With respect to the Propeller, what might "push-pull driving" be?

ElectricAye

I'm trying to design a stepper motor controller using the L6208 chip and on page 10 of its data sheet there's a caveat about how to wire up the ENABLE pin. I want to drive the L6208 directly with the Propeller so I'm assuming I would use the "push-pull" driving scheme it shows near the bottom of the page, but... I'm not really sure about this. As I (thought) I understand this, the +3 volt signals coming out of the Prop would be good enough to trigger the ENABLE pin. But why is it called "push-pull" when the signal is going from 0 to 3 volts and maybe back to 0?

Am I missing something here?

Leon

I think they mean driving the input from an output that can either source or sink current. The Propeller has CMOS outputs which should be OK

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Leon Heller
Amateur radio callsign: G1HSM

ElectricAye

Leon said...
I think they mean driving the input from an output that can either source or sink current. The Propeller has CMOS outputs which should be OK

Thanks Leon. That's comforting to know. I always get nervous when electronics people start using words like push and pull. You know, current runs in one direction, electrons run in another, and there are holes and positive hole migrations and... and.... it makes me nervous.

cheers,
Mark

Graham Stabler

It is rare that you have to consider electron currents so just stick to conventional current unless you are going into semiconductor physics [noparse]:)[/noparse]

You often see pairs of transistors called push-pull pairs. These let the output pin be connected to either the supply rail or ground and this is what allows the pin to push current out (by making the pin high and connecting through a load to ground) or pull it in, by connecting through a load to the supply. That is what the propeller can do.

The open collector type which has only one transistor can only connect the pin to ground so if you need it to go high when the transistor is off then you need a pull up resistor. The transistor and resistor form a potential divider where the transistors resistance goes from very high to very low as it switches on.

Graham

Peter Jakacki

I think push-pull was one of these really fancy electronic terms in elder days to describe the fact that the output stage used a whole two transistors (or tubes!). In the old audio amplifiers they used to use drive and output transformers and with really advanced technology you could drive the center-tapped transformer from both ends, pushing with one transistor, while you were pulling with the other, wow! The original topology is different from what we use now but the term has stuck so the output stages are only capable of pushing (sourcing) or pulling (sinking) but don't really push-pull at the same time.

EDIT: A slight correction as a center tap of the transformer was normally taken to the supply and the transistors pulled from either side but it still appears as push-pull through the transformer.

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*Peter*

Post Edited (Peter Jakacki) : 3/16/2010 1:21:18 AM GMT

ElectricAye

Peter Jakacki said...
I think push-pull was one of these really fancy electronic terms in elder days.... but the term has stuck so the output stages are only capable of pushing (sourcing) or pulling (sinking) but don't really push-pull at the same time.....

Okay, that helps explain it. I couldn't figure out why a microprocessor pin would want to do both at the same time to control the logic on a single pin on such a chip.

Thanks, Peter.

And thank you, too, Graham, for providing the transistor explanation.


Mark