GB1595842A - Guest-host liquid crystal display device system and its application - Google Patents

Abstract

An electronic clock with a guest-host liquid-crystal display is described. The specified drive electronics subject the guest-host liquid-crystal display to frequencies in excess of 32 Hz, preferably about 1 Hz. As a result, the power consumption of the guest-host liquid-crystal display is reduced to such an extent that there is no need for the previously used complex twisted nematic cell. Apart from the price advantage, the elimination of the previously necessary polarisers allows the proposed guest-host liquid-crystal cell to have a greater viewing angle for the display. <IMAGE>

Description

(54) GUEST-HOST LIQUID CRYSTAL DISPLAY DEVICE SYSTEM AND ITS APPLICATION (71) We, BBC BROWN, BOVERI & BR< COMPANY LIMITED, a body corporate organised and existing under the laws of Switzerland, 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: The present invention deals with a liquid crystal display which has a cholesteric guesthost layer between two cell plates and at least one front and one rear electrode and an integrated reflector inside the cell, as well as a driving circuit which supplies the liquid crystal display with alternating driving signals for the display of information.

Most of the modern electronic watches now employ twisted nematic liquid crystal displays to indicate the time. The commercially available liquid crystal mixtures that are used in these twisted nematic displays require driving potentials of about 3 volts. The display of an electronic wristwatch, for example, when driven with a 32 Hz square-wave at this potential draws an average current of 0.3 uA which results in an average power consumption of 0.9 ,uW that must be delivered by the battery.

In principle this power could be reduced somewhat by driving the display at lower frequencies because the capacitive current drawn by a twisted nematic cell at low frequencies is directly proportional to the driving frequency. In practice however, it is not possible to decrease the driving frequency below 22 Hz because at this frequency the display begins to show noticeable flicker to the viewer. This flickering is the consequence of a decrease in contrast resulting from a decrease of the electric field in the middle of the liquid crystal layer at the moment that the polarity of the driving potential is reversed.

The power consumption of various displays must be considered in relation to the power consumption of the whole device, in this case, for example, an electronic wristwatch. A modern electronic wristwatch with quartz oscillator and MOS integrated circuit consumes about 4.5 ,uW of power. The average current drawn by the twisted nematic cell discussed above therefore comprises about 20% of the total current requirement of the wristwatch.

Experience shows that a watch of this kind can be operated for at least a year before the batteries have to be replaced. Further advances expected in integrated circuit technology will permit a reduction in their current requirement and will make it possible to operate an electronic wristwatch with smaller batteries for as long or even a somewhat longer time without replacement.

A disadvantage of the present state of the art is the twisted nematic cell which results from the numerous production steps which are required for its manufacture.

Liquid crystal displays which employ the guest-host effect (see, for example, Physik in unserer Zeit, 1975, Nr. 3, page 71) are considerably cheaper to manufacture and, in the case of cholesteric guest-host displays, have the additional advantage of having a wider field of view than the twist cells.

According to the present invention there is provided a liquid crystal digital device system comprising a liquid crystal display cell with a cholestric guest-host liquid crystal layer contained between two cell plates and at least one electrode on each side of the layer and with a reflector integrated within the liquid crystal cell, and an addressing circuit adapted to supply addressing signals in the shape of unipolar pulses to each electrode, said addressing signals (Al - A4) being such that there results alternating voltages across the liquid crystal layer with a frequency (f) in the range of 32 Ek > f > 0.125 and a duty ratio of 50% and without any net DC voltage across the liquid crystal layer.

The quantitative specification of the display driving frequency overcomes the technical prejudice of always driving liquid crystal displays with higher frequency A.C. voltages be cause of irreversible electrochemical reactions which can occur under D.C. operation. (See, for example, Physik in unserer Zeit, 6th year, 1975, No. 3). For this reason the known integrated circuits employed in electronic watches all drive the liquid crystal display with a 32 Hz signal. A typical example is the CALTEX MOS integrated circuit CT 600" (Contiflex AG, CH-8700 Kusnacht-Zurich).

Various embodiments of the invention are illustrated with the help of the attached drawings. The figures show: Figure 1 The power consumption of a guesthost liquid crystal display device (G-H) driven according to the principles set forward in this invention with the power consumption of a conventional twisted nematic display (TN) given for comparison.

Figure 2 Block diagram of an electronic wristwatch employing a guest-host liquid crystal display.

Figure 3 Operation of the digital display indicating seconds in watch together with some of the appropriate driving signals.

Figure 4 Driving signals for the operation of a blinking point with a one second blinking rate in a guest-host liquid crystal display.

The principal advantage of the liquid crystal display system stated in this invention can be seen from the two curves of Figure 1. The curve designated by G-H shows the power consumption as a function of the driving frequency f.

