Connecting ARM controller to TTL logic levels

 Connecting ARM controller to TTL logic levels

I have an LPC2119 controller. The GPIO pins operate on 0-3.3V levels. I wish to interface this to an LCD controller which operates on 0 to 5V (the minimum input value recognised as high is 3.5V), therefore some level conversion is necessary to drive the LCD controller from the LPC2119.

One possible way to do this is to add pull-up resistors to the i/o lines.
An alternative is to use a level translator chip such as a 74LVXC3245MTC, but I'm reluctant to do this as I will require 2 of them and a lot of extra wiring.

So my question is: is using pull-up resistors to interface LPC2119 outputs with 0-5V logic advisable?

What value resistors should I use?

Regards
C

 RE: Connecting ARM controller to TTL logic levels

Yes, you can do that, as long as you are using 5-volt tolerant LPC2119 pins only (take care of ADC pins, there are usually *not* 5 volt tolerant even if they are not used as ADC inputs).

Note, that instead of switching the LPC2119 outputs from low to high you have to switch the port direction from output to input when using pullups to 5 volts.

A different approach would be lowering the power supply voltage of the LCD (usuall the spec says that this is possibly) by adding a series diode into the 5 volt line, thus reducing the threshold voltage of the LCD controller for detecting high. Some display controllers tend to work rather unreliable under reduced voltage conditions (although allowed by the spec), so I would not recommend the 2nd approach.

Regards
Herbert