WO2006051516A2

WO2006051516A2 - Display driver

Info

Publication number: WO2006051516A2
Application number: PCT/IE2005/000127
Authority: WO
Inventors: Stuart Norton; Pintér GÁBOR; David Mulroy; Nikolaos Vlachopoulos
Original assignee: Ntera Limited
Priority date: 2004-11-11
Filing date: 2005-11-09
Publication date: 2006-05-18
Also published as: IL183130A0; JP2008532055A; WO2006051516A3; EP1812925A2; AU2005303356A1; EP1657706A1; BRPI0516689A; KR20070085972A; CN101103389A; CA2587367A1

Abstract

Devices, methods and systems for controlling cells of an electrochromic display are disclosed. A device is connected to each cell of the display which is to be controlled to drive each cell according to a charging state associated with the cell; and to provide a characteristic of a selected cell to a sensor. The device can operate in a sense and a programming mode. In sense mode the device determines the characteristic of a selected cell; and in programming mode the device sets the charging state of the cells. Methods and systems for controlling display elements generally are also disclosed in which a varying drive signal (such as a sawtooth signal) is applied to the display element to increase the charge transfer over what would occur using a constant drive signal.

Description

Devices, Methods and Systems for Driving Displays

The present invention relates to devices, methods and systems for driving displays and display elements, in particular for driving electrochromic displays . A typical electrochromic display comprises a glass display screen, a substrate, tracks and electrochromic segments or pixels, which change colour upon application of an electrical potential.

In one embodiment, an electrochromic pixel comprises a first electrode made of nanostructured films of semiconducting metal oxides with a self-assembled monolayer of electrochromic viologen molecules. The charge to colour the electrochromic molecules is supplied by a second nanostructured counter electrode, comprised of a doped semiconductor. Between the electrodes there is a reflector made of a porous film of Titanium Dioxide.

Electrochromic displays are typically dc driven devices. A voltage can be applied to each individual segment or pixel of the display via a transparent conductive track leading to the pixel from the edge of the glass screen. The transparent conductive tracks are usually fabricated from Indium Tin-Oxide and as such behave in a manner similar to that of a resistor in series with the pixel .

The electrochromic pixel has similar characteristics to that of a capacitor in that it has the ability to store charge. The pixel is turned on or charged by applying a voltage to its anode. The charge capacity of a pixel is proportional to the area of the pixel. Once charged, the pixel can be left in an open circuit configuration and remains on. This characteristic of the electrochromic display is called bistability. Like a capacitor, however, the charge will slowly dissipate after time, resulting in deterioration of the pixel colouration.

This capacitor-resistor arrangement governs the rate at which the pixel can be charged according to the relationship dV/dt = V/RC. Thus the rate at which individual pixels turn on is inversely proportional to the area of the pixel and the resistance of the associated track. As such, individual pixels may charge at different rates. Pixels, like capacitors, can be damaged when exposed to applied voltages exceeding their capacity. Thus, due to this limitation on the applied voltage, and large ITO track resistances combined with large capacitances, the response time to the switching of electrochromic displays can be quite slow.

Exposure to UV light and voltage coupling from neighbouring pixels being switched on results in potentially dangerous variations within the individual pixels. As such, the pixels can reach their voltage capacity while still being driven, resulting in damage to the pixels US 5,973,819 discloses a driver for controlling the charge state (i.e., colour level) of an electrochromic device. After initiation the driver iteratively modifies and measures a charge level of the electrochromic device until a maximal or minimal charge level is achieved.

In particular, US 5,973,819 discloses connecting a single driver element through a switching matrix to each of a number of electrochromic (EC) elements in turn.

According to the present invention there is provided a device for driving a multi-cell display according to claim 1.

Using the present invention, the state of an EC element can be sensed while the remaining cells are driven, so providing more control of a display.

In preferred embodiments, potentially dangerous variations within the pixels, caused by photoelectric effect, voltage coupling from other pixels being turned on or the like, can be sensed and compensated for. The response time of the pixels is improved by using higher driving voltages in a safe controlled environment. Once charged the pixel can be left in an open-circuit state ensuring improved lifetime for the pixels. Leakage current from and between the pixels can be detected using voltage sensing and measures can be taken to maintain the correct appearance of the display.

