REVIEW OF SCIENTIFIC INSTRUMENTS
VOLUME 71, NUMBER 2
FEBRUARY 2000
High voltage multichannel wave form generator for liquid crystal research T. Matuszczyka) and R. Beccherellib) Department of Physics, Chalmers University of Technology, Division of Microelectronics and Nanoscience, S-41296 Go¨teborg, Sweden
共Received 20 July 1999; accepted for publication 27 October 1999兲 The article describes a wave form generator designed primarily for experiments on addressing all kinds of liquid crystal displays. It can also be used in any application requiring several simultaneous sources of high-voltage arbitrary pulse trains. The instrument has eight channels capable of pulse amplitudes of ⫾100 V at a slew rate better than 300 V/s. Its design differs significantly from a typical arbitrary wave form generator. First and foremost the wave forms are directly constructed from pulses with variable width as well as amplitude. Two interchangeable memory banks guarantee transient-free adjustments of generated wave forms. The generator is computer controlled with well integrated software and provides all functionality to assist in the creation of wave forms and experiments with liquid crystal addressing schemes in a straightforward and intuitive way. To point out its versatility we discuss a new mode of operation intended primarily for generation of analogue gray shades on a surface-stabilized ferroelectric liquid crystal display. The instrument creates conditions corresponding to driving a display of virtually any size with image frames changing at video rate. © 2000 American Institute of Physics. 关S0034-6748共00兲01502-1兴
I. INTRODUCTION
ages and analogue gray scale functions, is presented here. The eight-channel wave form generator 共Fig. 1兲 is capable of pulse amplitudes up to ⫾100 V at a high slew rate. Each pulse has individual amplitude and duration parameter, and occupies only one memory address. The generator is controlled by a computer 共Macintosh or Windows兲 using a dedicated extensive software that converts the creation and use of addressing schemes to a relatively simple task. The unique feature of the device is that the process of altering the wave forms is separated from their generation 共in the way described below兲, and that in this way undesired, intermediate pulse sequences are never delivered to the connected liquid crystal cell. Our recent experiments on analogue gray scale in FLCDs5 required further enhancements in performance relative to the previous instrument, which are discussed in this article. The most significant improvement is the new mode of operation allowing creations of analogue gray shades at video speed on a display with virtually unlimited number of electrodes.
Standard arbitrary wave form generators are not well suited to operational tests of liquid crystal displays 共LCDs兲 nor to general liquid crystal addressing experiments since they were designed with quite different applications in mind: the generation of advanced analogue functions, usually in one channel and at low voltage. Such functions are composed from a very large number of amplitude points that are equally spaced in time. The changes to the output signal are usually done by altering data directly in the same memory that is used for its generation. During this process the output is distorted. If such a generator drives a liquid crystal cell, the unbalanced voltage sequences disturb the electrooptic response measurements and may even damage the cell itself. Moreover, for meaningful experiments the wave form generator should have several channels with high output amplitude, well above the typical 10 V limit. The output amplitude should also be independent of the load. Since the early 1980s the Liquid Crystal Group at Chalmers University has been working on addressing surface-stabilized ferroelectric liquid crystal devices 共SSFLCDs兲. Several generations of dedicated drivers have been developed.1,2 The first prototype of the wave form generator discussed in this article has been presented at the 4th International Conference on Ferroelectric Liquid Crystals in Tokyo, 1993.3 Its functionality has since been continuously adjusted to the increasing needs. An improved version was described by Matuszczyk,4 while a more advanced and versatile version, specially adapted for generation of video im-
II. BASICS OF LIQUID CRYSTAL ADDRESSING
In a typical LCD the electrodes are arranged in a rectangular matrix of horizontal and vertical stripes, called rows 共or lines兲 and columns, respectively. The area where a row and a column electrode overlap forms a picture element, a pixel. Each pixel is exposed to a difference between voltage sequences delivered to both its electrodes. An image can then be created by scanning the rows with a selection wave form while supplying data wave forms simultaneously to all columns, as schematically shown in Fig. 2. Generally, the data wave forms consist of voltage sequences, each by superposition with row wave form leading to switching to either a bright or dark state. The control window 共CW兲 of addressing,
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Author to whom correspondence should be addressed; electronic mail:
[email protected] b兲 On leave from: Dipartimento di Ingegneria Elettronica Universita` di Roma ‘‘La Sapienza’’ e Istituto Nazionale di Fisica della Materia, via Eudossiana 18, I-00184 Roma, Italy. 0034-6748/2000/71(2)/563/4/$17.00
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FIG. 1. Photograph of the eight-channel wave form generator. The modules are 共from right to left兲: high voltage power supply, controller, time-base, output channel 1–8, summation module, and low voltage power supply.
