SparkFun Forums

[vbonachea]

I have a question about this product...

http://www.sparkfun.com/commerce/produc ... ts_id=9067

Let me make sure I have this straight the board is still powered by a 5v supply but it takes a 3.3 signal input or does it also require 3.3v power supply?

[redwire]

Run the whole shebang from 3.3V
The LCD display is a 3.3V part. 5V operation will not damage it, its chips will run from 2.7 to 5.0V but the LED backlight resistor (25R?) is for 3.3V operation and the LCD fluid too, the contrast goes wonky on 5V power.
The SerLCD runs from 2.0-5.5V (PIC16LF88/F688/F684), despite the silkscreen saying "5V" and I think it does not do 5V to 3.3V conversion so interfacing a 5V microcontroller to it needs resistors.

[vbonachea]

Sorry I am new to the micro controller world and trying to learn. So to make things clear your saying i can power the thing with either voltage but the interface need to be 3.3v? Would there be an advantage to running the LCD itself on a the 5v rather then 3.3v for example brightness?

[redwire]

It is kinda confusing. The SerLCD and LCD display should be powered from 3.3V
This must match your microcontroller, since it talks to the SerLCD.
If you run the SerLCD and LCD display from 5V, the backlight will be too bright and it is hard to adjust the LCD contrast control.

[vbonachea]

Ok well my problem is that I have a mBed microcontroller. It is powered by 5v-9v Vin (for all intensive purposes lets say 5v) but its I/O pins are all 3.3v. the way i have my circuit set up is I have a 5v bus powering the controller. Now i know that for the serials to talk i need both the communication output on the mBed to be 3.3v and the communication input to the LCD to be 3.3v. So regardless of this the LCD is also powered my 3.3v not 5v like the microcontroller meaning my best bet is to put a 3.3v regulator from my 5v power bus to power the LCD unit, correct?

Oh and thanks for all the help and patience!!

[redwire]

I looked at the mBed, it has an on-board 3.3V regulator for the MCU. You should be able to run the LCD display (60mA) from this.
From the mBed forums:

The Vout pin is supplied by a separate LDO (LD1117s33) on the board which is rated at 800mA.
If you are supplying your mbed from Vin then you can, in theory, draw the full 800mA through Vout assuming your power supply can source this. In practice this is limited by the heat dissipation of the LDO, and therefore by the actual voltage you are applying. The higher the voltage on Vin, the higher the heat dissipation for a given load, which might lead to thermal shutdown.
If your mbed is being powered by USB, the maximum you can draw through Vout is around 350mA. The mbed has a current limiter on the USB power supply that cuts off at 450mA to prevent damage to the host PC. The mbed itself take around 100mA, so a load of more than 350mA will cause the current limiter to step in.

[vbonachea]

Perfect!... exactly what i need to know. Thanks again!