In a further aspect, a method and system for driving a display element is presented in which a varying drive signal is applied to the display element to increase the charge transfer over what would occur using a constant drive signal . In one embodiment a sawtooth waveform can be used to drive the display element and achieve an approximately constant current charging/discharging of the display element. The method and system can be applied to a variety of display elements including electrochromic display elements which can exhibit differences in coloration between an edge portion and a center portion. In one embodiment the voltage at the edge portion is monitored and use of a sawtooth waveform allows for propagation of charge across the display element and a more accurate measurement of the state of charging at the edge portion.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a system diagram of a device of the present invention connected to a micro-control unit and an electrochromic display;

Fig. 2 illustrates a block diagram of the device of

Fig. 1 according to the present invention; Fig. 3 (a) is a timing diagram of the device of Fig. 1 operating in programming mode;

Fig. 3 (b) is a timing diagram of the device of Fig. 1 operating in sense mode;

Fig. 4 is a plot of an applied pixel voltage against time;

Fig.5 is a plot of an applied pixel voltage against time including sensing incidents;

Fig. 6 illustrates a display containing segments, with the segments having an edge portion and a center portion;

Fig. 7 illustrates a model for an electrochromic display element;

Fig. 8 illustrates a drive signal for an electrochromic display element;

Fig. 9 illustrates a block diagram for a display element driver; and

Fig. 10 illustrates an embodiment of a wave shaping circuit.

Referring now to Fig. 1 of the accompanying drawings, a system diagram of a device for driving a multi-cell display is indicated, generally at 10, connected to a micro control unit 12 and an electrochromic display 14.

The nanostructured film electrode of the electrochromic display pixel comprises an enormous surface area with a high number of electrochromic viologen molecules bound to the surface, enabling the viologens to be switched from colourless to coloured and vice versa very quickly. The high number of viologen molecules attached gives strong colouration and the high speed of electron transfer gives high switching speeds. Different colours can be achieved through using different viologen molecules. The doped semiconductor electrode can store charge due to its high capacitance and as such the display device is endowed with a memory, resulting in bistability and low power consumption.

According to the preferred embodiment of the invention, the device 10 comprises 65 output channels 16, labelled as 0[I] , O [2] , ...O[64] , O [65] . Each output channel 16 is connected via a corresponding transparent conductive track 18 to a cathode 20 of one of 65 segments or pixels 22 of the electrochromic display 14. It will be appreciated that fewer than the 65 pixels may be used. Likewise, more than 65 pixels can be used by joining or cascading a number of ICs together.

In one embodiment, the pixels 22 can be turned on or off by application of a dc voltage to the cathodes 20. A common anode 24, corresponding to the cathodes 20 is connected to a supply voltage Vcc. In one embodiment, by connecting the anode to a positive voltage relative to ground, the requirement of a negative pixel voltage can be avoided. When the pixel is on, the pixel voltage applied to the cathode 20 is positive but lower than Vcc.

The output channels 16 have been designed as voltage sources that source and sink current in order to get the connected pixel 22 to the applied voltage as quickly as possible. The 65 output channels 16 each support 4 voltage states; two 'on' voltage states, Vrefl and Vref2, an open circuit or high impedance (Hi- Z) state and an ^xoff voltage state.

The two ^λon' voltages are defined by the voltages at pins Vrefl and Vref2 , located on the device 10. An internal circuit and an external current reference resistor R3 define a constant current source that sinks through Vref2 allowing a pair of resistors, Rl and R2, to be used to accurately define the voltages at Vrefl and Vref2. The voltage drop at Vrefl and Vref2 will remain constant relative to Vcc as they will always have a constant current flowing through them, ensuring that the contrast of the electrochromic display 14 will not change if the supply voltage Vcc varies.

The constant current is defined by the value of the resistor R3, connected between ground and an Iref pin located on the device 10. In this embodiment, the equation for the constant current is 1.25/R3. For example, if resistor R3 has a value of 270KΩ, the current flowing through resistors Rl and R2 will be 4.6μA. Similarly, if resistor R3 has a value of 888KΩ, the current flowing through resistors Rl and R2 will be 1.43μA. The values of Rl and R2 are then set accordingly to provide the required voltage drop from Vcc to drive the display 14. In this embodiment, Vrefl should be set to a value of 0.8V below Vcc and Vref2 should be set between 1.5V and 2V below Vcc.

The pixels 22 assume the open circuit or Hi-Z State when the output channels 16 are disconnected from the pixels 22. Once the ^λturn on' voltage has been applied to the pixels 22, the pixels can assume the Hi-Z state without any change to the display image. This is due to the ability of the pixels to store charge and is known as bistability. The same display image will be maintained for a period of time before the voltage eventually changes due to charge leakage, causing the pixel image to fade. Depending on the characteristics of the pixel, the image could fade in a matter of minutes or days.

