVF - median value and averaging filter. The TSC2007 automatically collects 7 data units of
touch-screen data, and transmits a single refined data set to the host system.
16
Operation Schemes of Touch Screen Controllers
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