veryalive 49 Posted February 26, 2016 Share Posted February 26, 2016 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. cde, roadrunner84 and dubnet 3 Quote Link to post Share on other sites
cde 334 Posted February 26, 2016 Share Posted February 26, 2016 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. Quote Link to post Share on other sites
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