The ^Λoff state is achieved by setting the state for the output channel 16 to Vcc, thus eliminating the voltage drop across the terminals of the pixel, and causing the pixel to turn off. In general, once the pixels reach a voltage of approximately 40OmV or less, they are assumed to be off. Once the pixel has turned off, it should be set to the Hi-Z State. The device 10 further comprises three inputs, DATA_IN, SCLK and LOAD, which are connected to corresponding outputs, 26, 28, and 30 respectively, located on the micro control unit 12. An output, SENSE, provided on the device 10 is connected to an Analogue-to-Digital converter, A/D, located on the micro control unit 12.

The device 10 operates in one of two modes at any one time, programming mode or sense mode. In order to program the state of some or all of the pixels of the display 14, the device needs to operate in the programming mode. In programming mode, the device 10 is provided with information representing the pixels at its input DATA_IN in accordance with the clock signal provided to the input SCLK. The device 10 operates in the sense mode to monitor the behaviour of each of the pixels 22. In sense mode, a signal representing the state of a pixel is provided at the SENSE output and fed to the analog-to-digital converter A/D, where it is compared with a reference value. This mode enables the MCU 12 to sense variations in pixel voltage due to exposure to UV light, voltage coupling from neighbouring pixels being switched on, irregularities in the pixel, response to the applied voltage and other varying factors.

Referring now to Fig. 2 there is provided a block diagram of the device 10, according to the preferred embodiment of the invention. The device 10 comprises a control logic unit, 32 and a 130-bit shift register 34. The register 34 is connected to a 130-bit latch 36. The content of the 130-bit latch 36 is fed to 65 2-to-4-bit decoders 38, the outputs of which are connected to 65 corresponding CD (chromodynamic, i.e. electrochromic) drivers 40. The NCD drivers are in turn connected to the output channels 16. The truth table for the operation of each decoder 38 is depicted below as Table 1.

Table 1,

The 130-bit shift register 34 is also connected to a 7- bit latch 44. The content of the 7-bit latch 44 is fed to a 7-to-65-bit decoder 46. The outputs of the decoder 46 are connected to 65 respective switches 42, which control the monitoring of the pixels. The 65 NCD drivers 40 are connected to the 65 switches 42, which in turn provide an input to the output SENSE. The input DATA_IN is connected to the 130-bit shift register and the inputs SCLK and LOAD are connected to the control logic unit 32, which is in turn connected to the 130-bit shift register 34 at two points. The SCLK input is a clocked input and is controlled by the MCU 12. In the preferred embodiment, the maximum frequency of the SCLK is 10MHz. The LOAD input can assume a high or low signal value and is also controlled by the MCU 12. The value of the LOAD input determines whether the shift register 34 is filled with 7-bits or 130-bits.

For the device 10 to operate in the programming mode, the micro control unit must send a low signal value to the control logic unit 32 via the device input LOAD as illustrated in Fig. 3 (a) . The micro control unit 12 then feeds 130-bits into the register 34, via the input DATA_IN. Each 2-bit binary value of the 130 bits represents the desired state of one of the 65 pixels.

Data, representing the desired state for each of the 65 pixels, is shifted from the DATA_IN input into the register 34 at each low to high transition of the SCLK clock. Once the shift register 34 is filled with 130- bit binary values, the MCU 12 provides a high signal value at the LOAD input, causing the content of the shift register to be loaded into the 130-bit latch 36. The decoders 38 decode the data, and supply the corresponding NCD drivers 40 with the desired state information for each pixel. The NCD drivers 40 provide the output channels 16 with the requested voltage, according to Table 1, which is applied to the pixels.

When the device input LOAD is supplied with a high signal value, the mode of operation changes from programming mode to sense mode, as illustrated in Fig. 3 (b) . In sense mode, each bit of a 7-bit binary value representing the pixel number to be sensed is loaded into the shift register 34 on every low to high transition of the SCLK clock. After 7 clocked shifts, the LOAD input signal changes momentarily from high to low before returning to the high state. This causes the 7-bit binary value to be loaded into the 7-bit latch 44, from where it is decoded by the decoder 46 and applied to one of the 65 switches 42 corresponding to the pixel number. This switch 42 disconnects the corresponding NCD driver 40 from the corresponding output channel 16. This causes the pixel 22 to assume the Hi-Z state enabling its voltage to be sensed. The sensed voltage is applied to the SENSE output and fed to the Analogue-to-Digital Converter A/D located on the micro control unit. The A/D converts the signal to a digital value, which is compared with a fixed reference value, the outcome of which determines whether it is required to change the state of the pixel 22. When sensing is finished, the NCD driver 40 is reconnected to its associated output channel 16.

