GB1560103A - Method of driving electrochromic display segments of a display device of an electronic timepiece - Google Patents

Description

(54) A METHOD OF DRIVING ELECTROCHROMIC DISPLAY SEGMENTS OF A DISPLAY DEVICE OF AN ELECTRONIC TIMEPIECE (71) We, CITIZEN WATCH COM PANY LIMITED, a corporation organized under the laws of Japan, of No. 9-18, 1-chome, Nishishinjuku, Shinjuku-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of driving electrochromic display segments of a display device of an electronic timepiece and, to an electronic timepiece having a display device using electrochromic display segments.

Although liquid crystal display devices have been widely exploited as electro-optical display means, electrochromic display devices have been recently used in various applications such as electronic timepieces and calculators, etc. In general, liquid crystal display devices employ the application of electric potentials to alter the orientation of liquid crystals, with information being displayed by changing such electro-optical characteristics as the dispersion of light or by the effect of rotation of the polarization plane. Electrochromic display devices, on the other hand hereinafter referred to as EC display devices , use electric potentials to bring about a current flow through an electrochromic substance such as WO3 or MoO3, whereby reduction and colouration of the electrochromic substance is achieved.This state of colouration has a persistence which lasts for period of from one minute to one week in length, while the application of an opposite voltage or heat oxidizes the material and erases its colour.

In structure, an EC display device may be of solid state type in which a transparent electrode, an EC layer, an insulating layer and a thin counter electrode film are disposed on a single transparent glass substrate; a liquid type in which a transparent electrode, an EC layer or an insulating layer on a lead wire are disposed on an upper single transparent glass substrate and a counter electrode of carbon or the like is disposed on a lower substrate, with an electrolyte such as H2SO4 sealed between both substrates; or an organic liquid type in which an organic substance is coloured or bleached by suitably reversing an electric potential.

To drive such an EC display device, a common electrode of the EC display device is connected to ground. For colouration, a segment electrode is connected to a negative power source (-V) for a required period of time. For bleaching, the segment electrode connected to a positive power source (+ V) for a required period. Two power sources for +V and -V are thus required. To equip an electronic timepiece with two power sources entails either installing two individual batteries or providing a booster to raise the voltage on one side; both of these alternatives, however, offer major limitations and are disadvantageous when applied to electronic timepieces.

Liquid crystals employed in conventional timepieces do not exhibit persistence and their response speed is high; as a result, they can be caused to emit light by applying to them only for the required period of light emission, voltages. The voltages are applied by driver circuits which are controlled through counters and decoders. However, display elements which exploit electrochromic materials exhibit a colour persistence requiring that a voltage of a polarity opposite to that which produced the coloured state be applied in order to restore the bleached state.

According to a first aspect of the present invention, there is provided a method of driving electrochromic display segments of a display device of an electronic timepiece, comprising the steps of: selecting the display segment to be bleached; selecting the display segment to be coloured; applying a reverse polarity voltage to said display segment to be bleached at a first time instant; and applying a voltage to said display segment to be coloured at a second time instant; said second time instant being delayed from said first time instant by a time interval dependent upon the bleaching speed of said display segments.

According to a second aspect of the present invention, there is provided an electronic timepiece having an oscillator circuit, a frequency divider coupled to the oscillator circuit, counter means coupled to the frequency divider, and display means for displaying the content of the counter means and having electrochromic display segments, said electronic timepiece comprising: bleaching decoder means coupled to outputs of the counter means to select display segments of the display means to be bleached; colouration decoder means coupled to outputs of the counter means to select display segments of the display means to be coloured; first driver circuit means coupled between said display means and said bleaching decoder means; second driver circuit means coupled between said display means and said colouration decoder means; first timer means coupled to the frequency divider to provide a first timing signal; second timer means coupled to said first timer means to provide second timing signals; first gate means coupled between a power source and said first driver circuit means and second gate means coupled between said power source and said second time means; said first and second gate means being responsive to said first and second timing signals to couple said power source to said first driver circuit means and then to said second driver circuit means.

Embodiments of this invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram showing a conventional method of driving an electrochromic display device; Figs. 2A to 2D show a further driver circuit; Fig. 3 shows a modified form of the driver circuit shown in Figs. 2A to 2D.

Fig. 4A shows an example of a 7-segment display element; Fig. 4B is a view illustrating a driving method of the display element; Fig. 5 made up of Figs. 5A and 5B is a block diagram of an electronic timepiece illustrating another driving method of the present invention; and Fig. 6 is a timing chart corresponding to the colouration and bleaching operations performed by the circuit shown in Fig. 5.

