Using MCU LCD data line I/O as analog inputs

Mosaic

Well-Known Member

• Jun 15, 2012

• #1

Is it acceptable to dual use the lcd data out lines as analog inputs as well during program execution when the LCD is not being fed data?


I want to use 47K NTC thermistors as temp sensors on PORTB0,1,2,3 which are also the LCD data lines.

 

Last edited: Jun 15, 2012

ronsimpson

Well-Known Member

Most Helpful Member

• Jun 15, 2012

• #2

You need to read the LCD data sheet very carefully.
>Pull up resistors? 10k +/-30%?
>Input leakage? +/-20uA
How will this effect the 47K NTC and what ever circuit you add to drive the NTC.

If the NTC (+ resistor or whatever) are on the data lines, can the (micro and LCD) pull the data lines high and low.

 

Mosaic

Well-Known Member

• Jun 15, 2012

• #3

Well I am short just 2 adc channels. I guess I can reuse the analog inputs sharing a couple of the data lines once the thermistor doesn't load the line by more than 10K. I was intending to measure the temp by loading each line with a 10K pull up and the 47K thermistor pull down.

Judging from the datasheet:
**broken link removed**

The highest temp of the thermistor should not exceed 65 deg C. Which is ok for my app.

 

ronsimpson

Well-Known Member

Most Helpful Member

• Jun 16, 2012

• #4

Does the LCD have a 4 bit mode?

 

Mosaic

Well-Known Member

• Jun 16, 2012

• #5

I am using it in 4 bit mode.....still short 2 adc. Those data lines ought to be high impedance compared to the sub 50K impedance of the thermistor.

 

ronsimpson

Well-Known Member

Most Helpful Member

• Jun 17, 2012

• #6

So no data sheet on the LCD you are using....

I looked up two 3.3V and two 5V LCD displays and all have +/-1uA of input leakage on the data lines. Most likely the input current is much better than that.

Do you need help to know how much +/-1uA will effect your reading?

 

Mosaic

Well-Known Member

• Jun 17, 2012

• #7

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