Referring now to Fig. 4 there is illustrated a plot of the applied pixel voltage against time. In order to accelerate switching and increase the responsiveness of the display 12, the required pixels are driven by a voltage Vref2, which exceeds the safe voltage limit of the pixels. Ideally, before the pixels become fully charged, the safe voltage Vrefl is applied to ensure that the pixels don't exceed their voltage capacity for too long.

In a first embodiment, as depicted in Fig. 5, the device 10 will operate in the sense mode during charging until it is sensed that a pixel voltage is within a predefined range of a fixed reference voltage or Vrefl, and that the applied voltage thus needs to be changed from Vref2 to Vrefl to avoid overcharging. It will be appreciated however, that the sense mode can also be used to determine whether a pixel voltage in the Hi-Z state has drifted and thus requires a voltage, Vrefl or Vref2, to be applied to return the pixel voltage to the desired level. The MCU 10 will then send a low signal to the LOAD input causing the device 10 to change to programming mode and the required voltage

(including open circuiting) will be applied to the associated pixel output channel 16 by setting the input bits for the pixel to the required state. The device 10 will then return to sense mode.

In another embodiment, the MCU 12 contains timing information relating to each individual pixel of the display 12. This timing information is derived from the known capacitance of each pixel and the resistance of its associated ITO track and provides the MCU 12 with an estimated time period for the application of both Vrefl and Vref2. In this embodiment, the MCU 12 timing information also contains an estimated time for which the display 14 will remain coloured. This timing information is used to schedule the sensing of the pixels. If a pixel is sensed according to the schedule, and it is determined that due to voltage variations, it has not reached the predefined range which defines the necessity to change the applied voltage, the timing information associated with that pixel voltage is incremented by a predefined amount, and the schedule is updated accordingly. Similarly, if a pixel is sensed according to the schedule, and it is determined that due to voltage variations, it has passed the predefined range which defines the necessity to change the applied voltage, the timing information associated with that pixel voltage is decremented by a predefined amount, the schedule is updated accordingly and the required voltage (including open circuiting) is applied to the pixel. Likewise, if the MCU 12 detects that a pixel in the Hi-Z state has leaked charge, it will adjust the related timing information, update the schedule and change the mode of operation of the device 10 to programming mode in order to apply the required safe voltage to 'top up' the pixel.

In the preferred embodiment, the device 10 will operate in programming mode in order to change the applied voltage of one or more of the pixels in accordance with both the estimated timing information stored in the MCU 12 and the outcome of the sensing operation. In another embodiment, the timing information is incremented or decremented by an amount directly- related to the approximate rate of charge of the pixel at the time the pixel is sensed. In order to determine the rate of charge of a pixel, the MCU 12 stores the time at which each pixel enters each state.

In another embodiment, when the sensing function is carried out on a pixel, the MCU determines the time and associated voltage of the pixel. The same pixel is sensed again, and again the MCU determines the time and associated voltage of the pixel . The MCU can then use these two results to determine the rate of charge of the pixel and update the timing information as appropriate.

In another embodiment, each time the sensing function is used, the MCU determines a first time and associated voltage of the pixel, then momentarily reconnects the pixel output channel to its NCD driver before determining a second time and associated voltage for the pixel . These values are then used to determine the rate of charge of the pixel and update the timing information as appropriate.

In an alternative embodiment, the Analogue-to-Digital Converter located on the MCU 12 is replaced with a comparator, which compares the sensed voltage signal with the safe voltage Vref1. In the preferred embodiment, the device can be set to a standby state. This is achieved by setting all of the output channels to the Hi-Z state and setting the device to programming mode. In this state, the constant current source that provides the Vrefl and Vref2 voltages is shut down, enabling the device 10 to achieve very low power consumption.

