# How to drive 1 x 168 LCD using 4 x 32 LCD driver?

I have to drive an LCD that has only single COM and 166 segments:

Somehow using 4 x 32 LCD driver:

So I am going to use five such LCD drivers (32+32+32+32+32+32) by using only one COM0 from each of them tied all together with LCD's common but not sure whether it will work?

Do I understand correct that LCD driver operates in following way: it keeps COM constantly HIGH (5V) while asserting SEGMENTS pins down to GND he wants to turn ON?

EDIT: question changed - i would like to understand whether 1x168 LCD might be driven by 6 pcs of 4x32 LCD driver?

EDIT2: found some piece of schematic of 4x60 LCD driver with 1x54 LCD. Looks like driver's COMs may be tied all together:

as explained in comments looks like LCD driver can be used with LCD having less amount of segments and commons. Thank you.

EDIT 4:

Seems I found cascaded LCD driver that will do the trick as suggested from comments:

Thanks a lot guys! You saved me from a fatal mistake!

• 4x32 is 128, it seems you don't have enough pins to drive all the segments. Commented May 9, 2022 at 10:33
• @Lior Bilia, oh, sure, 5 x 32 + 6 directly from MCU. Will it work? Commented May 9, 2022 at 10:38
• Edit your question to reflect that, it still says 4x32. Alternately, for so many digits, a graphical LCD with a controller would be easier to drive. Commented May 9, 2022 at 15:40
• Why must you use the HT1621? That's making things more difficult than they need to be Commented May 9, 2022 at 17:01
• @Reinderien, was unable to find LCD driver that able to drive 1x168 LCD. Commented May 12, 2022 at 21:11

I don't have time right now to answer both things you're asking here ("does the driver work this way?" and "can I drive the remaining segments straight from the MCU?"), but I can answer one of them.

Driving an LCD directly from an MCU isn't a great idea.

LCD segments can be driven by a DC voltage, but this is not recommended as it will cause damage to the LCD over time (and not a terribly long time either, I believe. Maybe even just a few hours.). The proper way to drive an LCD is with an AC voltage, flipping polarity at a high frequency. The IC you mention generates appropriate drive signals using an internal oscillator, and if you wanted to drive the remaining segments from the MCU you would have to also use some sort of oscillator to produce the correct AC signal.

That's not impossible, and not really even very hard, but it would be easier to just use one more driver. Or even better, a different driver.

As @Justme commented below, it looks like the driver you propose might not be usable for this LCD anyway--the HT1621 expects multiple common terminals, and I'm not sure you can use it for something with a single common.

• The display has one common and 168 individual segments. Thus it can be driven with single HT1621 with 4 commons and 32 segments. And they have no syncronization pins so multiple chips can't be chained. At least a chainable chip should be used to get 1 common and 168 segments. Commented May 9, 2022 at 16:30
• @Justme I didn't give that last paragraph a whole lot of thought, nor did I worry much about whether the proposed driver would work at all--I was mostly wanting to advise the asker about how you need AC to drive an LCD. I'll remove the stuff at the end. Commented May 10, 2022 at 15:19
• This answer is actually a very important piece, because if they were ordinary digital signals then connect 128 individual segments to qty 32 of 1:4 mux, with the select bits generated from COM[1:4] by a priority encoder, and the single COM generated from COM[1:4] by an AND gate. That's a lot more trouble than using the correct driver chip, of course. But massively more complicated if signals are continuously varying instead of quantized. Commented May 12, 2022 at 20:57
• @BenVoigt They are quantized; an LCD is usually driven with a square wave because that's simpler. But it has to go both positive and negative, which logic circuits generally don't do. Commented May 12, 2022 at 21:08
• @Justme, could you please share some light why multiple LCD drivers can't be tied to same common of LCD? I was thinking that COMMON pin of LCD is constantly HIGH? Am I mistaken? Commented May 12, 2022 at 21:14