Referring now to Fig. 1, there is shown an example of electric circuitry adapted to drive an electrochromic display device 10. The electrochromic display device 10 comprises a common electrode 12, and segment electrodes 14 and 16. The common electrode 12 is coupled between two batteries 18 and 20 connected in series. Battery 18 has its positive terminal coupled to "bleach" contacts 22 and 24, and battery 20 has its negative terminal coupled to "colour" contacts 26 and 28. The segment electrodes 14 and 16 are coupled to switching means such as movable switch arms 30 and 32, which are normally open as shown in Fig. 1. In order to produce colouration in the display device 10, the switch arms 30 and 32 are thrown to the "colour" contacts 26 and 28 to permit the flow of electric current from the common electrode 12 toward the segment electrodes 14 and 16.Once complete colouration is induced, the switch arms 30 and 32 may be opened, disconnecting the batteries 18 and 20 from the segment electrodes 14 and 16.

To bleach or erase a previously coloured surface, the switch arms 30 and 32 are thrown to the "bleach" contacts 22 and 24, permitting the flow of electric current from the segment electrodes 14 and 16 toward the common electrode 12. After the switch arms 30 and 32 are held in these positions for a certain time interval, the switch arms 30 and 32 are opened. It will thus be seen that two batteries are required in the above circuit arrangement in order to selectively produce or remove colouration in the device 10. It is disadvantageous when an electronic timepiece employing an electrochromic display device is provided with two batteries, because it causes serious space requirement problems in the timepiece.

Another examplary driver circuit is shown in Figs. 2A, 2B, 2C and 2D, in which like or corresponding component parts are designated by the same reference numerals as those used in Fig. 1. This example has a driver circuit utilizing a single power source by which an electrochromic display device is selectively controlled.

The driver circuit of Figs. 2A to 2D, includes a third switching means such as a movable switch arm 34 coupled to the common electrode 12 of the electrochromic display device 10. The switching means 34 is normally opened, and connectible to either the positive or the negative terminal 36 or 38 of a single power source 18. While, in actual practice, the display device 10 has a number of segment electrodes, it is to be noted that only two segment electrodes are shown for the sake of simplicity of illustration. It is herein assumed that Fig. 2A shows that a display element (hereinafter referred to as segment 14) corresponding to the segment electrode 14 is in the coloured stated and a display element (hereinafter referred to as a segment 16) corresponding to the segment electrode 16 is in the bleached state. Figs.

2B, 2C and 2D illustrate how the segment 14 is bleached and how the segment 16 is coloured.

In Fig. 2A, the segments 14 and 16 are maintained in the coloured and bleached states, respectively, by the persistent characteristic of the electrochromic material while the segment electrodes and the common electrode are open circuit. Fig. 2B shows a case in which the segment 14 is bleached. In this instance, the switching means 30 is coupled to the positive side of the battery 18 through the contact 22, and the switching means 34 is coupled to the negative side of the battery 18 through the contact 38. Thus, the segment electrode 14 has an applied voltage opposite that of the voltage which produces colouration, so that the colouration is removed. In this situation, the switching means 32 remains in its open state. Fig. 2C shows a case in which the segment 16 is coloured.In this case, the switching means 32 is coupled to the negative terminal of the battery 18 through the contact 28, and the switching means 34 is coupled to the positive terminal of the battery 18 through the contact 36. Thus, a voltage is applied across the segment electrode 16 and the common electrode 12 in a direction to produce colouration. In this instance, the switching means 30 is maintained in its open state. Fig. 2D shows that the writing in of the display information has been completed. In this condition, while the segment electrodes 14 and 16 and the common electrode 12 are maintained in their open state, the segments remain in the coloured and bleached state, respectively, by the persistence effect of the electrochromic material.

It will now be understood that the common electrode 12 is not fixed to a given potential and instead thereof it can be selectively coupled to either the positive or the negative potential side of a single battery. As seen from Figs. 2B and 2C, voltages of opposing polarity may be applied to a display segment to be coloured and a display segment to be bleached, respectively, at different times. It should be noted that the writing in of the display information may be achieved in the order of Figs. 2A, 2B, 2C and 2D or in the order of Figs. 2A, 2C, 2B and 2D.It should also be borne in mind that if the segment electrodes or the common electrode remain in the open state when a display condition is maintained by the persistence effect of the electrochromic material, viz., in a state shown in Figs. 2A and 2D, no change will be effected even when the other electrode side is coupled to the battery. In Figs. 2A and 2D, for example, the memory function of the electrochromic (EC) display device 10 is not adversely affected even when the common electrode 12 is coupled to the battery 18.

