Writing drivers for common touch-screen interface hardware - Embedded.com

Writing drivers for common touch-screen interface hardware

Although touch screens are rapidly becoming more popular most developers have never created one before. Here is a step by step design guide that leads you through the hardware and software required to make touch screens work the first time.

Touch screens are everywhere. Industrial control systems, consumer electronics, and even medical devices are commonly equipped with touch-screen input. We use touch screens every day without even thinking about it. You might get cash at your ATM, sign for a package, check in for your flight, or look up a telephone number all by using a touch screen.

This article describes two relatively new CPU offerings that provide built-in support for touch-screen input. I'll show you how to write a software driver that will configure, calibrate, and continuously respond to touch-screen input using either of these microprocessors. The result will be working code that you're free to download and use as a baseline for your own efforts.

Good but not perfect
No input method is ideal for all situations, and touch screens are not a good fit in certain applications and product types. I'd be remiss if I didn't at least summarize the good and not-so-good factors associated with using a touch screen as a user-input method.

First, the good: touch screens have an undeniable coolness factor and can instantly make a product more enjoyable to use. Touch screens are also intuitive. When your user wants Option A, he reaches out and touches Option A. What could be more intuitive than that? A two-year-old knows how to reach out and touch what he or she wants.

Finally, touch screens are attached to the system for which they provide input. The user can't misplace the remote or the mouse. If he has the device in front of him, he has the touch screen ready to start providing input.

The bad? Touch screens can also be misused and applied to products where they really don't belong. I'm referring here to safety-critical devices, where a touch screen can be dangerous if used without appropriate precautions. I'll summarize a couple of the most obvious potential problems and leave you to the references to learn more.


Figure 1: Parallax (cross-section view)

The first problem is parallax, which is the difference between the apparent position of an object on the screen and its actual active position on the touch panel. This problem is illustrated in Figure 1. The best example I can think of is the typical drive-up ATM. The machine doesn't raise and lower itself based on the height of your car, so if you pull up in a tall SUV or truck you'll likely be looking down at the display from an elevated position. ATMs are designed to protect the expensive parts (the display) from hostile users by placing layers of strengthened glass between the user and the display.

The touch screen cannot be so protected. If it were, you would not be able to touch it, would you? So the touch screen sits right on the surface, and the display some layers of glass behind the surface. This creates a physical separation between the touch layer and the display layer. If you're looking at the screen at any angle, this means that where you press the touch screen to make a selection may be physically quite a distance from the input location expected by the user interface software.

Humans adapt to this offset fairly quickly. You learn to mentally project the displayed information to the surface of the touch screen and touch the correct location after a little trial and error. ATM designers also account for this by using large buttons and separating them liberally to help prevent accidental activation of the wrong button. Of course, mistakenly pressing the wrong ATM button won't give you cancer or cause you to go blind. A mistake like this on a medical control device could certainly do either if the system designer doesn't build in extensive safety precautions.

Parallax is minimized by reducing the physical distance between the display and the touch layers. There will always be some glass at the front of a CRT or covering an LCD. The best case is for the touch-sensitive electronics to be built into this glass, and for this glass to be as thin as possible. This reduces the separation between touch-input layer and display layer. Handheld devices such as Palm organizers use this strategy because they don't have to be nearly as concerned about mechanical strength or hostile users. The separation is minimized (you feel like you're actually touching the graphical elements), and the accuracy is greatly improved.

A second obvious problem is that during the period of time the user is touching the screen, the object touching the screen (a stylus, a finger) is obscuring at least some small part of the display from the user's eye. This is more of a factor in factory automation applications where the user is more likely to use a finger or glove than a thin stylus, but even with a stylus the act of selecting something on the screen momentarily obscures part of the information you're presenting to the user. As an example, imagine you are displaying a slider-type control for the user to adjust a value such as speed or volume, and you display the chosen value numerically just to the left of the slider control. It works great until a left-handed user operates your system and can't see the chosen value until he removes his finger. You have to factor this sort of thing into your user-interface design.

