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Long time gone that I did more than occassional reading across the forum... but it's nice to know to always have a place to go with questions :) (and answers, if I have of course)
Now I'm messing around with a mobile phone display, the one of the Siemens S65 and similar. It is available for around 10€ here in Germany, has a resolution of 176x132 pixel and 16bit colordepth. Unfortunately it isn't very well documented, but with the information on Christian Kranz' page, I was able to get it to work with a atmega324 µC as well in software as in hardware spi.
Now, since the data can only be written to the display and not be read, the whole screen must be refreshed at once in certain situations, requirering at decent spi clock. In my case this is 8Mhz with the delay between the bytes as short as possible. The problem is that, to run stable on 16Mhz, the atmega must be provided with 5 Volts with the display only being tolerant to 3,3 Volts at maximum.
I have testet 3 methods (all work) to link the µC to the display:
1. voltage divider with resistors: pro: easy, cheap con: consumes much space, also consumes rather much power, especially on the control lines that are high on idle
2. 3,3V zener diode to ground with preresistor pro: easy as 1. con: same as 1. (used in my current setup)
3. no voltage shifting, simply a preresistor pro: easiest con: voltage on inputs of the display much higher than it's operating voltage, display controller gets quite warm after some minutes
Additionally all the above setups, at 8Mhz, are only working (no data loss) with "small" resistors (specifically on the clock and data line) below 5kOhms respectively 1kOhms which drives the sinking capabilities of the atmega to it's maximum and above.
Finally to the question: What kind of level shifter do you recommend for this application? Maybe a unidirectional line buffer supplied with 3,3 volts and 5 volt-tolerant inputs?
(I apologize all the information, I know the problem's description could have been shorter. Maybe they're useful for further discussion the display ;))
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