Single resistor or dual voltage div for TFT level shift?

Using Arduino Displays
1

I've seen many (probably the majority) of articles that just have a single resistor between Uno and TFT pins.

Surely these can't be correct. It takes a pair of resistors to form a voltage divider to level shift from 5V down to 3.3V

All a single resistor will do is reduce the current, not change the voltage.

What am I missing here?

2
  • For anyone to comment, we need to see the articles you referenced.
3

What happens to the voltage drop across the resistor as the current changes? Look at Ohm's Law as the reference.

Some displays only need Vcc to that pin to work properly. Changing that voltage in a very small range will change the contrast. Changing the Vcc will also change the operation of the LCD. That is from experience, not from any book.

4

@gilshultz,
Actually, I measured the input impedence to one of the TFT pins and it's greater than 20MOhm, so a 1k resistor doesn't even drop the current. I don't see how a single resistor makes any difference.

5

@LarryD,
Here are a sample of posts/articles that propose a single resistor:
TFT 1K Resistor
Another 1K resistor
Yet another proposing a single Resistor
And here is one that has a true voltage divider:
Voltage Divider solution

A simple google search brings up many, many more examples with a vast majority just using/proposing a single resistor solution.

Not sure how a single 1K resistor makes any difference when the TFT Pin has a resistance greater than 20M

6
  • Resistors on a these pins can sometimes be a simple point of isolation from one component to another.
  • However, resistors on inputs in this case are used as a current limiters.
    5v signal current from the Arduino goes through the series resistor (RSER) then through the display input protection diode to 3V3 (VDD) on the display.
    Without the resistor, 5v on an Arduino output pin would only see the display’s input protection diode to 3V3, hence a short.

IMG_3055
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  • A Better design would be to use gates for level translation.
  • LVC can be used as 5V to 3V3 logic level shifter. i.e. 5V logic --> 3.3V.
  • Use the SN74AHCT1G125 for 3V3 to 5V.
8

Hello Larry,
Having thought more about this ( and your illustrative circuit diagram was very helpful ), I can see how this can work with a single resistor.
I had been measuring the resistance between the TFT logic input and ground. This was an incorrect assumption on my part. The diode to 3.3v makes all the difference.

Thanks