MSP430FR4133: Segment LCD contrast

Hi furieux,

Thanks for interest in FR4133. The power supply applied at Vcc should be in specification so one AAA battery is not working with MSP430FR4133. In your case maybe need add a Boost.

regards
KC

I probably didn't wrote correctly what I meant.
For MSP430FR4133 I'm applying 3.3V (not 1.5V) but looks like that is not enough for that particular LCD.
What I mentioned before was my wonder - how originally (with the original thermometer) LCD is working with only one AAA battery (with its own unknown mcu) but not working with 3.5V from MSP430FR4133?

Hi Furieux,

Sorry for reply late,  for LCD contrast tuning need to check VLCDx setting, please refer to the description on UG 17.2.7.3. and also here is another App Note for your reference: Designing With MSP430™ MCUs and Segment LCDs

A typical approach to determine the VLCD for good contrast, is to use the threshold voltage when the contrast is 10% and use this with the VRMS,OFF/VLCD ratio from the user's guide table to calculate a recommended VLCD using this equation: VLCD = Vth,10%/(VRMS,OFF/VLCD). The Vth,10% is a characteristic of the fluid used in the LCD display, so it varies with the display. In your case VLCD=3.5V maybe not work well with the LCD, if not able to get the Vth from datasheet of the LCD, maybe you can try with different setting on VLCD.

Another idea is the fLCD(frequency of the LCD)setting, the required fLCD can be calculated from fFRAME(frame frequency) which also from the datasheet for the LCD.  please refer to UG 17.2.4 LCD Timing Generation.

regards

KC

That's the main problem - I don't have a datasheet for that LCD. As I already said, I took it off from a cheap digital thermometer which works on one 1.5V AAA battery.
I used oscilloscope to play with original microcontroller from that thermometer and I can see that on COM0 port it has like 32 Hz and also it shows 4.8 volts! If using multimeter to measure voltage between GND and COM0, it shows 2.3V. But MSP430FR4133 is showing 1.3 V which is not enough for that particular LCD even with charge pump enabled.
Also I did a test and attached this LCD to different MCU (from Microchip) and different LCD driver (HT1621) and applied 5V to the circuit. Then this LCD worked as expected with good contrast.
So what else can I measure to understand why this LCD works good on 5V, but not on 3.3V and how is it possible that originally it works with 1.5V?

Hi furieux,

Form the data you have measured I will say this is a 5V LCD, that's why it works well as expected while applied 5V to the circuit, could you please try to applied 3V to HT1621 and see the LCD behavior? Thanks.

Regards
KC

Hi KC,

By applying 3.3V to HT1621 I get the same contrast problem.

So that means initially I misunderstood the purpose of charge pump. Since HT1621 does not have charge pump inside, I thought that MSP430FR4133 will do the job and bring up the voltage for LCD.

Now, if I understand correctly it is not possible to use this particular LCD with MSP430FR4133 mcu, right?

Hi KC,

Thanks for the clarification.

But just because of curiosity - how did they manage to bring up voltage from 1.5V (1xAAA battery) to almost 5V for that LCD and in the same time keep continuously approx. 14 micro-amps for that particular device? That's a perfect current draw for a battery powered device which probably has some dc/dc upconverter?

Hi KC,

Ok, I got it.

Thank you for the support!