43oh

# Using 5volt LCDs on 3.3volt MSP430s - with no extra power supply

## Recommended Posts

Hi forum,

I have many 5 volt LCDs.   Here's an idea for using them on 3.3 volt MSP430 systems, other 3 volt systems and LaunchPads.

Let's think of the typical 16 char  x 2 row LCD, with a 16 pin connector (pin 1 = GND, pin 2 = Vcc, pin 3 = Vee, etc) and operate the MSP and LCD off only one 3volt source.    No extra power supplies.

I've been using this technique for a long while, its nothing new, you may have already seen it.

PROBLEM
STATEMENT - how to use a 5volt LCD unit on a 3.3 volt system without creating a separate 5v supply.

SOLUTION
SUMMARY - use a simple circuit to create a negative voltage from the MSP 430 ACLK.  This negative voltage then drives the LCD Vee on pin 3.   The ACLK ***must*** be enabled in your software.   You can enable ACLK in either CCS or Energia.

Current measurements and waveforms are shown below.

Clearly, you have to sacrifice use of the PORT P1_0 (red LED) pin, but this can be a reasonable trade off.   If you can't do this, then you can use another MSP output to generate the negative voltage.

HARDWARE -

a-- create a simple negative voltage charge pump (two capacitors, 2 diodes).
- input is from MSP ACLK pin +- 12 KHz.
- output is to LCD Vee, which is almost always pin 3.

b-- here is a simple charge pump circuit:
(please do google this if you've a question, it is really simple)

| |                    ||
ACLK -->---| |---------O----------||<|------O--------->---- to Vee, pin3 on LCD.
| |         |          ||        |
C1         |        D2          |
|                    |
===                 =====
D1    \ /             C2  =====
=======                 |
|                    |
|                    |
|                    |
GROUND               GROUND

Typical component values are not critical. I used:  C1 = C2 = 1 uF.   D1 = D2 = Germanium 1N569.
Results are below.

c--  then connect the LCD module as follows:
- pin1 to GROUND
- pin2 to +3.3 volts (same line as the MSP430 positive supply)
- pin3 to the charge pump inverter circuit, above in para 'b'.
- all the other pins (RS   R/W    E   DATA0-7) to PORT pins suitable for your application.

d--  the scope output of the waveform in actual operation is shown in the scope screen shot below.

SOFTWARE -  you must enable the ACLK / VLO to be output from the MSP430:

P2SEL |= 0x07; // aclk P2_0 pin8.  smclk P2_1 pin9.   ta0 3
P2DIR |= 0x07;
BCSCTL3 = LFXT1S_2; // aclk = vloclk abt 12khz

This can be done in CCS or in Energia 'SETUP'.

CURRENT CONSUMPTION -
.... With this configuration:
- MSP430F2272 at 1MHz,  PORTs 1,3,4 all set to OUTPUT LOW.  PORT 2 set up as above.
4 x 20 LCD with blue backlight (powered by 3.3volts to a reasonable back light output)

....  here is the typical current consumption I measured:

- MSP430F2272 running tight loop, 1MHz    1.1  mA
- loop + charge pump                      1.55 mA   <<<< so, only 0.45 mA for the charge pump
- loop + charge pump + LCD backlight      9.44 mA    (these are high efficiency blue LEDs)

PLUS POINTS -
- only one power supply needed for a simple 430 + LCD project.
- rather low current consumption
- even lower current consumption by switching ACLK off
(but then you should turn off the LCD Vcc +3.3 - use another PORT pin as the LCD Vcc)

MINUS POINTS -
- you have to use a MSP430 port pin

ALTERNATIVES -
- use a timer / int driven PORT pin to replace the ACLK.
- LCD too much contrast? :
- insert series resistor to LCD Vee from charge pump to reduce neg volts
- LCD not enough contrast? :
- use a negative voltage triple / quadrupler.

Hope someone can use this idea.....

Cheers.

##### Share on other sites

Nice. Of course, the launchpads run at 3.5V, and have USB/5V available, which in my experience is enough to power the lcd, with logic at 3.5V talking just fine with them. But that's kludgy so nice write up.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
• Blog

• #### Activity

×
• Create New...