Also, if the display element has been bleached, the corresponding segment electrode may be short-circuited with the common electrode. For example, the segment electrode 16 shown in Fig. 2B may be coupled through the switching means 32 to the negative terminal of the battery 18, and the segment electrode 14 of Fig. 2C may be coupled to the positive terminal of the battery 18.

Fig. 3 shows a modified form of the driver circuit shown in Fig. 2A to 2D, and like or corresponding component parts are designated by the same reference numerals as those used in Figs. 2A to 2D. In Fig. 3, the switching means 30 comprises a P-channel metal oxide semiconductor field-effect transistor (P-channel MOS FET) 30a and an N-channel metal oxide semiconductor fieldeffect transistor (N-channel MOS FET) 30b.

The source terminal of the P-channel MOS FET 30a is coupled to a positive terminal denoted H of a single power source, while the source terminal of the N-channel MOS FET 30b is coupled to a negative terminal denoted L of the single power source. The drain terminals of the P-channel MOS FET 30a and the N-channel MOS FET 30b are coupled together and connected to the segment electrode 14. Similarly, the switching means 32 comprises a P-channel MOS FET 32a and an N-channel MOS FET 32b. The source terminal of the P-channel MOS FET 32a is coupled to the positive terminal denoted H of the single power source, with the source terminal of the N-channel MOS FET 32b is coupled to the negative terminal denoted L of the single power source. The drain terminals of the P-channel MOS FET 32a and the N-channel MOS FET 32b are coupled together and connected to the segment electrode 16.Likewise, the switching means 34 comprises a P-channel MOS FET 34a and an N-channel MOS FET 3b, serving as a circuit means for coupling the common electrode 12 to either the positive or the negative terminal of the single power source.

The source terminal of the P-channel MOS FET 34a is coupled to the positive terminal of the single power source while the source terminal of the N-channel MOS FET 34b is coupled to the negative terminal of the single power source. The drain terminals of the P-channel MOS FET 34a and the N-channel MOS FET 34b are coupled together and connected to the common electrode 12 of the display device 10. In a modification shown in Fig. 3, gate terminals of the P-channel MOS FET 34a and the N-channel MOS FET 34b are shown as connected to each other to provide an inverter, whereby when an alternating signal is applied to them the common electrode 12 may be alternately coupled to either the positive or the negative side of the power supply. However, it should be noted that the gate terminals may be disconnected from each other if desired.

With the arrangement mentioned above, the segment electrodes 14 and 16 are coupled to the positive side of the power supply when the voltage applied to the gate terminals of the P-channel MOS FET 30a and 32a becomes low level. When, in contrast, the voltage applied to the N-channel MOS FET 30b and 32b becomes high level, the segment electrodes 14 and 16 are coupled to the negative side of the power supply.

Referring now to Figs. 4A and 4B, when the count value in the counter of an electronic timepiece of the liquid crystal display type varies from "0" to "1" display segments b and c are displayed by normally applying drive signals thereto. However, in a previously proposed EC display device, when this count value in the counter of the electronic timepiece varies from "0" to "1", the display segments b and c are maintained in their previously coloured states and the display segments, a, d, e and f which have been coloured are bleached by applying a drive current to the corresponding segment electrodes. The drive signals are applied to the display elements a, b, d, e and g in the liquid crystal type display device when the count value in the counter varies from "1" to "2".

In the EC display device, however, no drive current is applied to the display segments b as shown, which consequently remains in its previously coloured state. Further, a drive current is applied to the segment electrode corresponding to the display segment c in the direction which removes colouration, and a drive current is also applied to the segment electrodes corresponding to the display segments a, d, e and g in the direction which produces colouration. Finally, a numeral "2" is displayed by the segments a, b, d, e and g.

In this manner, the display segment or segments which will be commonly displayed for the following order of numerals are caused to remain in their previously displayed conditions by open circuiting the corresponding segment electrode or electrodes and a drive current is applied only to the segment electrode or electrodes necessary to provide the display of a desired numeral.

Turning now to Fig. 4B, there is shown a manner in which display segments are coloured or bleached. When changing a displayed digit from 1 to 2 as shown in Fig. 4B with conventional display elements the segment b receives a signal to continue to colour it whereas c ceases to receive a signal. In this instance, a drive signal is applied to colour segments a, d, e and g. This holds for conventional display elements such as liquid crystals but when electrochromic materials are used their persistence makes it unnecessary to continuously drive the segment b while the instantaneous application of a potential bleaches segment c and an appropriate potention colours segments a, g, e and d.