Touch-screen technologies
Before we can begin writing a touch-screen driver we have to have some basic understanding of how the hardware works. Many different touch technologies convert pressure or touch at a screen location into meaningful numerical coordinates. Some of these technologies include resistive, surface wave, infrared, and capacitive touch screens. For an excellent overview of the available technologies, you can go to www.elotouch.com and www.apollodisplays.com.

For this article I'll focus on resistive touch screens. Resistive touch screens are very popular, and they're the type of touch screen you'll find integrated with many evaluation boards and development kits. Resistive touch screens are popular mainly because they're inexpensive and electrically straightforward to add to your system.

Resistive touch screens are so named because they are basically resistive voltage dividers. They're composed of two resistive sheets separated by a very thin insulator usually in the form of plastic micro-dots. When you touch the screen, you deform the two resistive sheets just enough to make electrical contact between them. Your software can figure out where the sheets are shorted together, and hence the touch location, by examining the voltage produced across the voltage dividers.

There are several types of resistive touch screens with names like “four-wire,” “five-wire,” and “eight-wire.” The additional wires improve accuracy and reduce temperature drift, but the basic operation doesn't change. In the simplest four-wire design, one resistive layer, the “x-axis” layer, is powered and the second layer, the “y-axis” layer is used as the pickup to measure voltage corresponding to the x-axis position. The process is then reversed and the y-axis layer is powered while the x-axis layer is used as the voltage pickup.


Figure 2: Touch-screen circuit diagram

Figure 2 shows the simple circuit equivalent of a resistive touch screen. Note that two completely separate readings must be taken, x-axis position and y-axis position, and these readings cannot be taken in parallel with a four- or five-wire resistive touch screen. Your software must read one axis, then the other. It doesn't matter in which order you take these readings.

To convert the voltage produced by a resistive touch screen into a number we need an analog-to-digital converter (ADC). Until quite recently this ADC was nearly always external to the main CPU. An example of such an ADC controller is the Burr Brown NS7843 or NS7846. This device is a 12-bit analog-to-digital converter with built-in logic to control a touch screen by powering alternate planes and converting from the other. You could accomplish the plane power switching using general-purpose I/O lines and such, but this device offloads much of the work and also provides means for generating a touch or pen-down interrupt.

More recently several CPU manufacturers have started to bring the analog-to-digital conversion block and specialized touch-screen control circuitry inside the main CPU. This makes perfect sense when a CPU is intended for consumer devices, telematics, or other markets where LCD displays and touch screens are prevalent.

The reference boards
For this article we'll examine two reference boards featuring CPUs that have integrated touch-screen support. Both of these CPUs are based on the ARM processor architecture.

The first board is the Freescale MX9823ADS evaluation board, which features Freescale's MC9328MX1 processor. These can be ordered directly from Freescale distributors. The evaluation kit includes a QVGA (240×320) color LCD and touch screen.

The second board is based on the Sharp LH79524 ARM processor. The Sharp reference boards can be ordered from LogicPD Corporation (www.logicpd.com), along with integrated display and touch kits. Several interchangeable display kits are available at resolutions ranging from QVGA to 800×600 pixel resolution.

Rather than listing the code for each driver within this article, I'll instead describe the design and flow of the drivers and highlight the important bits. You can download the full source code for each driver at ftp://ftp.embedded.com/pub/2005/07maxwell.

Taking a top level view, the software provides functions to accomplish these things:

  1. Configure the controller hardware;
  2. Determine if the screen is touched;
  3. Acquire stable, debounced position measurements;
  4. Calibrate the touch screen;
  5. Send changes in touch status and position to the higher-level graphics software.

Now I'll walk you through each step in detail.

