To Level Translate or not to

AtomSoft · Jun 17, 2011

Ok im planning on using a 3.3v MCU to control a 5v Character LCD. I can supply the LCD with 5 volts no problem. But not even sure if the MCU i plan to use has 5v tolerant pins.

Can someone suggest a simple (CHEAP) way to ensure i wont damage the MCU ? Can i simply use a resistor on each pin needed?

be80be · Jun 17, 2011

I use a 5 volt Lcd with 3.3 pic the LCD works fine at 3.3 it the contrast that's the problem. so I made this just use diodes that can handle the current

hook your contrast to the - 2.4 volt end **broken link removed**

AtomSoft · Jun 17, 2011

Can i supply LCD with 5v still for power ? I plan to use the NCP1402 5v version to boost the 3.3v to 5v but will it hurt my MCU ? if the RW pin is GND and its in write mode only. I dont plan to use read mode.

be80be · Jun 17, 2011

That's not going to hurt It should work fine tie the gnds and hook the LCD to the 5 volt end. The data pins and E RS should read a high at 3.3 volts it can't make you give them more LOL.

Then I was playing with the LCD at 3.3 I found that if I raised VDD to 3.6 most where more then useable

Gobbledok · Jun 17, 2011

Depending on which MCU you use, you can use pull-ups on a 5v tolerant, open-drain output in order to have 5v outputs from a 3.3v MCU.

be80be · Jun 17, 2011

Gobbledok said:
Depending on which MCU you use, you can use pull-ups on a 5v tolerant, open-drain output in order to have 5v outputs from a 3.3v MCU.

That's so true

Mike - K8LH · Jun 18, 2011

Jason,

Just ran across these **broken link removed** (including shipping) from China. I haven't tried them yet (no money) but I suspect they might be perfect for some 3.3v projects.

Cheerful regards, Mike

AtomSoft · Jun 18, 2011

Mike cool!, even tho the image shows a 5v sticker on it

I might just use a level translator as not to cause any damage. Would suck. I dont know which MCU i will use since this is just a main PCB. I might use it on ARM, ST, PIC not sure

Mike - K8LH · Jun 18, 2011

AtomSoft said:
... I dont know which MCU i will use since this is just a main PCB. I might use it on ARM, ST, PIC not sure ...

I'm in a similar situation. I've been playing with a few of the $4.30 TI LaunchPad boards which use 16 bit MSP430 micro controllers running at 3.3 to 3.6 volts. Buying a 3.3v LCD seems easier than using level conversion and dual regulators on a 5v LCD.

Good luck. Let us know what you end up doin'?

Cheerful regards, Mike

AtomSoft · Jun 18, 2011

in fact to save money i will use transistors... 6 to be exact and simply switch the 5v.. 6 transistors cost me like $0.18 cents
might take up slightly more board space but heh .. ill use the BC846 its a General Purpose PNP... nice and cheap and simple

3v0 · Jun 18, 2011

AtomSoft what processor you are using.

AtomSoft · Jun 18, 2011

Not sure. This was going to be for sale in my store... i wanted it to be usable by any MCU...

Here is a image of my schematic ... i think it will work well. If i use a 1k resistor for the base of each and a 100 ohm i should be able to get about 20mA for each pin even tho they need like 2mA .... but it wont pull more than it needs i guess...

While it will be for sale the schematic will be OPEN SOURCE just not the PCB since i did that work really

AtomSoft · Jun 18, 2011

Oh wait! i found a nice cheap IC:

https://www.mouser.com/ProductDetai...GAEpiMZZMs9F6aVvY09bk/fb8R74De/GSQeU%2b1MCBo=

$0.26 heh i will use this instead. Saves board space and cost about 8cents more heh

be80be · Jun 18, 2011

Here a nice Link looks like there was a level problem using the 3.3 on some LCD have a look https://delphys.net/d.holmes/hardware/levelshift.html**broken link removed**

an ATMEL AT90S8515 (at 3.6864Mhz) controlling a Seiko L203400J000 20x4 character LCD module. The 8515 is connected to a 3V supply while the LCD requires 5V. The control lines, RS, RW and E and the data lines D0-D7 are each level-shifted with the BSS138 MOSFETs. The level-shifting imposes no directionality on the bus so both reading from and writing to the LCD are possible.

AtomSoft · Jun 18, 2011

HEH COOL! Of course it would cost more to do that. Counting resistors and board space. Here is my current schematic using that IC.. i just need to order some to test it out now heh... ill get a couple DIP ... oh wait i think i have some here ! YAY!

Anyway here is the schematic:

(AS A NOTE: i dont connect the POT to VCC seems to work way better just limiting it to GND)

3v0 · Jun 18, 2011

THAT IC is a 74xx245 variant. We used it a lot when we only had micro processors. Great chip with the paired pins physically located across from each other on the chip.

AtomSoft · Jun 18, 2011

yeah, i even forgot about these and i have about 10 on hand

AtomSoft · Jun 18, 2011

YAY! it works take a look. all running from 3.3v

be80be · Jun 18, 2011

You have some of them cheap LCD to they work fine I like the blue I got 6 of them for $1.50 each and shipping was cheap to ended costing less then 2.50 each

O and I want my screwdriver back LOl Looks nice.

AtomSoft · Jun 18, 2011

LOL on the scewdriver. These LCDs cost me $6 each lol not cheap for me. You got lucky! I just uploaded a video:

https://www.youtube.com/watch?v=buBa96gg9D0

The cool thing is i can now use this LCD on 3.3v with no problem! so i can make the PCB using a SSOP and make it smaller Also might make the POT SMT and on the back so you can adjust while its plugged in

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