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In a display device using a time gradation method, power consumption at a time when multigradation display is unnecessary is reduced. In a second display mode in which the number of gradations is set to two as compared with a first display mode of multigradation, writing of a digital video signal of

In a display device using a time gradation method, power consumption at a time when multigradation display is unnecessary is reduced. In a second display mode in which the number of gradations is set to two as compared with a first display mode of multigradation, writing of a digital video signal of a lower order bit into a memory is eliminated by a memory controller of a signal control circuit included in the display device. In addition, readout of the digital video signal of the lower order bit from the memory is eliminated. The amount of information of a digital video signal inputted to a source signal line drive circuit is reduced. In accordance with such operation, a display controller sets a display period for conducting display to be long. When the gradation is reduced, a frame period can be also set to be longer than the first display mode.

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What is claimed is: 1. A display device comprising: a display having a display portion comprising a plurality of pixels; and a display controller for dividing a frame period of a first display mode into a plurality of subframe periods for which one of lighting and non-lighting is set to each of the

What is claimed is: 1. A display device comprising: a display having a display portion comprising a plurality of pixels; and a display controller for dividing a frame period of a first display mode into a plurality of subframe periods for which one of lighting and non-lighting is set to each of the subframe periods and expressing gradation in accordance with a total lighting time during the frame period of the first display mode, and for setting to one of lighting and non-lighting during the frame period of a second display mode, wherein the display controller can switch between the first display mode and the second display mode, wherein each of the plurality of pixels comprises at least one thin film transistor, wherein each of the plurality of subframe periods of the first display mode includes a first write-in period and a first display mode, wherein the frame period of the second display mode includes a second write-in period and a second display mode, wherein the second write-in period is longer than the first write-in period, and wherein the second display period is longer than the first display period. 2. A display device according to claim 1, wherein the display device further comprises a frame memory, n-bits data (n is natural number of two or more) is written and read out of the frame memory so that display is conducted in the frame period of the first display mode, and 1-bit data is written and read out of the frame memory so that display is conducted in the frame period of the second display mode. 3. A display device according to claim 1, wherein the display controller is integrally formed on the same substrate as the display. 4. A display device according to claim 1, wherein the display device further comprises a light emitting clement for each pixel, a specific voltage is applied to the light emitting element, and a voltage applied to the light emitting element in the frame period of the first display mode is higher than a voltage applied to the light emitting element in the frame period of the second display mode. 5. A display device according to claim 1, wherein the display device further comprises a light emitting element for each pixel, a specific current is supplied to the light emitting element, and a current supplied to the light emitting clement in the frame period of the first display mode is larger than a current supplied to the light emitting element in the frame period of the second display mode. 6. A display device according to claim 1, wherein the frame period of the first display mode is composed of three periods of a wiring period, a display period, and an erasing period. 7. A display device comprising: a display having a display portion comprising a plurality of pixels; and a display controller for dividing a frame period of a first display mode into a plurality of subframe periods for which one of lighting and non-lighting is set to each of the subframe periods and expressing gradation in accordance with a total lighting time during the frame period of the first display mode, and for setting to one of lighting and non-lighting during the frame period of a second display mode, wherein the display controller can switch between the first display mode and the second display mode, wherein each of the plurality of pixels comprises at least one thin film transistor, and wherein the frame period is set longer in the second display mode than in the first display mode, wherein each of the plurality of subframe periods of the first display mode includes a first write-in period and a first display mode, wherein the frame period of the second display mode includes a second write-in period and a second display mode, wherein the second write-in period is longer than the first write-in period, and wherein the second display period is longer than the first display period. 8. A display device according to claim 7, wherein the display device further comprises a frame memory, n-bits data (n is natural number of two or more) is written and read out of the frame memory so that display is conducted in the frame period of the first display mode, and 1-bit data is written and read out of the frame memory so that display is conducted in the frame period of the second display mode. 9. A display device according to claim 7, wherein the display controller is integrally formed on the same substrate as the display. 