SparkFun Forums

[ploedoff]

Hi all,

Will i fry my nokia lcd if i use 5v levels on the I/O pins?

I'm trying to hook up the sparkfun color lcd to a sparkfun mega128 board. the problem is, one is a 5v part, the other a 3.3 v part. I'm trying to use Jimbo's avr code, but he doesn't say anything about the actual connections.

Does anybody know whether the display is 5v-tolerant on the data lines?
I tried to connect them via 10k Resistors (as stated in the bascom example), but

Code: Select all

  __  __  __
  | | | | | |
__| |_| |_| |

on the avr side of the resistors
looks like

Code: Select all

    ,
   / \    /\  /\
__/   '--'  '-'  '

on the display side.
So, the lcd, of course, stays blue all the time.
But since i don't want to destroy the lcd, i don't want to just hook it up directly and hope it survives...

so, does anybody KNOW, or has already tried that?

Thanks, Nico

PS: if i had a 3.3V µC, i'd be using it....

[JamesC]

Based on the datasheet, the answer is no, it is NOT 5v tolerant, and you will likely fry it you try.

Assuming you've already got a voltage regulator to supply the 3.3v power that the LCD needs, and it shares the ground with your 5v supply, you can create a voltage translation circuit very easily using some signal diodes and pull up resistors.

Code: Select all

                     Pull-up  
                     to 3.3v  
                      (10k)        
     3.3v supply  ---/\/\/\--+
                             |
                             |               
                             |     Diode         
     3.3v Input pin   -------|------>|------------- 5v CPU pin

Hope that ascii art makes sense. The idea is that when the CPU pin is high, the diode is reverse-biased and blocks current flow, and the resistor pulls the 3.3v line high. When the CPU pin is low, the diode is foward-biased and current flows from 3.3v side to ground (at the CPU). A key point here is that the 3.3v and 5v circuits MUST share a common ground.

A more robust variation is documented here...
http://delphys.net/d.holmes/hardware/levelshift.html

...which uses pullups on both sides and a mosfet in place of the diode to allow bi-directional I/O. For the LCD, you only need to communicate from the CPU to the LCD, so a signal diode should be sufficient. Don't use a power diode, they don't switch fast enough. I haven't used this technique on the LCD display, but the circuit I describe and the one described at the URL worked for interfacing to a pair of 3.3v uMIRFs. (diodes for uni-directional lines, mosfets for bi-directional lines)

I highly recommend bread-boarding it first and making sure it looks like it will work before hooking it up to your display, as I just woke up and may be confused.

One final note: You may get signal degradation problems with this technique as well, for the same reasons. Decreasing the value of the pull-up (a little!) may help, but make sure it is a large enough value that you dont draw too much current and fry the display. The web site above has o-scope capture of the mosfet circuit.

[ohararp]

I use a 5V pic to talk to a 3.3V sd card using voltage dividing resistors. See the schematic below. This is a really simple solution and works pretty well for SD cards.

http://www.sparkfun.com/cgi-bin/phpbb/v ... php?t=3188

[ploedoff]

Thank you,

I tried the voltage resistors (don't have diodes at hand, though it might be the more elegant solution) , 100 and 220 Ohm, gives me nice clean 3 voltish signals, and i can actually see a circle when i look at a special angle.

but the contrast or brightness / color temperature changes about every second. is that the flicker everybody complains about? i thought it would be a lot quicker... i added the 100uF cap, and from what my scope shows, it's a pretty stable 3.3v.

when the display is uninitialized, it shows no flicker whatsoever, just a plain blue color. but when i initialise it, draw some stuff on it, and then STOP sending any commands/data, it still flickers... Any ideas? might there be some lcd controller settings i missed?

Bye, Nico

[ploedoff]

Hello,

if somebody is interested, I got everything to work, ported Jimbo's Nokia LCD Example Code to avr-gcc and ATMega128 (pgm_read_byte!), stripped basically everything out except the 6x8 font, and will soon put it in the bootloader section together with the butterfly bootloader! Wouldn't that be cool

The voltage dividers did the trick with the signal levels.
But although I used a seperate 3.3v regulator for the backlight booster, the electronic volume/brightness/contrast changes sometimes (a "flicker" with a period measured in minutes, if not hours ), so i added two pushbuttons to brighten/dim the display..
Sometime in the future, I will pust the graphic lcd bootloader..
Bye, nico

[Grimm Spector]

I'd really love to see that bootloader, see if maybe it could be useful in an upcoming project I'm working on.