Hardware configuration
The first thing the touch drivers need to do is configure the hardware. For these integrated controllers, this means writing to memory-mapped registers to configure the controllers to a known state. This is done in a function named TouchConfigureHardware() in each driver.

In order to configure the hardware, we already have some decisions to make. Should the driver be interrupt driven? What type of conversion rate is required to get responsive and accurate touch position information? Let's walk through the thought process used to make these decisions.

As for whether or not the touch driver should use interrupts, the example drivers do in fact use the interrupt-driven approach. I did this mainly because, to be honest, it's fun to use interrupts. Don't infer from this implementation that it's always the best or correct design, and don't let anyone tell you that your touch driver is “done wrong” if it's not interrupt-driven.

I bring this up only because it seems “polling” has become a dirty word to embedded systems programmers. I once asked a client if he was polling or using interrupts to service an input device. The response was “this is an embedded system, we don't do any polling.” I felt (temporarily) like an idiot for asking the question, but on further reflection polling is a very reasonable thing to consider. If you're using an RTOS and all of your tasks are often blocked waiting for an external event of some sort, the processor is sitting in an idle-task loop, doing nothing constructive. Maybe a better design in this situation is to have your idle task poll for input from your touch screen. It's a reasonable design and is worth considering depending on your overall system requirements.

How to go about configuring an interrupt varies from one operating system to another. You'll find sections of the code have been #ifdef'ed in for each of the many supported RTOSes. In all cases the drivers actually use two distinct interrupts:

  1. An interrupt to wake up when the screen is initially touched, known as the PEN_DOWN interrupt and
  2. A second interrupt to signal when the ADC a set of data conversions.

I'll describe these interrupts and how they're generated in the following paragraphs.

The next question is how fast do we want to receive sample input readings from the ADC. The rate will affect how we need to configure the clock that will drive the touch controller and ADC. We want the clock to be fast enough to provide responsive input and accurate tracking but not so fast that the conversion is inaccurate or the system is consuming more power than required.

In my experience, a touch screen needs to provide position updates to the higher-level software at a minimum 20Hz rate, or every 50ms. Faster is better, assuming the higher-level software can keep up, and we aren't too concerned with power usage. If the touch input response is much slower than this, there will be a noticeable and annoying lag between touch input by the user and visual response on the display.

The 20Hz update rate might not sound too challenging, but providing updates at 20Hz actually requires sampling at approximately 200Hz, depending on how many readings we are going to take before deciding we have a stable input. We need to oversample in order to debounce and average the touch input position values. Resistive touch screens, especially the inexpensive variety, are notoriously noisy and bouncy.

The driver needs to sample the input for each axis several times before sending a position update to the higher-level software. The provided drivers default to configuring ADC clocks on the respective processors for a minimum 200Hz (5ms) sampling rate. This allows the driver to sufficiently debounce and filter the incoming raw data and still provide a 20Hz true position update rate to the high-level user interface software.

For the Freescale i.MX processor the touch controller module is named the Analog Signal Processor (ASP). The processor provides two peripheral clocks that are derived by dividing the core CPU clock. The course input to the ASP block is PERCLK2 (Peripheral Clock 2) , which can again be divided to produce the final input clock to the ASP. Note that PERCLK2 drives other sub-modules in addition to the ASP block, including the internal LCD controller, and therefore the touch driver cannot program PERCLK2 just for a good fit for touch sampling. PERCLK2 is programmed to the highest rate required by all attached peripherals, which in most cases would be the LCD controller, and further divided as required for the slower peripherals. The MC9328MX1 reference manual includes a table that specifies the clock programming values needed to achieve a 200Hz output data rate.

Hardware configuration for the Sharp LH79524 requires programming a few GPIO pins so that they are assigned to the ADC function, programming and enabling the ADC clock, and programming the ADC sequencer.

