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Topic: Pulling pins high or low, cap or resistor? (Read 2644 times) previous topic - next topic

GilchristT

Complete newbie question here but I've seen a few different ways of pulling pins high and low and was wondering what was the standard or recommended way?

For example, I've build my own Arduino with a pre-bootloaded ATMega328p 28 pin DIP and have the RESET pin connected to a capacitor and then on to the DTS line of the FTDI cable.

However, I've seen other schematics where the pin is tied to the 5V line via a 10K resistor.

I'm looking at another component (a GL850G USB hub IC) and need to tie some of the pins high or low. How would this typically be done? It'll be a fixed solution, no need for the input to be switchable or anything like that.

Thanks in advance for any help, it's probably a stupidly simple query but I haven't been able to find anything definitive online.

Tommy

cmiyc

The function of the cap on the RESET line is to allow the reset signal to be pulsed when DTR goes low.  The cap keeps the signal low for a short period of time before it recharges and the signal is pulled up by the resistor.

So that is a special case.

As for tieing a signal high or low, the data sheet will generally say if a resistor needs to be used.  ( generally it isn't.)

Pull up resistors are used to keep non-connected inputs from floating.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

MarkT

If an INPUT signal is not connected to anything else simply connect to GND or +5V - All logic chips these days are CMOS which is completely happy with this.  If its an output, a resistor is mandatory to prevent overloading the pin.  If the pin might be either input or output (such as with a microcontroller) then a resistor is the safe way. 

The Arduino chip has built-in pullup resistors, you simply enable them thus:
Code: [Select]

  pinMode (pin, INPUT) ;
  digitalWrite (pin, HIGH) ;

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GilchristT

Thanks guys, much appreciated.

The data sheet is light on detail and the manufacturer is ignoring my e-mails (which is probably to be expected, my requirements might run to a grand total of 100 units  :))

Time to experiment, they're only a pound or so each but it's getting the damn things onto a board so I can play with them...

Tommy

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