Pullup/pulldown resistors

Larry C.

I'm wondering if the propeller chip has internal pullup (or pulldown) resistors built in, or if unused inputs are just left floating?
Can't seem to find anything definitive on the data sheet.

If no internal terminations, what's the recommended treatment for unused inputs? Most CMOS logic families call for unused
inputs to be terminated with high-value resistors, but with all the pins on the propeller chip, this could eat up a lot of circuit
board real estate.

Thanks,

Larry C.

Phil Pilgrim (PhiPi)

The Propeller has no internal pull-up/-down resistors. If you have unused pins and don't wish to terminate them, you can always make them outputs so the pins don't float.

-Phil

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'Still some PropSTICK Kit bare PCBs left!

Larry C.

I was more concerned with the issue of possible damage to the chip. Most logic chip manufacturers warn against
letting CMOS inputs float in the "no man's land" between logic 1 and logic 0, as this can introduce a boat load of
noise into the system and excessive power dissipation, leading to potential chip failure in time.

(The project I'm working on requires that the board with the Propeller chip be able to be disconnected from the
rest of the system, thus leaving the inputs open -- so I can't just make everything an output.)

Any thoughts on this?

Larry C.

Phil Pilgrim (PhiPi)

I see. In that case, you should add pullups or pulldowns, as appropriate. These don't necessarily have to take up a lot of room. For surface mounting, resistor arrays in multi-pin 0603s are quite small and cheap:



For thru-hole boards, bussed SIP resistor arrays fill the bill:



Note that SMD arrays of discrete resistors are way less expensive than the bussed variety.

-Phil

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'Still some PropSTICK Kit bare PCBs left!

Post Edited (Phil Pilgrim (PhiPi)) : 8/7/2008 9:17:41 PM GMT

Larry C.

Thanks for the quick reply, Phil (and for the pix). I am trying to keep the parts count down on this project, but a bunch of pullup resistors are certainly in order. Didn't know that SMD resistor arrays were available. I'll probably go that way.

Larry C.

Ken Peterson

I don't think letting the inputs float would cause excess power dissipation on the Prop. With many logic devices a floating input will cause an output to switch endlessly, which will cause excessive power consumption and electrical noise.

I have not seen anything in the Propeller documentation about tying unused inputs.

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Mike Green

Letting inputs float will modestly increase the power demand of the Propeller. Since the input circuitry is such a small amount of the overall power demand of the Propeller, it normally won't be an issue. When you're trying to run the Propeller off batteries and are idling most of the chip and running the rest of it off the RCSLOW internal clock to save power, this extra power demand can become a significant part of the power "budget". One simple and cheap solution would be to set all the unused I/O pins to outputs in your initialization routine. That will prevent them from being floating inputs. Any I/O pins that are actually used as inputs should have either a pullup or pulldown resistor or should be connected to a logic output of some other device that's always powered up when the Propeller is powered.

Paul Baker

Mike is correct, if a pin is left floating and is an input it will consume more power than if it were strapped or made an output. The reason the datasheet didn't mention it is that it is a common property with cmos inputs which do not have internal pullups, and it causes no harm to the chip. I will add it to the list of things to do for the datasheet.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

johnfl68

OK, so what is Parallax's recommendation for unused pins in a finished design, sink to ground with 1K resistor, and set unused pins to output? Or something else?

Thanks!

John

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Inigo Montoya: Let me 'splain. (pause)
Inigo Montoya: No, there is too much. Let me sum up.
The Princess Bride (1987)

Mike Green

I can't speak for Parallax, but I'd use 10K resistors to either Vss or Vdd. If you forget and make one of the terminated pins an output, it won't draw much current, yet won't be too sensitive to induced noise on the PCB trace.

Paul Baker

Either will work just fine. Some people like the hardware solution, since it will be handled regardless of what the program does. Some people like to set the pin to output because it saves them the cost of the resistors in thier final design.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.