The LH79524 ADC is a full programmable state machine and sequencer that is quite an amazing bit of circuitry all by itself. The ADC can be programmed to drive one touch plane; delay; take a measurement; drive the opposite plane; delay; take a measurement, and so on all without any core CPU intervention whatsoever. Understanding how to program the LH79524 ADC Sequencer control banks could be a challenge but this is made much easier by an application note provided by Sharp (www.sharpsma.com). The provided driver precisely follows the recommendations of this application note regarding how to configure the Sharp ADC sequence controller.

Is the screen touched?
Once the basic hardware setup is complete, we need a reliable method to determine if the screen is touched. It makes no sense to run the ADC and get conversion readings if the user is not touching the screen. Both controllers provide a mechanism to detect if the screen is touched, and optionally to interrupt the main processor when a touch down event occurs. The driver function that determines if the screen is touched or not is named WaitForTouchState() .

When the controller is in the touch detection mode the y-axis touch plane is tied high through a pull-up resister. The x-axis touch plane is tied to ground. When the user touches anywhere on the screen, the planes are shorted together and the y-axis plane is pulled low. This can be connected internally to an interrupt generation mechanism known as the PEN_DOWN IRQ .

During normal operation the drivers use the PEN_DOWN IRQ to wake up the touch driver task when a touch-down event occurs. This allows the driver task to block itself and not consume any CPU time when the screen is not touched and wake up and go into conversion mode once the user touches the screen. We can also save power by disabling the ADC clock while not in active conversion mode.

During calibration and active sampling the drivers use the same basic mechanism to detect a screen touch; however, in these modes the drivers mask the actual interrupt and simply check the touch status manually. For the Freescale processor, this requires programming the controller to touch detect mode and checking the PEN_DOWN IRQ bit. For the Sharp processor, touch detection is built into the ADC command sequence and no extra steps are required.

Reading touch data
During calibration and normal operation, we need a procedure to read and debounce the x- and y-axis raw data values and determine if we have a stable reading while the screen is touched. This procedure is named TouchScan() in both drivers. The outline of this procedure is:

  1. Check to see if the screen is touched.
  2. Take several raw readings on each axis for later filtering.
  3. Check to see if the screen is still touched.

While performing analog-to-digital conversions, both controllers provide means to program a delay between powering the touch planes and beginning an actual analog-to-digital conversion. Freescale calls this delay the Data Setup Count (DSCNT) , and it's a number of ASP input clocks to delay after switching between planes. Sharp calls this the pre-charge delay .

In either case this time delay is needed because the resistive touch panel is two large conductors separated by a thin insulator, which is the textbook definition of a capacitor. A delay is required when switching which plane we're powering and from which plane we'll perform an analog-to-digital conversion to allow this capacitor to settle to steady state.

For the Freescale i.MX1 processor, once we initiate conversions the data produced by the ADC is stored in a 16-bit wide by 12-entry deep FIFO. The ADC produces 9-bit unsigned results, so the upper seven bits of each 16-bit entry are discarded. This means the full-scale data range of this touch controller is 0 to 511, although in reality no ADC or resistive touch screen produces results near the limit values.

We can program the processor to generate an interrupt when the FIFO has any data available or program to interrupt when the input FIFO is full. Since we always want to take multiple readings, the driver programs the FIFO to interrupt when full. When this interrupt occurs, we have 12 raw analog-to-digital conversions ready for processing, corresponding to six readings for the x-axis and six readings for the y-axis.

The Sharp LH79524 processor allows us to program a precise sequence of steps to complete before generating an interrupt. As each step is performed, results are likewise stored in an input FIFO to be retrieved by our driver software. Results are stored as 16-bit values. The most significant 10 bits of each result are the analog-to-digital conversion value, and the least significant four bits are the sequence index. The 10-bit conversion result means that this touch controller has a full-scale range of 0 to 1,023 counts, although again you will never observe results exactly at the limit values.

Once the sequencer control words are programmed on the LH79524, all the driver needs to do to acquire raw readings is command the sequencer to go. When the EOS (End Of Sequence) interrupt is generated, our results are ready to be picked up and examined. The sequencer can be configured to trigger automatically when the screen is touched, trigger on software command, or trigger continuously.