This curve was measured for a cholesteric guesthost liquid crystal display of standard dimensions for a wristwatch which is driven at a potential of 5.5 volts. The adjacent curve designated by TN shows the behaviour of a twisted nematic cell which is driven at a potential of 3 volts. This curve shows that the so-called flicker effect, designated on the curve by F-E, begins a to appear at operation below 22 Hz. For this reason as well as for reasons of lifetime, the known liquid crystal displays are operated with driving signals of 32 Hz.

A comparison of these two curves shows that a typical power consumption of 09 ,uW for a twisted nematic cell at 32 Hz is first attained with a guest-host liquid crystal cell only by operation above 4 Hz. The advantage of this invention is that the cholesteric guest-host display can operate without problems at frequencies below 4 Hz and hence requires less power than the twisted nematic display. The significance of this invention can be appreciated when it is taken into consideration that the cholesteric guest-host liquid crystal display operates without polarizers and so has considerably lower manufacturing costs.

The practical example of the electronic wristwatch of Figure 2 shows that the usage of a guest-host liquid crystal cell does not require any additional expenditure in the driving electronics. As is usual with electronic watches, the construction of this wristwatch also consists of the basic components: timekeeping circuitry 20, display driver logic 30, guest-host liquid crystal display 40, and battery 10 with voltage converter V-C for powering the whole system.

The timekeeping circuitry 20 comprises a quartz oscillator 1, which oscillates at a frequency of 32,768 Hz, a frequency divider chain 2, a setting logic SL, a seconds counter 7, a minutes counter 8, an hours counter 9 as well as a seven-segment decoder 7-Sg. The display driver logic consists of an inverting driver 3, a non-inverting driver 4 and exclusive-OR drivers 5 and 6 of which only two have been shown here for the sake of clarity.

The guest-host liquid crystal display 40 has a common back electrode R, as well as front electrodes Fl to Fe, only three being shown here for clarity.

The arrangement shown in Figure 2 operates in the usual way, namely, that the oscillations at 32,768 Hz generated in the quartz oscillator 1 are divided down to a frequency of 2 Hz or 1 Hz by the frequency divider chain 2 composed of conventional flip-flop stages.

The 2 Hz-signal is led to the inverting driver 3 and appears at its output as the addressing signal A, which is fed to the front electrode F, of the display element taking the form of a point or colon. The 1 Hz signal from the frequency divider chain 2 is led to the driver 4 and appears at its output as addressing signal A2 which is fed to the back electrode R. The remaining front electrodes F2, Fe, which correspond to segments 2 and e, respectively, are driven from the outputs of exclusive- OR drivers 5 and 6. The l-Hz signal is also fed to the first input of each of these exclusive-OR drivers 5, 6 is fed by one of the selection signal S-S3 or S-S4 produced by the seven-segment decoder 7-Sg.

This seven-segment decoder 7-Sg is controlled by the secondscounter 7, the minutescounter 8 and the hours-counter 9, which delivers the actual time signals. Setting the time is accomplished by the setting logic S-L which is connected to the counters 7-9 and which may be connected additionally to the frequency divider chain 2 to permit counting in sequence at a l-Hz rate.

The complete arrangement according to Figure 2 is powered by a battery 10 having a terminal voltage of 1.5 V. The electronic voltage converter V-C transforms this 1.5 V potential to the value of 5.5 V which is necessary to activate the liquid crystal layer.

According to the kind of liquid crystal substance or to the desired level of activation of the liquid crystal layer, voltages from 2 to 16 volts are applicable, so that for example the voltage converter can be eliminated by series connection of two 1.5 V batteries.

The arrangement according to Figure 2 has proven itself in life tests: no decomposition of the cholesteric liquid crystal could be identified;the display functioned throughout the test period absolutely free from flicker.

In Figure 3, the individual segments of one of the digital displays presenting the time in seconds are designated by a-g: the available selection states corresponding to 0,1 through 9 are shown consecutively in the first line.

As an example, the individual segment e may be considered. The LC layer behind it is activated only during the display of the numbers 0,2, 6 and 8. The waveforms of the addressing signals are shown normalized to the values + 1 and - 1. A2 is the l-Hz addressing signal applied to the back electrode R; A4 is the addressing signal applied to the front electrode Fe (segment e), while the voltage appearing between the front electrode Fe and the rear electrode R is VeR.

The waveform of the selection signal S-S4 delivered by the seven-segment decoder 7-Sg, which contains the actual time information, is evident in Figure 3 as the uppermost, unnormalized curve.

In Figure 4 are depicted the addressing relationships on the front electrode F, which is a point which blinks in seconds rhythm. Herewith is A, the 2-Hz addressing signal on the front electrode F1; A2 is again the l-Hz addressing signal on the front electrode F1; A2 is again the l-Hz addressing signal applied to the rear electrode R, while the voltage appearing between the front electrode F1 and the rear electrode R is designated VF1R andt in the present case represents one second.