as indicated in Fig. 2, is defined for each line as the time period when those pulse sequences, which decide the state of each pixel in this line, appear on the column electrodes. The selection wave form applied to the rows is invariable and independent of the information to be displayed. The pulse trains are only phase shifted with respect to each other by the size of the control window. The selection wave form is built in such a way that only the data 共i.e., the choice of the pulse sequence兲 within the control window can determine the optical state of each pixel in the addressed row. A set of data and selection wave forms for a display is called the addressing scheme and its proper design is quite involved. The wave form generator discussed in this article has eight channels and can physically drive eight distinct electrodes. Nearly all important effects can be studied by driving just a 4⫻4 matrix while pretending to drive a display with thousands of lines 共i.e., each pixel in the matrix receives the same wave forms as if it was a part of a large display兲. Display drivers with thousands of real outputs have a very limited flexibility and do not offer the possibility of varying
FIG. 2. Matrix arrangement of electrodes in a typical SSFLC display. An image is created by sequentially applying a selection wave form 共scanning兲 to horizontal electrodes while supplying data wave forms simultaneously to all vertical electrodes. The control window of addressing is defined as the time period when the data intended for a given row of pixels control vertical electrodes.
T. Matuszczyk and R. Beccherelli
FIG. 3. Functional concept of the wave form generator. Memory banks 1 and 2 are isolated from each other in each module. In this example memory 2 is used for wave form generation, while memory 1 is being updated by the computer. Memory banks are interchanged 共swapped兲 at certain times.
the pulses in addressing wave forms on line. Typically, such drivers are used first in the final stage of the display system development. III. THE DOUBLE-MEMORY CONCEPT
Figure 3 shows the main functional blocks of the wave form generator. It consists of a microprocessor-based controller, one time-base module, and eight analogue output channels. The controller acquires data and commands from the computer and governs all functions of the instrument. The time-base Module defines the duration of each generated pulse, during which every output channel transforms the pulse amplitude data to a corresponding voltage level. An additional Summation module, visible in Fig. 1, allows a difference between two chosen wave forms to be observed on an oscilloscope, which corresponds to the driving signal seen by a pixel in liquid crystal addressing. There are two memory banks in the time-base module and in each output channel. Initially both memory banks store identical data. At any time one of them is used in the process of the wave form generation, while the second one can be freely accessed by the controller module. Such a configuration is the prerequisite for undisturbed wave form modification during operation. This can be clarified by the following example. Suppose that we want to change the width of a pulse N in a wave form M. We do it with a mouse on a computer screen. The information about the position (N,M ) and the new width of the pulse is automatically sent to the generator. The controller interprets the received information and changes the data in the corresponding location of the memory bank 1 of the time-base module 共in this example兲. It also stores the data internally for future use. So far, the operating waveform is not affected since the memory bank 2 is solely used for its generation. The system now waits for the last pulse of the wave form and then interchanges 共swaps兲 the memory banks. The swapping operation does not introduce delay nor disturbance. The wave form generation proceeds using data from memory bank 1 and the
Rev. Sci. Instrum., Vol. 71, No. 2, February 2000
controller gains access to memory bank 2, which can now be updated using preserved information about the performed change. In such a way the initial state, where both memory banks hold the same data, is restored. This operation is not limited to one data feature at a time, but can be performed on groups of pulses or even groups of wave forms. The existence of two memory banks with separate address buses functionally divides the instrument in two nearly independent parts: 共i兲 共ii兲
the microprocessor-based controller communicating with the host computer and having access to one memory bank at a time; the hardware-controlled wave form synthesis circuitry using data stored in the other memory bank.
