JPH0316031B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field The present invention relates to a liquid crystal display drive circuit that forms an alternating current voltage to be applied between electrodes based on on/off information of segment electrodes and common electrodes of a liquid crystal.

(b) Prior art and its drawbacks To drive a liquid crystal (hereinafter abbreviated as LCD),
Generally, an AC rectangular wave of several tens of Hz is applied between the common electrode and the segment electrodes. For a segment to be displayed, the phase of the rectangular wave applied to the segment electrode is reversed to the phase of the rectangular wave applied to the common electrode. For a segment whose display is to be erased, the phase of the rectangular wave applied to that electrode and the phase of the rectangular wave applied to the common electrode are set to be the same phase. Figure 1 shows this relationship. A in the figure shows a rectangular wave applied to the common electrode. B shows a square wave applied to the segment electrodes. Further, C indicates the state of the voltage applied between the common electrode and the segment electrode.
The range of period P indicates a period during which rectangular wave A and rectangular wave B are in opposite phases. Further, in other periods, both rectangular waves are in the same phase. Therefore, during period P, the voltage applied to the liquid crystal display is 2 (VDD-
VSS), and the segment to which that voltage is applied is displayed, and is not displayed in other periods. FIG. 2 is an example of a circuit for making the rectangular waves applied to the segment electrodes have an opposite phase to the rectangular waves applied to the common electrodes. Exclusive OR circuit as shown
Segment control signal S1, which is segment electrode on/off information, is input to EOR input terminals a and b, respectively.
And a rectangular waveform oscillation pulse S2 is derived, the oscillation pulse is directly applied to the common electrode, and the output of the exclusive OR circuit EOR is applied to the segment electrode. With this configuration, the phase of the rectangular wave applied to the segment electrodes can be made opposite to the rectangular wave applied to the common electrode only during the period when the segment control signal is turned on.

A conventional liquid crystal display drive circuit is an exclusive OR circuit in which the above-mentioned exclusive OR circuit EOR is provided corresponding to each segment, and the segment control signal S1 is set to "H" based on display information.
I was trying to select EOR. However, in such a configuration, it is necessary to input a segment control signal equal to the number of segments to select the exclusive OR circuit EOR based on the display information, and when the number of display digits increases, even if a decoder is used The drawback was that connections with peripheral circuits were unavoidably complicated. In addition, circuits that minimize the number of connection lines by setting the number of decode combinations to a minimum have been put into practical use, but such drive circuits have the disadvantage that they cannot display segments other than the decoded combinations, and the display format The drawback was that the design was limited.

(c) Purpose of the Invention The purpose of the present invention is to eliminate the above-mentioned drawbacks, to simplify connection with peripheral circuits, and to eliminate the need to use oscillation pulses to apply rectangular waves to the common electrode and segment electrodes. Another object of the present invention is to provide a liquid crystal display driving circuit in which the duty ratio of the rectangular wave is exactly 1/2.

(d) Structure of the Invention The present invention comprises an external input circuit that synchronizes on/off information of each electrode of a liquid crystal display to a shift register operated by the clock, and an external input circuit that synchronizes with a clock and serially leads the output information of the shift register. a cyclic circuit that inverts a signal at regular intervals and serially feeds it back to the input of the shift register; a switching circuit that switches the operation of the external input circuit and the cyclic circuit; and a counter that counts the number of shifts of the shift register; a latch circuit that stores output information of the shift register when the counter counts the total number of shift stages of the shift register;
Of the information stored in the latch circuit, the on/off information of the common electrode and the on/off information of each segment electrode are applied as a common control signal and each segment control signal to the common electrode and each segment electrode of the liquid crystal display, respectively. Features. That is,
The external input circuit serially inputs the common control signal and each segment control signal to the shift register, the cyclic circuit inverts the contents of the shift register at fixed time intervals, and the switching circuit inputs the common control signal and each segment control signal to the shift register by the external input circuit. inputting operation of on/off information of the common electrode and on/off information of each segment electrode, and signal inversion of the contents stored in the shift register by the circulation circuit. Also,
A counter counts the number of shifts of the shift register, and a latch circuit stores an output signal of the shift register when the counter has counted the total number of shift stages of the shift register. Therefore, the common control signal and each segment control signal are latched in this latch circuit, and the respective signals are applied to the common electrode and each segment electrode of the liquid crystal display. Since the contents of the latch circuit are inverted at regular intervals by the action of the circuit, the liquid crystal display is statically driven by the application of a so-called alternating voltage.