Be aware that there will always be some noise and variation in raw converter readings; this is normal. You'd be hard-pressed to take two consecutive readings on a resistive touch screen and get back identical 9-bit or 10-bit raw data values. You will find however that as the stylus or finger enters or leaves the touch screen, the readings vary much more than if you are holding steady pressure. Remember that the user is mechanically connecting together two flat resistors, the touch planes. Some small amount of time will pass during which the electrical connection between the two planes is marginal, as the user presses and releases the touch panel. We need to reject these readings until the system stabilizes, otherwise our reported touch position will jump about wildly and the higher level software will not act appropriately.

There is an unavoidable tradeoff here. If we require a narrow stability window, the driver won't be able to track fast “drag” operations. This is important for things like scrolling or pen-tracking during signature input. If we widen the stability window, we run the risk of accepting touch values that are inaccurate and the result of the marginal plane connection described above. You will need to experiment to determine the best values to use on your system. Intelligent touch controllers likewise allow you to tune these parameters via software commands.

The number of readings to take for each sample, the allowable variation in consecutive readings, and the speed at which samples are taken are all progammable parameters of each driver. These parameters can be adjusted via #defines to produce the best results on your system. An intelligent external touch controller will often take dozens or hundreds of readings at a very fast rate for improved accuracy. Since we are doing this filtering using our core CPU, we need to decide how much time we can reasonably afford to spend in our touch-sampling task. Embedded systems involve tradeoffs, and it's your job to make good compromises to produce a system your user is happy with.

As a sort of game, I like to test the commercial touch systems I run into in daily life. The next time you sign for a purchase or package using a touch screen, try flailing away with fast broad pen movements. Watch the result and see how well the screen tracks your movements. If you see nice smooth tracking, you know the driver is sampling pretty fast, probably 200Hz or more. Often you will observe your strokes turned into straight lines (slow sampling) or missed entirely (rejected input due to large value changes). Try not to shout “yee haa” while you're performing this little test in a retail store or you might get some strange looks. Normal people just don't understand what excites engineers.

Calibration
To this point we have been describing support functions of the drivers, the dirty work that has to be done and working before we can get to the cool stuff. Now that these functions are in place we're ready to actually ask the user to touch the screen.

Resistive touch screens require calibration. We need some reference values to be able to convert the raw A-to-D numbers we'ill receive into screen pixel coordinates required by the higher-level software. In an ideal case the calibration routine might be run once during initial product power-up testing, and the reference values saved to nonvolatile memory. I've organized the touch drivers to run the calibration routine once on entry, but keep in mind that you can save the reference values and not bother the user with calibration on subsequent power-up cycles. In any case you'll want to provide the user with a method of entering the calibration routine just in case the calibration becomes inaccurate due to temperature drift or other factors.

The calibration routine, named CalibrateTouchScreen() , is a simple step-by-step procedure that provides the user with a graphical target on the screen, asks the user to touch the target, and records the raw ADC readings for use later in our raw-data to pixel-position scaling routine. The graphical target and user prompts are displayed by using the Portable Embedded GUI (PEG) graphics software API, but this can be implemented using any similar graphics software.

In a perfect world we'd need only two sets (x and y ) of raw values, the minimum and maximum values read at opposite corners of the screen. In reality many resistive touch screens are notably nonlinear, meaning that simply interpolating positions between the min and max values will yield a highly inaccurate driver.

By nonlinear, I mean that equidistant physical movements across the screen won't return equal increments in the raw data. Worse still, the value read for one axis, say the y-axis, might vary significantly even if we only change the x-axis touch position. To demonstrate this phenomenon I charted y-axis data readings as I moved a stylus across a typical resistive touch screen from left to right, keeping the y-axis position as constant as possible. You would reasonably expect the y-axis readings to remaining somewhat constant as we slide from left to right across the x-axis, but as Figure 3 shows this is not the case.


