Simple, Passive Interface from 1.8V Output to 3.3 volt circuit

I am looking at a chip which only uses 1.8 volt power and has diode clamps to the power and ground rails. I want to drive a 3 volt LCD display. The chip maker tells me, "Swinging 3.3v open drain is pretty easy". Maybe I am having a brain cramp, but I can't think of how to do this in a way that doesn't require a transistor or buffer. That was the context, I stated I could use other devices that don't require the use of buffers for 3.3 volt drive.

Obviously the "easy" way is to drive an SPI or I2C port device with enough outputs to control the LCD, or even to use an LCD driver chip. That's fine if that is what I have to do. I just don't want to add another chip or two or three driving up the cost by another several bucks. At that point I would just use a chip with LCD compatible drivers built in.

Am I missing something? Is this easy to do without buffers or external driver chips?

Rick

Reply to
rickman
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It's rather easy to use a low voltage uC to signal/drive higher voltage logic.. If the IO line needs to be converted then a simple NPN can be used with the emitter to common and the base being driving from the IO line via a small bias R. This NPN on the collector side will then be the sink for the 3.3 volt device.

If the IO output can not be inverted then you can use a NPN and connect the emitter to the IO output. The base will then be biased with a dropping R from the 1.8V supply.. With this, it'll only conduct when the Emitter gets pulled low via the IO line and the collector will act as the pull down.. The only problem with this is, the current limit of what you pull down is limited by what the uC IO output can handle, but, it'll give you the level shift you desire..

In any case, I like serial LCD displays, it limits the required IO lines. The only problem there, is response time and it could be important if you are doing graphics on the LCD display in real time.

This maybe of some interest to you.

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Jamie

Reply to
Jamie

Thanks for the reply, but I am aware that you can use a buffer or transistor to buffer the output signal. I have no interest in that since I can just select an MCU that will directly drive an LCD if that is what the best approach is.

I was told by the maker of a 1.8 volt I/O MCU, as I stated above, "Swinging 3.3v open drain is pretty easy". I am assuming by this he means it is pretty easy without adding a small truck load of parts and none of them costing over a penny. The LCD I am looking at driving has over 30 segments if I use a static device. If I bail on the 1.8 volt part and use a standard MCU with LCD drive I can even use a multiplexed drive bringing the pin count below 20 for an even more complex display.

If he were talking about switching a 3.3 volt TTL level input, then I could agree since the input only needs to swing from below 0.8 volts to

2 volts and I might be able to do that with a combination of resistors (I haven't actually checked to see how much noise margin is left, if any). But an LCD needs to swing close to 3 volts minimum.

I think this company is a bit unusual... to put it nicely. They think it is better for you to add external devices than it is to "complicate" their chip with real world I/O voltages and hardware I/O peripherals. That may work for async serial ports and SPI, but so much of today's hardware needs more performance or just plain needs to simplify the bill of materials.

Rick

Reply to
rickman

He meant as you understood it, you need to use a truck load of parts if you plan on implementing many IO lines up to 3.3 volt levels. Or, use a dense level shifter logic chip.

I think it would be cheaper to use a 3.3 capable processor. There are DC DC converters that will swing a single cell source up to 3.3V if the power source is a concern.

Jamie

Reply to
Jamie

Yes, I have come to the same realization. No, the power rail is not the issue. The issue is with the MCU with lots of internal horsepower, but very, very poor I/O. I can live a rich full life with a cheaper MCU with LCD drivers built in. But it won't be as much fun I guess... Isn't fun what it is supposed to be all about?

Rick

Reply to
rickman

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