It will be appreciated that the timing information may be derived from a number of factors such as the size of the pixel, its proximity to a crossover point, the conductivity of the substrate resistance, the context within which it is switching or a combination thereof. For example, if a pixel is the only one being turned on when all the others are bleached (being turned off) , the effect of the bleaching pixels (turning off) will be to ^vpush up' the apparent voltage of the substrate to such a level that the pixel that is turning on will colour quicker than if it was one of many pixels to be turning on.

In particular, when implementing delayed sensing, the pre-programming or currently calculated delay value could be factored with a coefficient value that is determined by the amount of active area turning off compared with the amount of active area turning on at that time.

For example, if a large aggregate area is being switched off while a small area is being switched on, a relatively large amount of charge may be freed up from the area being switched off. This charge may then be available to colour the small area being switched on. This large amount of excess charge is likely to result in the small area being driven to the desired voltage more quickly than normal, and if care is not exercised, will result in the small area being overdriven.

Accordingly, if the driver would normally wait a delay time tl before sensing if a given pixel, which is being switched on, has reached the target voltage, this time tl may be adjusted by a coefficient to reduce the time, to take account of the charge available from the large area being switched off.

The coefficient, which may usefully be calculated as a function of the area being switched off and the area being switched on, causes the pixel to be sensed earlier than normal, in order to anticipate the quicker-than-normal charging time and thereby sense the pixel before it has been overdriven. Conversely, if a large area is being switched on with only a small area being switched off, the reduced charge availability on the anode (or backplane) may result in the pixels taking longer than normal to be fully charged, and this can be compensated for by adjusting tl upwards using a coefficient greater than unity.

Precise details of how a coefficient will be calculated will depend on the characteristics of the display and driving circuitry, but at a first level of approximation the coefficient may be proportional to [(area being switched on) /(area being switched off)] .

Furthermore, because of the resistive nature of the substrate, pixels close to the pixel that is turning on will have more of an effect on the switching time of that pixel than pixels that are further away, i.e. the microcontroller would internally know the size and position of each pixel . In each switch it may calculate for every pixel a coefficient value based on an equation that relates the transition of every other pixel with the location of every other pixel.

Again due to the resistive nature of the substrate after a switch, pixels can be at different contrasts even though their sensed voltage is the same. This is due to the local fluctuations in the apparent voltage of the substrate. It is possible to compensate for these different contrasts by setting different threshold voltages for each turning-on pixel. These threshold voltages may be calculated by the microcontroller as a function of what has previously been on and turned off, what was previously off and has turned on and the locations of these pixels.

Figure 6 illustrates a 7 segment display which can be realized using electrochromic elements such as those described herein. As illustrated in Figure 6, the segments can be considered to have an edge portion 60 and a center portion 61. In an alternate embodiment the display elements are electrophoretic elements which serve as electronic ink. When used herein, the term element refers generally to a display element, with segments such as those shown in Figure 6 being one type of display element. Other types of display elements of different shapes and configurations, including electrophoretic display elements, are understood to be elements as well.

As illustrated in Figure 7, an electrochromic display element can be modelled as a set of distributed variable resistors and capacitors. The elements of the model are made variable because their values change over time based on the state of charge and discharge at the various points in the electrochromic display element. Referring again to Figure 7, anode 70 represents the anode of the electrochromic display element along which are distributed anode resistances 72, 74, 76, 78, and 80. Vdrive 71 represents the conducting element on which the drive signal is applied. In one embodiment anode 70 is common to the entire electrochromic display element with Vdrive 71 representing an individual electrode which addresses a pixel or segment in the display. Vdrive 71 can be modelled as having distributed drive track resistances 112, 114, 116, 118, 120, and 122.

The display elements illustrated in Figure 6 having an edge portion 60 and a center portion 61, will, when realized as electrochromic display elements, charge differently at edge portion 60 than at center portion 61. Thus, the electrochromic display element has spatial varying properties as well as time varying properties. The spatial variations can be modelled as distributed variable capacitances and resistances which appear between the anode 70 and Vdrive 71 and are shown in figure 7 as edge capacitance 81 and edge resistance 100, to center capacitance 91 and center resistance 110, with intermediate capacitances 83, 85, 87 and 89, in series with intermediate resistances 102, 104, 106, and 108 respectively.

The electrochromic display element behaves more similarly to a transmission line than to a lumped resistance and capacitance. In addition to its transmission line like properties, the fact that the impedance at different points in the electrochromic element will vary depending on its state of charge causes the electrochromic display element to act as a time varying transmission line. As a result, it can be difficult to obtain uniform charging and coloration of an electrochromic element. A time varying waveform can be utilized to obtain uniform coloration by essentially launches a wave into the element, with that waveform being matched to the spatial and time varying impedance of the electrochromic element .