10. A display device according to claim 7, wherein the display device further comprises a light emitting element for each pixel, a specific voltage is applied to the light emitting clement, and a voltage applied to the light emitting element in the frame period of the first display mode is higher than a voltage applied to the light emitting element in the frame period of the second display mode. 11. A display device according to claim 7, wherein the display device further comprises a light emitting element for each pixel, a specific current is supplied to the light emitting element, and a current supplied to the light emitting clement in the frame period of the first display mode is larger than a current supplied to the light emitting element in the frame period of the second display mode. 12. A display device according to claim 7, wherein the frame period of the first display mode is composed of three periods of a wiring period, a display period, and an erasing period. 13. An electronic appliance using the display device according to claim 7. 14. A method of driving a display device including a display having a display portion comprising a plurality of pixels, and a display controller, comprising: dividing a frame period of a first display mode into a plurality of subframe periods for which one of lighting and non-lighting is set to each of the subframe periods and expressing gradation in accordance with a total lighting time during the frame period of the first display mode; and setting to one of lighting and non-lighting during the frame period of a second display mode, wherein the display controller can switch between the first display mode and the second display mode, wherein each of the plurality of pixels comprises at least one thin film transistor, wherein each of the plurality of subframe periods of the first display mode includes a first write-in period and a first display mode, wherein the frame period of the second display mode includes a second write-in period and a second display mode, wherein the second write-in period is longer than the first write-in period, and wherein the second display period is longer than the first display period. 15. A method of driving a display device according to claim 14, wherein the display device further comprises a frame memory, n-bits data (n is natural number of two or more) is written and read out of the frame memory to conduct display in the frame period of the first display mode, and 1-bit data is written and read out of the frame memory to conduct display in the frame period of the second display mode. 16. A method of driving a display device according to claim 14, wherein the display device further comprises a light emitting element for each pixel, a specific voltage is applied to the light emitting element, and a voltage applied to the light emitting element in the frame period of the first display mode is higher than a voltage applied to the light emitting element in the frame period of the second display mode. 17. A method of driving a display device according to claim 14, wherein the display device further comprises a light emitting element for each pixel, a specific current is supplied to the light emitting element, and a current supplied to the light emitting element in the frame period of the first display mode is larger than a current supplied to the light emitting element in the frame period of the second display mode. 18. A method of driving a display device according to claim 14, wherein the frame period of the first display mode is composed of three periods of a wiring period, a display period, and an erasing period. 19. An electronic appliance using the display device according to claim 1. 20. An electronic appliance using the display device driving method according to claim 14. 21. A method of driving a display device including a display having a display portion comprising a plurality of pixels, and a display controller, comprising: dividing a frame period of a first display mode into a plurality of subframe periods for which one of lighting and non-lighting is set to each of the subframe periods and expressing gradation in accordance with a total lighting time during the frame period of the first display mode; and setting to one of lighting and non-lighting during the frame period of a second display mode, wherein the display controller can switch between the first display mode and the second display mode, wherein each of the plurality of pixels comprises at least one thin film transistor, wherein the frame period is set longer in the second display mode than in the first display mode, wherein each of the plurality of subframe periods of the first display mode includes a first write-in period and a first display mode, wherein the frame period of the second display mode includes a second write-in period and a second display mode, wherein the second write-in period is longer than the first write-in period, and wherein the second display period is longer than the first display period. 22. A method of driving a display device according to claim 21, wherein the display device further comprises a frame memory, n-bits data (n is natural number of two or more) is written and read out of the frame memory to conduct display in the frame period of the first display mode, and 1-bit data is written and read out of the frame memory to conduct display in the frame period of the second display mode. 23. A method of driving a display device according to claim 21, wherein the display device further comprises a light emitting element for each pixel, a specific voltage is applied to the light emitting clement, and a voltage applied to the light emitting element in the frame period of the first display mode is higher than a voltage applied to the light emitting element in the frame period of the second display mode. 24. A method of driving a display device according to claim 21, wherein the display device further comprises a light emitting element for each pixel, a specific current is supplied to the light emitting element, and a current supplied to the light emitting element in the frame period of the first display mode is larger than a current supplied to the light emitting element in the frame period of the second display mode. 25. A method of driving a display device according to claim 21, wherein the frame period of the first display mode is composed of three periods of a wiring period, a display period, and an erasing period. 26. An electronic appliance using the display device driving method according to claim 21.