Author Topic: Understanding the Vih specification driving a 5V character LCD with 3.3V logic  (Read 5233 times)

PJoyce

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I am powering a 2x16 LCD with 5V but interfacing it to a processor powered by 3.3V.  The pins on the processor are 5V tolerant so the D0-D7 lines are safe to interface the processor to the display.

My question is the Vih specification, on some data sheets (ex.  NHD-0216PZ-FL-YBW) the Vih min. specification is 2.2V so driving it with 3.3V logic should be fine.  On another (ex. NHD‐0216T2Z‐FSY‐YBW‐P) the Vih min specification is 0.7*Vdd = 3.5V so not OK.  The data sheets for both reference either the SPLC780D driver (2.2Vih min) and ST7066U (0.7*Vdd).

Is there a way to know if a particular build has the SPLC780D driver chip?  The main page for all of the 2x16 character LCDs state that the controller is the ST7066U for all of the modules, so I run the risk of bad operation with 3.3V logic?

Michael_L

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Can you provide a full part number of the display you are using?

PJoyce

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The part numbers in the original post ( NHD-0216PZ-FL-YBW &  NHD‐0216T2Z‐FSY‐YBW‐P) were taken from the data sheets and are believed to be complete numbers.  I am not using either yet; I need to be sure that the Vih spec will not change from batch to batch in production.

Michael_L

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My apologies, I thought you were just referring to them as examples.  From batch to batch of the same display part number, the Vih spec will not change.  The Vih spec can vary between different parts, but not between 2 displays of the same part number.

Andy Peters

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I am powering a 2x16 LCD with 5V but interfacing it to a processor powered by 3.3V.  The pins on the processor are 5V tolerant so the D0-D7 lines are safe to interface the processor to the display.

My question is the Vih specification, on some data sheets (ex.  NHD-0216PZ-FL-YBW) the Vih min. specification is 2.2V so driving it with 3.3V logic should be fine.  On another (ex. NHD‐0216T2Z‐FSY‐YBW‐P) the Vih min specification is 0.7*Vdd = 3.5V so not OK.  The data sheets for both reference either the SPLC780D driver (2.2Vih min) and ST7066U (0.7*Vdd).

Is there a way to know if a particular build has the SPLC780D driver chip?  The main page for all of the 2x16 character LCDs state that the controller is the ST7066U for all of the modules, so I run the risk of bad operation with 3.3V logic?

I have the same question. The specific display I want to use is the NHD-0216HZ-FL-YBW-C, which I like because of its smallish size. But the data sheet shows typical CMOS logic thresholds (0.7 * VDD for logic high), which doesn't work for a 3.3V processor.

The data sheet says that it has the ST7066U controller. The data sheet for that part says that it supports a supply rail ranging from 2.7 V to 5.5 V, and the DC characteristics are given as the same levels noted in the display module datasheet. So it seems to me that I should be able to run this guy from a 3.3 V rail. What am I missing?

And there's this, the NHD-0216HZ-FSW-FBW-33V3C. It's the same size as the -FL-YWB-C, and has a VDD spec of 3.3V. Same controller -- the ST7066U. The only difference I see is in the backlight LED supply requirement.

So based on this, I think I should be able to run the NHD-0216HZ-FL-YBW-C on a 3.3V logic supply, with a 5V backlight supply through an appropriate resistor. Yes? No?

Thanks!

-a

PS: I also like the NHD-0220FZ-FSW-GBW-P-33V3, which is even smaller, but I can't find any of the 0.050"-pitch cables anywhere.

Michael_L

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The logic levels the ST7066U controller can accept are not the only determining factors for being able to use a display with different voltages.  Based just off the controller specification regarding the voltage/logic level range, it seems like the 5.0V display would work fine with 3.3V.  However, the liquid crystal (the display glass) is different between the two.  Therefore, the voltage required for the glass is different.  You will notice a different VLCD rating in the datasheets, this refers to what I have just described.

If you are interested in a custom display to better suit your needs, feel free to email nhtech@newhavendisplay.com with your requirements and we can work with you to determine feasibility and cost.

Andy Peters

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The logic levels the ST7066U controller can accept are not the only determining factors for being able to use a display with different voltages.  Based just off the controller specification regarding the voltage/logic level range, it seems like the 5.0V display would work fine with 3.3V.  However, the liquid crystal (the display glass) is different between the two.  Therefore, the voltage required for the glass is different.  You will notice a different VLCD rating in the datasheets, this refers to what I have just described.

Ah, I see that. VLCD is spec'ed on the mechanical drawing page of each data sheet.

The key then is that the glass sets the required drive, so I can't modify the oscillator frequency or the drive biases to have it work with a 3.3 V supply.

Quote
If you are interested in a custom display to better suit your needs, feel free to email nhtech@newhavendisplay.com with your requirements and we can work with you to determine feasibility and cost.

I would imagine that I wouldn't have the volume required for a custom part.

-a

Sylvain

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Hello,

Like PJoyce, I  want to use a 2x16 LCD powered with VDD=5V (NHD-0216K1Z-FSW-FBW-L) but with 3V logic interface. The specification of this module give me Vih input level = 2.2V mini (p5 of this specification) regarless of the built-in controller that can be either SPLC780D or ST7066U (p2). However, ST7066U has Vih input level = 0.7xVDD = 3.5V (see DC characteristics at page 32/45 of datasheet http://www.newhavendisplay.com/app_notes/ST7066U.pdf). So, is the module really compatible with a 2.2V input voltage level when built-in controller is ST7066U and power supply is 5V ? If not, it seems better to distinguish Vih depeding on the controller.
Note : competitors have also 2x16 LCD with ST7066U built-in and input level = 2.2V mini!

Saurabh_B

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The ST7066U can be used with either 3.3V or 5V depending on which LCD display it is used in. In this case the VDD is 5V so your minimum Vih would be 3.5.

 

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