Use of a modified (and potentially matched) waveform is preferable over a constant waveform applied as a drive signal, because the constant waveform drive signal can cause the segment to color very quickly but not evenly. By varying the drive signal to the display element over time, increased charge transfer over which would be obtained using a constant drive signal can be obtained. Additionally, varying the Vdrive waveform can prevent unsafe charging of the display element. In one embodiment the waveform is varied to achieve an approximately constant current charging or discharging of the electrochromic display element. Constant current charging permits more uniform coloration of the electrochromic display element than can be obtained using a constant voltage drive signal with a charging current that may initially be very large but which decreases as the element charges.

Referring to figure 8, a sawtooth waveform can be used to accomplish an approximately constant current drive signal for the electrochromic display element. A positive going sawtooth signal such as OFF 151 of Figure 8 can be used to turn the electrochromic display element off while a negative going signal such as ON 153 can be used to turn the electrochromic display element on. In one embodiment, the drive signal is varied from a voltage referred to as V-safe 160 to a voltage of a higher magnitude such as V-attack 162. In one embodiment V-attack 162 is equal to Vdrive-ON 154. In one embodiment V-safe 160 is equal to Vrefl, which has a value of approximately 500 mV. In this embodiment V-attack 162 is equal to Vdrive-ON 154 which has a value of approximately 1000 mV. A signal of a similar magnitude but opposite polarity, such as OFF 151, can be used to turn the element off. In one embodiment these signals are referenced to the level established by a Virtual-GND 152. As illustrated in Figure 8, a rise time 163 and decay time 165 can be associated with the sawtooth waveforms.

One of the advantages of using a sawtooth waveform is that the decreasing edge of the sawtooth waveform can be used to draw charge off of the edge portion 60 on the display element thus creating more even charging across the display element and resulting in more uniform coloration. Another advantage of using the sawtooth waveform illustrated in Figure 8 is that the safe voltage for the electrochromic display element, in this case V-safe 160, can be exceeded momentarily while the drive signal is ramped up to V-attack 162, and then ramped back down. Because of the transmission line nature of the display element, ramping ON 153 up to Vdrive-ON 162 does not result in the entire display element reaching a voltage which is above the safe voltage, but instead allows the charge to propagate through the display element to achieve uniform coloration.

In one embodiment the voltage at the edge portion 60 of the display element is sensed to determine the state of charge of the display element. This allows for monitoring of the amount of charge placed on the display element and ensures that the element is not damaged. Because of the transmission line like nature of the display element, the use of the variable waveform not only allows for the charge to be effectively propagated along the display element, but also ensures that when the edge portion 60 is monitored it provides a voltage reading which is representative of the average voltage and amount of charge on the entire electrochromic element. By using a variable waveform, such as the sawtooth waveform illustrated in Figure 8, it becomes possible to allow charge to propagate to the center portion 61 of the display element while measuring a safe voltage at edge portion 60. Even though what appears to be a quote "unsafe" voltage has been applied to the display element it simply results in charge propagating to the center the display element and does not result in an unsafe long- term voltage being applied to the entire display element .

Figure 9 illustrates a block diagram for one embodiment of a driver system in which MCU 12 is used in conjunction with digital-to-analogue converters (DACS) 170, voltage to constant current circuits 172, a wave shaping circuit 174, a current control capacitor 182, a phase reversal circuit 180, window comparator 176, a virtual ground generator 178 (producing VIRTUAL-GROUND 152) and output channels 16. In one embodiment, MCU 12 is used to control DACS 170 to produce waveforms that are used by voltage to constant current circuits 172 and window comparator 176, the outputs of both being used to drive wave shaping circuit 174 which works in conjunction with current control capacitor 182 to produce waveforms which are inverted by phase reversal circuit 180 and applied to the output channels 16 for application to the electrochromic display 181. Figure 10 represents one embodiment of wave shaping circuit 174. In this embodiment, a first current mirror transistor Q6 190 is used in conjunction with a second current mirror transistor Q16 192, with 190 and 192 forming a matched pair. The current in load 191 is set using a preset resistance (not illustrated) and is mirrored on the collector of current mirror transistor Q6 190. This produces a constant current which is used to charge current control capacitor 182 through Q9 199. When the voltage on current control capacitor 182 reaches V-SAFE 160, comparator IC2A 197 switches Q13 200 off, Q9 199 on. Current control capacitor 182 now . discharges to V-ATTACK 162. Current control capacitor 182 is discharged by a constant current. When the voltage on current control capacitor 182 discharges to V-ATTACK 162, IC2A 197 switches Q13 200 off and Q9 199 on, and the cycle repeats.