In this way the speed of wave form generation does not depend on the internal speed of the microprocessor nor on the data communication, which are much slower processes. The controller interferes with the wave form synthesis hardware only by raising two ‘‘flags’’: RUN and SWAP. The flags are checked by the hardware during generation of the last pulse in a wave form and a corresponding action is taken. Naturally, when the wave form generator is stopped the controller has free access to both memory banks. Except for the buffers, the wave form generator does not contain any traditional, small TTL-type circuits. All digital logic was constructed using large programmable logic devices 共MACH from Vantis兲. This approach greatly simplified the layout of printed circuit boards, facilitated the entire development process, and opened a way for future upgrades of the hardware. The transformation of binary data describing the pulse amplitude to a voltage signal is done by an ultrafast 12-bit digital-to-analog converter 共DAC兲 AD668 and an operational amplifier AD844 共both from Analog Devices兲. The DAC settles to 1% within 25 ns. The final stage of each channel is made using a high voltage amplifier PA85A 共from Apex兲 that brings the output voltage up faster than 300 V/s. The amplifier has a very low output impedance, less than 0.1 ⍀, and the pulse amplitude is independent of the connected load within the current limit.
Multichannel wave form generator
V. NEW OPERATION MODE
The previously described procedure of updating data in two memory banks represents the direct mode of the wave form generator operation. The user has full control of the parameters of each individual pulse in every channel and can freely change these parameters without interrupting the generation of wave forms. This full control can easily be extended to groups of pulses and to groups of wave forms. Any addressing scheme can be studied in detail and adjusted. In a black-and-white ferroelectric liquid crystal 共FLC兲 display, there are two predefined pulse sequences to be applied to the column electrode when a corresponding row electrode receives a selection signal and they are selected by a single data bit. To obtain observable switching, wave forms corresponding to at least two consecutive display frames have to be programmed. In the simplest case of two-slot column sequences, one can investigate driving conditions of a 6144 row display, since the total limit of 12 288 pulses is imposed by the available memory, which is sufficient for most experiments. However, this limit becomes prohibitive when using very advanced column sequences combined with a large number of consecutive frames. For example, an addressing scheme for analogue gray scale generation may contain 64 sequences built out of 16 pulses each. A detailed discussion of this and similar schemes will be published separately.5,7 The scheme should then address a fewthousand row display and span over hundreds of frames. It is obvious that such an amount of pulses is not manageable in a direct way. We have, therefore, created a new mode of the wave form generator operation. The amount of required information may be greatly reduced by observing the following facts: 共i兲 共ii兲 共iii兲
共iv兲 IV. SOFTWARE
A LCD is driven by an extremely large amount of pulses and wave forms in continuous sequence. To handle this stream of pulses an appropriate software that also decides of the usefulness of the equipment is crucial. The wave form generator software6 has been developed concurrently with the hardware and also concurrently with the authors’ research on liquid crystals. Thus, both the hardware and the software of the generator offer all desirable features and make it very straightforward to work with. Its design and construction leave room for expansions to accommodate research progress. Such a major expansion, involving a new mode of operation, is presented in this article. The original software has been thoroughly described elsewhere.4
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all sequences have the same time steps; all row wave forms are alike, they are only time shifted by one control window; a row wave form consists of an active part, which spans over one or more sequences, and one passive or ‘‘nonselect’’ sequence repeated over the remaining part of the wave form; and the background image data, far enough from the control window, is irrelevant and may remain unchanged.
