Pins with pullup resistors built in

[bigone5500]

On the 328P-PU, do all digital and analog pins have pullup resistors built in? If not, which pins do have them?

 

[dorke]

The answer to your question is "hidden" on the AVR datasheet on chapter 14 (page 75).

Why on earth do you think an analog input should have a pullup resistor?

 

[bigone5500]

The answer to your question is "hidden" on the AVR datasheet on chapter 14 (page 75).

Why on earth do you think an analog input should have a pullup resistor?

Because I am new to this???

And I have looked at the datasheet on the chip. Didn't really know what I was looking at, hence this question.

Thanks though.

 

[dorke]

O.K
The term pull-up(down) resistor is a term coined in the digital world.

It is related to the inputs of digital circuits-in which we would want to decide the input logic state of an un-used input as well as a default logic state for a used input(weak pull).

It is also related to the outputs of digital circuits in devices(sometimes used for wired-or/and) which have open-collector(drain),open-emitter(source),
as well as devices with Tri-State ,the state the output isn't driven(buses etc.)

The term pull-up(down) resistor is meaningless in the analog circuit world.

Hope this helps clarify things

 

[bigone5500]

X-IV!

I did follow the link above and carefully read through a couple pages on the digital pins. I finally saw what you referenced.

Thanks!!

 

[(*steve*)]

In the PIC environment, making a pin an analog input disables the input buffer, so the setting of a weak pull-up is mooted.

However, it appears that atmel don't follow the same strategy because there is a warning that setting weak pull-up on a pin set for analog input will affect the values reported by analogread.

I would recommend a deep read of the datasheet(s). I have read the datasheet again, and while I can see that the pull-up can be functional on an analog input pin, I didn't find the warning. However I did find people using this feature to read light levels using an LDR (where the weak pull-up forms the upper part of a voltage divider).

Note that this doesn't detract from anything @dorke has said. However, it also doesn't mean that you can't think up a situation in which this pull-up could be used for some purpose in conjunction with analogread.