The constant current applied to current control capacitor 182 produces a linear voltage waveform (both rise and decay) on current control capacitor 182, with that signal being buffered by IC4C 206 and push-pull transistor pair comprised of Ql 201 and Q12 202. The resulting voltage, VDRIVE-ON 154 drives the segments, and is inverted by IC4D 203, and transistors Q5 204 and Q8 205 to produce VDRIVE-OFF 150. VDRIVE-OFF 150 is used to drive the segments off.

Wave shaping circuit 174 can be used to vary the parameters of rise time 163, decay time 165, V-SAFE

160, and V-ATTACK 162. By using a sharp rise time the segments can be colored from the edge portion 60 to the center portion 61. A softer rise time produces a more uniform fill and colors from the center portion 61 to edge portion 60.

Using the drive waveforms described herein allows for constant current charging of the display element, with the ability to ramp the voltage down and avoid misleading readings regarding the state of the charging as detected by a voltage sensor located in edge portion 60. In one embodiment if the segment has not reached the correct voltage with the applied charge (from the applied waveform) an additional charge is provided. When the segment has reached the correct voltage the driver goes to a high impedance state.

In one embodiment the system shown in Figure 9 can be used to learn the electrochromic display 181 so that it can drive different sized segments by varying the amount of charge applied to each segment. Since a linear relationship exists between the voltages applied to the current mirror transistors and the area underneath the sawtooth voltage waveform, the sawtooth voltage waveform can be modulated by MCU 12 on a segment by segment basis.

The present invention is not limited to the embodiments described herein, which may be amended or modified without departing from the scope of the present invention.

Claims

1. A device for driving a multi-cell display, the device being arranged to connect to each cell of the display to be driven, and the device comprising: a cell driver for driving each cell according to a charging state indicator associated with the cell; and a signal path for receiving a characteristic of a selected cell and providing said characteristic to a sensor; and wherein said device is arranged to operate in a programming mode and in a sense mode; said programming mode in which said charging state of the cells is set according to said charging state indicator; and said sense mode in which said characteristic of a selected one of said cells is received via said signal path while one or more of the remaining cells are driven according to said charging state indicator associated with the cell.

2. The device according to claim 1, wherein said device is operatively associated with a controller which acts as the sensor.

3. The device according to claim 2 wherein said device is operable in said sense mode to provide a signal indicative of said characteristic of said selected cell to the controller.

4. The device according to claim 2 or 3 wherein said device is operable in said programming mode to receive a signal indicative of a required charging state of one or more of the cells from the controller.

5. The device according to any one of claims 2-4 wherein the operating mode of said device is determined by a logic signal received from said controller.

β. The device according to any one of claims 2-5 wherein the mode of operation changes from the sense mode to the programming mode when at least one of the characteristics of one or more of the cells is determined to be within a reference range.

1. The device according to any one of claims 2-6 further comprising a shift register for storing information received from the controller.

8. The device according to claim 7 wherein said device is responsive to a clock signal to successively load information into the shift register.

9. The device according to claim 8 wherein, when said device is operating in programming mode, said information comprises cell charging state information indicative of each of the cells required charging state; and, when said device is operating in sense mode, said information comprises an identifier of said selected cell.

10. The device according to claim 1 wherein said charging state is one of an ^λoff state, an 'on' state and a high impedance state.

11. The device according to claim 10 wherein said ^λon' state can be one of a slow charging state and a fast charging state.

12. The device according to claim 10 wherein, when said device is operating in sense mode, the cell to be sensed is set to the high impedance state.

13. A control system for a display comprising a device according to any previous claim and a controller, wherein said controller is arranged to store timing information for the or each cell derived from said characteristic for said cell.

14. A control system as claimed in claim 13 wherein said controller is arranged to schedule the device to switch to sensing mode according to said timing information.

15. A control system according to claim 13 wherein said controller is arranged to adjust said timing information periodically.

16. A control system according to claim 13 wherein said device is the device as claimed in claim 3, wherein said controller is arranged to adjust said timing information in response to said signal received from said device.

