To display pixels on a TFT hsync/vsync is used. But when the displayed image does not change do I still have to update the TFT regularly? I like to save the precious bus bandwidth for other things (there is no dedicated framebuffer available with its own memory).
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You most definitely do. The LCD will go blank within 1 frame period or less if you stop supplying data.
If you need a static image without refresh, you need an e-ink type display. All LCD technologies require constant refresh. You could consider an LCD with on-board controller and frame buffer, and maybe SPI or I2C interface (but of course, you lose performance in the interface bottleneck).
Just to better understand it. For STN like displays the capacitor can't store the information long enough (therefore constant refresh is a must). But on TFT LCDs there is a transistor for each pixel AFAIK, which should store this information.
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The transistor relies on its gate capacitance to store that information; this capacitance gets discharged because of leakage. What happens to a TFT when you stop to refresh it is it loses its image, goes through sort of rainbow colours to totally white IIRC (it's been years since I last saw this because of error or experiment :-) ). So you may not lose the image on the next frame as larwe suggests, but within a second or so you will. And you will certainly at least begin to lose image quality on the very next frame.
Dimiter
------------------------------------------------------ Dimiter Popoff Transgalactic Instruments
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A transistor is not a flip-flop. The purpose of the transistor is to provide local amplification of the drive signal, so it is not subject to the capacitance of a long row or column.
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