However, since the electrochromic material has a slow bleaching speed, an image of segment c remains and can be seen along with the coloured segments a, g, e and d with the result that the digit 2 is not correctly displayed. Segment c must therefore be bleached before segments a, g, e and d are coloured. This is achieved by applying reverse polarity voltage for bleaching segment c before the application of a voltage which colours segments a, g, e and d. The segment b, meanwhile, is allowed to remain in its displayed state as governed by its persistence.

Fig. 5 represents an example of a block diagram of a drive system arranged to achieve the above concept. The numeral 140 denotes an oscillator circuit, 142 a frequency divider, 144 a seconds counter, 146 a minute counter 148, an hours counter, 150, 152 and 154 bleaching decoders, 156, 158 and 160 colouration decoders 162, 164, 166, 168, 170 and 172 driver circuits, 174 a seconds display device, 176 a minutes display device, 178 an hours display device, 180, 182, 184, 186, 188 and 190 gates associated with respective driver circuits, 192, 194 and 196 timers, and 198 a timer coupled to an output of frequency divider 142 and associated with the bleaching operation. The timer 198 generates a first timing signal which is applied to the timers 192, 194 and 196, which generate second timing signals delayed in phase from the first timing signal.

One-second signals from divider 142 are counted by seconds counter 144 and count signals are applied as inputs to bleaching decoder 150 and colouration decoder 156.

Bleaching decoder 150 selects the segment to be bleached by the application of a voltage having a reversed polarity. Accordingly, in a case where the count value in seconds counter 144 changes from "1" to "2" as shown in Fig. 4B, the segment cis selected by bleaching decoder 150. At the same time, a first timing signal from timer 198 opens gate 180, thereby impressing voltage Vcc upon driver 162. At this instance, driver 162 generates a drive signal which is applied to seconds display device 174. Thus, the bleaching of the segment c is started in response to the first timing signal from timer 198. After a predetermined time interval, timer 192 generates a second timing signal which is applied to gate 182 to cause this gate to be opened.

Therefore, the voltage Vcc is applied to driver 164 which generates a drive signal.

This drive signal is in turn applied to seconds display device 174. Thus, the segments a, d, e and g which have been selected by colouration decoder 156 are applied with electric current in the colouring direction. In this manner, the electric current is first applied to the segment c in response to the first timing signal from timer 198 and subsequently applied to the segments a, d, e and g in response to the second timing signal from timer 192. Under these circumstances, the segments a, d, e and g are coloured during the latter part of the bleaching operation of the segment c such that bleaching of the segment c and colouration of the segments a, d, e and g will be completed substantially at the same time as shown in Fig. 6 and, therefore, the digit "2" is correctly displayed.

The segment to be bleached by bleaching decoder 150 is selected by the signal from seconds counter 144, and driver circuit 162 impresses a voltage of an opposite polarity upon this segment in a manner as previously stated. The time required for the segment to bleach depends upon the particular electrochromic material employed although this can be set to the most suitable value by timer 198. In other words, a signal from seconds counter 144 causes timer 198 to operate at a time best suited for bleaching, whereby a signal is impressed upon gate 180 after this period of time has elapsed. This causes gate 180 to cut off, removing voltage Vcc from driver circuit 162 while the signal from timer 198 is also impressed upon timer 192 which in turn selects a time suitable for achieving colouration of the electrochromic material.

The timing at which the second timing signal is generated is preferably determined in dependence on a bleaching speed of the particular electrochromic material used for the EC display device. The timers may be of any suitable construction such as multivibrators, counters employing flip-flops, etc.

Fig. 6 illustrates a time chart corresponding to the colouration and bleaching processes mentioned above. A represents the bleaching voltage which, when impressed upon segment c, causes the colour of the segment to fade as shown by C. B represents the colouration voltage which, when impressed upon segments a, g, e and d, causes these segments to colour as shown by D.

In the illustrated electrochromic display drive system of this invention as thus described, segments are caused to conduct during time intervals which are delayed in phase so as to prevent unnecessary segments from appearing and hence interferring with the display of the desired digits which is extremely advantageous for displays produced by electrochromic materials. This method has been described with reference to the seconds counter but is also applied in identical manner for the minutes and hours displays as well. Moreover, it is possible to shorten the time required for bleaching by impressing a higher voltage upon the driver circuits for the bleaching operation.

Attention is directed to Applications Nos.

27717/76 (Serial No. 1560102) and 7902864 (Serial No. 1560104); this application and application No. 7902864 (Serial No.

1560104) were divided from 27717/76 (Serial No. 1560102).