17. A control system according to claim 13, wherein said controller is arranged to vary the timing information for a cell in dependence on the charging state of one or more other cells.

18. A control system according to claim 17, wherein said controller is arranged to vary the timing by a coefficient factor that is determined by an aggregate area of cells turning off compared with an aggregate area of cells turning on at that time.

19. A method for driving a multi-cell display, comprising the steps of: connecting to each cell of the display which is to be controlled; driving each cell according to a charging state indicator associated with the cell; receiving a characteristic of a selected cell; and providing said characteristic to a sensor; and wherein said method comprises operating in a programming mode and in a sense mode; said programming mode in which said charging state of the cells is set according to said charging state indicator; and said sense mode in which said characteristic of a selected one of said cells is received via said signal path, while one or more of the remaining cells are driven according to said charging state indicator associated with the cell.

20. A method for driving a display element having a time varying impedance comprising:

a) applying a drive signal to the display element; b) varying the drive signal to the display element over time to substantially increase the charge transfer into the display element over the charge transfer occurring using a constant drive signal.

21. The method of claim 20 wherein the step of varying prevents unsafe charging of the display element.

22. The method of claim 20 wherein the varying of step b) is accomplished using a sawtooth signal waveform.

23. The method of claim 20 wherein the varying of step b) is accomplished using the negative portion of a sawtooth signal waveform to achieve charging of the display element.

24. The method of claim 20 wherein the varying of step b) is accomplished using the positive portion of a sawtooth signal waveform to achieve discharging of the display element.

25. A method for driving an electrochromic display element with a time varying impedance comprising:

a) applying a voltage to the electrochromic display element; and

b) varying the voltage to the electrochromic display element to achieve an approximately constant-current charging/discharging of the electrochromic display element.

26. The method of claim 25 wherein the voltage to the electrochromic display element is varied in a sawtooth fashion.

27. The method of claim 25 wherein the varying of step b) is accomplished using the negative portion of a sawtooth voltage waveform to achieve charging of the display element.

28. The method of claim 25 wherein the varying of step b) is accomplished using the positive portion of a sawtooth voltage waveform to achieve discharging of the display element.

29. A method for charging an electrochromic display element having an edge area and a center area comprising: a) applying a drive signal to the display- element: and

b) varying the drive signal to the display element to achieve a higher uniformity of coloration between the edge area and the center area over what is obtained using a constant drive signal.

30. The method of claim 29 wherein the drive signal is a voltage drive signal.

31. The method of claim 29 wherein the drive signal is a current drive signal.

32. The method of claim 29 further comprising:

c) sensing the level of charge at the edge area to determine if there is sufficient charging.

33. The method of claim 29 further comprising:

d) sensing the level of charge at the edge area to determine if a safe charging level has been exceeded.

34. A method for driving a display element comprising:

a) applying a drive signal to the display element to charge the display element;

b) sensing the display element to determine the level of charge of the display element;

c) determining if the level of charge of the display element has passed a predetermined charge range; and

d) varying the drive signal when the charge of the display element has passed the predetermined charge range.

35. The method of claim 34 wherein the determining in step c) is accomplished using voltage monitoring of the display element.

36. The method of claim 34 wherein the varying of step d) is accomplished by varying the voltage of the drive signal.

37. The method of claim 34 wherein the varying of step d) is accomplished by varying the timing of the drive signal.

38. The method of claim 37 wherein the timing is varied according to the rate of charge of the display element.

39. A method for driving a display element having an edge area and a center area comprising:

a) applying a drive signal to the display element;

b) varying the drive signal to remove a portion of the charge from the edge area; and c) sensing the charge at the edge area to obtain a measure of the overall charge on the display element.

40. The method of claim 39 wherein the drive signal is a voltage signal.

41. The method of claim 40 wherein the varying of b) is accomplished using a voltage level that is below a charging threshold.

42. A driver for a display element having a time- varying impedance comprising:

a) a control circuit producing a control voltage substantially matched to the time varying impedance of the display element; and

b) a variable voltage drive circuit containing a waveform generator wherein the waveform generator is controlled by the control voltage.

43. The driver of claim 42 wherein the control circuit produces a control voltage resulting in an approximately constant current signal to the display element.

44. A driver for an electrochromic display element comprising:

a) a driver circuit producing a time varying waveform for application to the electrochromic display element; b) a sensing element for measuring the voltage on the electrochromic display element; and

c) a control circuit for controlling the driver circuit dependant on the voltage measured on the electrochromic display element.