The Figures 3 and 4 show clearly that pure dc voltages which could destroy the cholesteric liquid crystal never appear in the display device of this invention. The duty ratio of the pulses is 50% in both cases.

WHAT WE CLAIM IS: 1. A liquid crystal digital display device system comprising a liquid crystal display cell with a cholesteric guest-host liquid crystal layer contained between two cell plates and at least one electrode on each side of the layer and with a reflector integrated within the liquid crystal cell, and an addressing circuit adapted to supply addressing signals in the shape of unipolar pulses to each electrode, said addressing signals (A1 - A4) being such that there results alternating voltages across the liquid crystal layer with a frequency (f) in the range of 32 Hz > f > 0.125 and a duty ratio of 50% and without any net DC voltage across the liquid crystal layer.

2. A digital crystal display system according to Claim 1 with digital display in an electronic time measuring apparatus with quartz oscillator and frequency divider, wherein the addressing signals (A2 - A4) are pulsating direct voltages with a duty ratio of 50% and a frequency (f) of 1 Hz and which are produced by a frequency divider chain.

3. A liquid crystal display system according to Claim 1 or 2, wherein the addressing signals (Al - A4) exhibit amplitudes such that voltages of from 2 volts to 15 volts appear across opposing front and rear electrodes in the selected state.

4. A liquid crystal display system according to Claim 1 or 2, wherein the addressing signals (A1 - A2) which serve to indicate seconds are unipolar pulses with 50% duty ratio whereby the signal (A2) applied to the rear electrode exhibits a frequency (f) of 1 Hz and the signal (A1) applied to the front electrode exhibits a frequency (f) of 2 Hz.

5. Application of a liquid crystal display system according to one of the Claims 1, 2, 3 or 4 as display element in portable time measuring instruments, especially wristwatches.

6. A liquid crystal display system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

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

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. In Figure 3, the individual segments of one of the digital displays presenting the time in seconds are designated by a-g: the available selection states corresponding to 0,1 through 9 are shown consecutively in the first line. As an example, the individual segment e may be considered. The LC layer behind it is activated only during the display of the numbers 0,2, 6 and 8. The waveforms of the addressing signals are shown normalized to the values + 1 and - 1. A2 is the l-Hz addressing signal applied to the back electrode R; A4 is the addressing signal applied to the front electrode Fe (segment e), while the voltage appearing between the front electrode Fe and the rear electrode R is VeR. The waveform of the selection signal S-S4 delivered by the seven-segment decoder 7-Sg, which contains the actual time information, is evident in Figure 3 as the uppermost, unnormalized curve. In Figure 4 are depicted the addressing relationships on the front electrode F, which is a point which blinks in seconds rhythm. Herewith is A, the 2-Hz addressing signal on the front electrode F1; A2 is again the l-Hz addressing signal on the front electrode F1; A2 is again the l-Hz addressing signal applied to the rear electrode R, while the voltage appearing between the front electrode F1 and the rear electrode R is designated VF1R andt in the present case represents one second. The Figures 3 and 4 show clearly that pure dc voltages which could destroy the cholesteric liquid crystal never appear in the display device of this invention. The duty ratio of the pulses is 50% in both cases. WHAT WE CLAIM IS:

1. A liquid crystal digital display device system comprising a liquid crystal display cell with a cholesteric guest-host liquid crystal layer contained between two cell plates and at least one electrode on each side of the layer and with a reflector integrated within the liquid crystal cell, and an addressing circuit adapted to supply addressing signals in the shape of unipolar pulses to each electrode, said addressing signals (A1 - A4) being such that there results alternating voltages across the liquid crystal layer with a frequency (f) in the range of 32 Hz > f > 0.125 and a duty ratio of 50% and without any net DC voltage across the liquid crystal layer.

2. A digital crystal display system according to Claim 1 with digital display in an electronic time measuring apparatus with quartz oscillator and frequency divider, wherein the addressing signals (A2 - A4) are pulsating direct voltages with a duty ratio of 50% and a frequency (f) of 1 Hz and which are produced by a frequency divider chain.

3. A liquid crystal display system according to Claim 1 or 2, wherein the addressing signals (Al - A4) exhibit amplitudes such that voltages of from 2 volts to 15 volts appear across opposing front and rear electrodes in the selected state.

4. A liquid crystal display system according to Claim 1 or 2, wherein the addressing signals (A1 - A2) which serve to indicate seconds are unipolar pulses with 50% duty ratio whereby the signal (A2) applied to the rear electrode exhibits a frequency (f) of 1 Hz and the signal (A1) applied to the front electrode exhibits a frequency (f) of 2 Hz.

5. Application of a liquid crystal display system according to one of the Claims 1, 2, 3 or 4 as display element in portable time measuring instruments, especially wristwatches.

6. A liquid crystal display system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.