WHAT WE CLAIM IS: 1. A method of driving electrochromic display segments of a display device of an electronic timepiece, comprising the steps of: selecting the display segment to be bleached, selecting the display segment to be coloured; applying a reverse polarity voltage to said display segment to be bleached at a first time instant; and applying a voltage to said display segment to be coloured at a second time instant; said second time instant being delayed from said first time instant by a time interval dependent upon the bleaching speed of said display segments.

2. An electronic timepiece having an oscillator circuit, a frequency divider coupled to the oscillator circuit, counter means coupled to the frequency divider, and display means for displaying the content of the counter means and having electrochromic display segments, said electronic timepiece comprising: bleaching decoder means coupled to outputs of the counter means to select display segments of the display means to be bleached; colouration decoder means coupled to outputs of the counter means to select display segments of the display means to be coloured; first driver circuit means coupled between said display means and said bleaching decoder means; second driver circuit means coupled between said display means and said colouration decoder means; first timer means coupled to the frequency divider to provide a first timing signal; second timer means coupled to said first timer means to provide second timing signals; first gate means coupled between a power source and said first driver circuit means and second gate means coupled between said power source and said second timer means; said first and second gate means being responsive to said first and second timing signals to couple said power source to said first driver circuit means and then to said second driver circuit means.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   applied to the segments a, d, e and g in response to the second timing signal from timer 192. Under these circumstances, the segments a, d, e and g are coloured during the latter part of the bleaching operation of the segment c such that bleaching of the segment c and colouration of the segments a, d, e and g will be completed substantially at the same time as shown in Fig. 6 and, therefore, the digit "2" is correctly displayed.

   The segment to be bleached by bleaching decoder 150 is selected by the signal from seconds counter 144, and driver circuit 162 impresses a voltage of an opposite polarity upon this segment in a manner as previously stated. The time required for the segment to bleach depends upon the particular electrochromic material employed although this can be set to the most suitable value by timer 198. In other words, a signal from seconds counter 144 causes timer 198 to operate at a time best suited for bleaching, whereby a signal is impressed upon gate 180 after this period of time has elapsed. This causes gate 180 to cut off, removing voltage Vcc from driver circuit 162 while the signal from timer 198 is also impressed upon timer 192 which in turn selects a time suitable for achieving colouration of the electrochromic material.

   The timing at which the second timing signal is generated is preferably determined in dependence on a bleaching speed of the particular electrochromic material used for the EC display device. The timers may be of any suitable construction such as multivibrators, counters employing flip-flops, etc.

   Fig. 6 illustrates a time chart corresponding to the colouration and bleaching processes mentioned above. A represents the bleaching voltage which, when impressed upon segment c, causes the colour of the segment to fade as shown by C. B represents the colouration voltage which, when impressed upon segments a, g, e and d, causes these segments to colour as shown by D.

   In the illustrated electrochromic display drive system of this invention as thus described, segments are caused to conduct during time intervals which are delayed in phase so as to prevent unnecessary segments from appearing and hence interferring with the display of the desired digits which is extremely advantageous for displays produced by electrochromic materials. This method has been described with reference to the seconds counter but is also applied in identical manner for the minutes and hours displays as well. Moreover, it is possible to shorten the time required for bleaching by impressing a higher voltage upon the driver circuits for the bleaching operation.

   Attention is directed to Applications Nos.

   27717/76 (Serial No. 1560102) and 7902864 (Serial No. 1560104); this application and application No. 7902864 (Serial No.

   1560104) were divided from 27717/76 (Serial No. 1560102).

   WHAT WE CLAIM IS: 1. A method of driving electrochromic display segments of a display device of an electronic timepiece, comprising the steps of: selecting the display segment to be bleached, selecting the display segment to be coloured; applying a reverse polarity voltage to said display segment to be bleached at a first time instant; and applying a voltage to said display segment to be coloured at a second time instant; said second time instant being delayed from said first time instant by a time interval dependent upon the bleaching speed of said display segments.
2. 2. An electronic timepiece having an oscillator circuit, a frequency divider coupled to the oscillator circuit, counter means coupled to the frequency divider, and display means for displaying the content of the counter means and having electrochromic display segments, said electronic timepiece comprising: bleaching decoder means coupled to outputs of the counter means to select display segments of the display means to be bleached; colouration decoder means coupled to outputs of the counter means to select display segments of the display means to be coloured; first driver circuit means coupled between said display means and said bleaching decoder means; second driver circuit means coupled between said display means and said colouration decoder means; first timer means coupled to the frequency divider to provide a first timing signal; second timer means coupled to said first timer means to provide second timing signals; first gate means coupled between a power source and said first driver circuit means and second gate means coupled between said power source and said second timer means; said first and second gate means being responsive to said first and second timing signals to couple said power source to said first driver circuit means and then to said second driver circuit means.