(e) Embodiment FIG. 3 is a block diagram of a liquid crystal display driving circuit according to an embodiment of the present invention.

In the figure, reference numeral 1 denotes an external input circuit that receives on/off information of each electrode of the liquid crystal, that is, display information from a CPU (not shown). When the display information is sent in parallel from the CPU etc., this external input circuit 1
It is provided in the display information generation section (CPU side).
If the display information is serial, it is provided on the display information generating section (CPU side) or on the switching circuit side (LCD side) to which its output is guided. Display information includes common electrode on/off information (common control signal) and all segment electrodes on/off information (common control signal).
Consists of off information (segment control signal).
In addition to this display information, the external input circuit 1 also includes a clock.
CL1 is input, and the on/off information of each electrode is outputted to input terminals A1 and A2 of the switching circuit 2 in synchronization with the clock CL1.

The switching circuit 2 has first input terminals A1 to A4, second input terminals B1 to B4, and Y1 to B4.
It has an output terminal of ~Y4 and a switching terminal S. A switching signal is input to the switching terminal S from the CPU side. When the signal is "H", input terminals A1 to A4 are connected to output terminals Y1 to Y4. Further, when the switching signal is "L", the input terminals B1 to B4 are connected to the output terminals Y1 to Y4.

The output terminals Y1 and Y2 of the switching circuit 2 are connected to the data input terminal D and the clock input terminal CLK of the shift register 3, respectively. The total number of shift stages of the shift register 3 is set equal to the total number of segment electrodes of the LCD 4 + the number of common electrodes. The output information of the shift register 3 is led in parallel to the latch circuit 5, and the latch information is further led to each electrode of the LCD 4.

Also, the top output terminal of the shift register 3
Qmax is connected to a signal inversion circuit 6, and the output of the signal inversion circuit 6 is guided to the second input terminal B1 of the switching circuit 2.

Furthermore, a counter 7 is provided which outputs an overflow signal from an output terminal Q2 when the total number of shift stages of the shift register 3 is counted. This counter 7 counts the clock CL1, introduces a pulse synchronized with the clock CL1 to the input terminal B2 of the switching circuit 2, and also introduces an overflow signal to the input terminal B4 when the counted value overflows. Further, it is reset when the output terminal Y4 of the switching circuit 2 rises.

In the above configuration, the signal inversion circuit 6 constitutes a cyclic circuit of the present invention.

Next, the operation of the liquid crystal display driving circuit will be explained.

First, the switching signal is set to "H" to connect the input terminals A1 to A4 and the output terminals Y1 to Y4 in the switching circuit 2. Also, external input circuit 1
The display information and clock CL1 are input to the input signal. External input circuit 1 serially outputs on/off information of each signal to input terminal A1 of switching circuit 2 in synchronization with clock CL1.
CL1 is also output to input terminal A2 at the same time. Since the input terminals A1 to A4 and the output terminals Y1 to Y4 are connected, the on/off information of each electrode is guided to the data input terminal D of the shift register 3. Clock CL1 is also led to clock input terminal CLK. This allows the shift register 3 to clock the ON/OFF information that is input serially.
Shifts sequentially in synchronization with CLK. The switching signal is all display information (on/off information for each electrode) from external input circuit 1 to switching circuit 2.
It is set to "L" when outputting is completed. This connects the input terminals B1 to B4 of the switching circuit 2 to the output terminals Y1 to Y4. On the other hand, the counter 7 counts the clock CL1, and when the counted value matches the total number of shift stages of the shift register 3, it outputs an overflow signal to the input terminal B4 of the switching circuit 2. Since the input terminals B1 to B4 are connected to the output terminals Y1 to Y4, the overflow signal resets the counter 7 at its rising edge and is output to the latch circuit 5 as a latch signal. When the counter 7 is reset, the overflow signal also falls, but when the overflow signal falls, that is, when the latch signal falls, the output information of the shift register 3 is transferred to the latch circuit 5. Latched. The timing at this time is shown in FIG. When the latch circuit 5 latches the output information of the shift register 3, the
Turn on and off each segment electrode and common electrode of LCD4.

