A display device includes a source device to output image data in a normal mode and in a re-synchronization mode, and refrain from outputting the image data in a panel self-refresh mode, and a sink device to perform a displaying operation based on the image data in the normal mode, store the image data at a time when an operating mode is changed from the normal mode to the PSR mode, perform the displaying operation based on the still image data in the PSR mode, and perform a frame-timing synchronization operation in the re-synchronization mode in response to a PSR-exiting command, wherein the frame-timing synchronization operation includes a first period in which a length-change of a vertical blank period is measured, a second period in which a temporary panel on-off clock is determined, and a third period in which the temporary panel on-off clock is applied to the adjustment-target frame.