Figure 3: Y-axis variation with x-axis motion

The result of this is that the more calibration points you can take the better, to minimize the span of your interpolation windows and produce the best accuracy possible. If you can calibrate once in the factory, taking a lot of sample points isn't a big deal. If that's not possible you'll have to decide how many points you want to force your user to enter to produce an accurate calibration. The provided calibration routine uses four data points, one at each corner of the screen. This produces results accurate to within a pixel or two on a VGA resolution (640×480) display screen on the reference boards described. For higher screen resolutions or other touch screens this may either be overkill or be not nearly enough data points to produce an accurate driver. The only way to determine this for certain is to work with your real hardware and do lots of trial-and-error testing.

In any case, my advice is to err on the side of taking too many calibration points. The user will be less annoyed by a long calibration procedure done infrequently than by a normal-state system that doesn't accurately respond to touch input.

Normal operation
Once the calibration sequence is complete, we're ready to begin normal operation and start sending touch events to the higher-level software. I've organized each of the provided touch drivers to run as a low-priority task within each supported RTOS environment.

The task entry point is named PegTouchTask , because the drivers are written to operate with the PEG graphics software. These drivers can be modified to work with another graphics package or even your own home-brew user-interface environment. In any case PegTouchTask first calls the hardware-configuration routine, then calls the calibration routine, and finally enters a forever loop waiting for touch input.

In the MX1 driver, the forever loop blocks itself by waiting for the PEN_DOWN interrupt event described earlier. While the screen is touched, the task continuously reads raw values, converts them to screen pixel coordinates, and sends changes in touch position or status to the higher-level software. I call this the “active tracking” mode.

The LH79524 driver works in a similar fashion. When a PEN_DOWN interrupt is generated, we command the ADC sequencer to start doing conversions. The driver does this at a 20Hz rate, checking for position changes, until the screen is no longer touched.

While the screen is touched, we continuously read multiple conversion values for each axis to determine if the touch position is stable. If the delta or change in any two consecutive readings are outside of a #defined noise window, we start over. We do this until either we can read multiple consecutive values that are within this #defined stability range, at which time we scale the results and report an update to the higher-level software, or the screen is no longer touched, at which time we again block the task and wait for input.

Before and after each conversion sequence, the driver must check to ensure that the screen is still touched. We don't want to report a stable reading to the higher-level software that is actually an “open state” reading. I've also seen drivers that automatically discard N number of readings after the screen is initially touched. I didn't find discarding some number of initial readings to be necessary or beneficial with either of the example boards.

While the screen is touched, the driver takes each stable reading and coverts the raw data to pixel coordinates using simple linear interpolation. The routine to read raw values and convert them to screen coordinates is named GetScaledTouchPosition() .

The final piece
OK, we've tuned the driver and have accurate, scaled, reliable touch information. What do we do with all this great data? If you're running with a graphical user interface system like PEG, the heavy lifting is done. You simply pack up the touch data into a message and send the message into the PEG message queue. The PEG software figures out what to do with it from there.

PEG recognizes three touch-input event types corresponding to touch down, touch up, and drag. Sending drag events is optional but is required if you want to present smooth scrolling operations to the user. The logic to decide what type of message to send into the PEG message queue is contained in the function named SendTouchMessage() in the provided source code.

One item of note here is the use of a function named Fold() for sending drag (PM_POINTERMOVE) messages. This is a handy PEG API function that prevents the user interface from getting behind in response to user input. For example if the user is scrolling a large window on a high-resolution display, it's possible for the user interface to get behind in re-drawing the scrolled window. You don't want the screen to continue scrolling after the user releases the scroll bar while the user interface catches up. Instead, if the message queue already contains a PM_POINTERMOVE message, we just want to update that message to the latest position instead of posting a new message. The effect is t

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