Further, when the counter 7 starts counting the clock CL1 again after being reset, the signal inversion circuit 6 outputs the output terminal Qmax of the shift register 3 at the top stage.
The output information is inverted and output to the input terminal B1 of the switching circuit 2. Therefore, the signal inversion information of the display information that is first input from the external input circuit 1 is input to the shift register 3 in synchronization with the clock, and the shift operation proceeds in the same manner as described above.
When the counter 7 counts all the shift stages of the shift register 3, the counter 7 is reset at the rising edge of the overflow signal, and the overflow signal is supplied as a latch signal to the latch circuit 5, and the shift stage is shifted at the falling edge of the latch signal. The output information of register 3 is latched again. Thereafter, the above operation is repeated, and the output information of the shift register 3 is circulated and the output information of the shift register 3 is latched.

In the above operation, if we pay attention to one output information of the shift register 3, the output information is repeatedly inverted every time a latch signal is generated, and the duty ratio is 1/
It can be seen that the result is a square wave of 2. Similarly, other output information repeats inversion and becomes a rectangular wave with a duty ratio of 1/2. Therefore, according to the LCD driving conditions, if the rectangular waves applied between the common electrode and the segment electrode are in the same phase, the segment will not be displayed, and if the rectangular waves are in opposite phase, the segment will be displayed. On the other hand, when transmitting display information from the outside for the first time, common electrode display information (on,
If the off information) is set to "L", only the information of the segment desired to be displayed needs to be set to "H". FIG. 5 is a diagram showing the timing relationship between the output information of the shift register 3 and the latch signal. Output terminal Q1 corresponds to common, output terminal Q2, Q1
If 0 corresponds to segment 1 and segment 2, respectively, then in the example shown in the figure, the display information of segment 1 is in the opposite phase to the common information, so segment 1 is in the display state. Furthermore, since the information of segment 2 is in the same phase as the common information, segment 2 is in a non-display state.

(f) Effects of the Invention As described above, according to the present invention, since the on/off information of each electrode of the liquid crystal is synchronized with the clock and serially guided to the shift register operated by the clock, the static drive method is not possible. However, the number of connection lines with peripheral circuits is extremely small. Furthermore, a special timing signal generation circuit or drive circuit for generating a common control signal to be applied to the common electrode of the liquid crystal display becomes unnecessary. moreover
While it is necessary that the LCD drive waveform does not generate a DC component, the duty ratio of the square wave is 1/1.
However, according to the present invention, the output information of the shift register is latched as inverted information every time it goes around, so the period is constant and the segment and The voltage applied to each common electrode becomes a perfect rectangular wave with a duty ratio of 1/2.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing driving waveforms of liquid crystal, and FIG. 2 is a circuit diagram for forming the driving waveform shown in FIG. 1 using a conventional liquid crystal display driving circuit. FIG. 3 is a block diagram of a liquid crystal display drive circuit according to an embodiment of the present invention, FIG. 4 is a timing chart when shift register output information is latched, and FIG. 5 is a timing relationship between a latch signal and shift register output information. FIG. 1...External input circuit, 2...Switching circuit, 3
...Shift register, 4...Liquid crystal display (LCD), 5...Latch circuit, 6...Signal inversion circuit, 7...Counter.

Claims (1)

[Claims] 1. An external input circuit that synchronizes the on/off information of each electrode of the liquid crystal display with a clock and serially leads it to a shift register operated by the clock, and an external input circuit that inverts the output information of the shift register at regular intervals. a cyclic circuit that serially feeds back to the input of the shift register, a switching circuit that switches between the operation of the external input circuit and the cyclic circuit, and a counter that counts the number of shifts of the shift register;
a latch circuit that stores output information of the shift register when the counter counts the total number of shift stages of the shift register, and among the information stored in the latch circuit, on/off information of the common electrode and each segment electrode A liquid crystal display drive circuit characterized in that on/off information is applied as a common control signal and each segment control signal to a common electrode and each segment electrode of a liquid crystal display, respectively.