The user has to control the generated addressing scheme from a higher level of abstraction and can no longer focus on parameters of individual pulses. He simply has to define the building blocks of the addressing scheme and provide data for the image frames. The new operating mode is based on look-up tables containing the basic components of the addressing scheme. Only these tables, and not the wave forms, are transmitted to the generator, which then internally creates the entire addressing scheme. The following information is sent to the generator: 共i兲 共ii兲
definitions 共pulse amplitudes兲 of all sequences for the column wave forms; definition of the row wave form 共all active sequences and one inactive sequence兲;
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timing information for the sequence, together with the clock speed and the position of the oscilloscope trigger pulse; and image data for the first frame, also indicating in which area of the display the data will change in succeeding image frames.
All the above information is stored in the controller module and is used by its microprocessor to construct the addressing scheme, which is a very rapid process since it does not involve further communication with the computer. The addressing scheme is created in such a way that the first memory bank contains half of the scheme, which includes the active part, and the second memory bank contains only the background part of the scheme. Thus, the second memory bank does not require further updating. The data for the video image can have an arbitrary length and has to be provided during operation, one frame at a time. Consecutive frames are created as follows: when the wave form generator starts outputting the first frame using data already stored in the first memory set, it receives data from the computer for the next frame and prepares for updating memory bank 1, which becomes accessible for the controller as soon as the memories are swapped and the remaining background part of the image is being generated. Then the controller moves only the selected data blocks into memory bank 1, creating the column wave forms for the second frame. Furthermore, the background part of the addressing scheme can be repeated several times. In this way one creates the conditions corresponding to driving a display with any number of rows, bypassing the memory limitations of the wave form generator. In spite of such expansion the user does not need to sacrifice much of the ability of detailed control over generated wave forms. Adjustments of the timing speed and oscilloscope trigger position can be performed transparently to the user. Even the row wave form can be altered in real time. Changing the column signals, on the other hand, require time for the wave form generator to recreate these wave forms in both memory banks. Therefore, the generation is paused and all channels briefly output 0 V.
T. Matuszczyk and R. Beccherelli
VI. DISCUSSION
When studying the electro-optic behavior of a liquid crystal cell one has to be able to freely adjust the applied electric signal. Similarly, when matrix addressing a LCD one needs to adjust the applied wave forms. Additionally, since these wave forms are functionally related to each other, one requires the means to perform simultaneous adjustments of many pulses in a simple way. Finally, when creating images on a display, one wishes to work on a higher level of abstraction, providing simply the image data. The wave form generator has been designed to fulfill all these, sometimes contradicting, requirements in an optimized way. The wave form generator has been successfully tested in several industrial and academic liquid crystal research laboratories. It has proven particularly simple to use even for persons with no previous experience in electronic addressing of displays. The scope of applications of the instrument is not limited solely to addressing all kinds of liquid crystals. It can be equally well utilized whenever synchronous, high voltage trains of pulses are required. ACKNOWLEDGMENTS
This work was supported by the Swedish Foundation for Strategic Research. The authors are grateful for rewarding discussions with Dr. M. Matuszczyk, Professor S. T. Lagerwall and Professor P. Maltese. R.B. would like to sincerely thank Professor S. T. Lagerwall for the invitation to Go¨teborg and Professor P. Maltese for guidance and supervision in his doctoral studies. J. Wahl, T. Matuszczyk, and S. T. Lagerwall, MCLC 146, 143 共1987兲. J. Wahl and T. Matuszczyk, J. Phys. E 21, 460 共1988兲. 3 T. Matuszczyk and S. T. Lagerwall, Ferroelectrics 163, 5 共1995兲. 4 T. Matuszczyk, Ph.D. thesis, Chalmers University of Technology, Sweden, 1995 共available from the author or from http://fy.chalmers.se/ ⬃tomasz兲. 5 R. Beccherelli, V. Ferrara, P. Maltese, and T. Matuszczyk, Displays 共submitted兲. 6 Freely available at http://www.flce.se 7 F. Campoli, R. Beccherelli, A. d’Alessandro, V. Ferrara, and P. Maltese, Displays 共in press兲. 1 2
