What is the purpose of "Symmetrical Drive" on Port A ?

Bean

I was wondering what the purpose of the "symmetrical drive" characteristics of the port A pins is for ?

What is it, and what is it used for ?

Inquiring minds want to know [noparse];)[/noparse]

Bean


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JonnyMac

Found this comment: Outputs can sink and source 30 mA (subject to overall device power limit), with those on Port A featuring symmetrical drive (i.e., centered about VDD/2 under any load). This feature is useful for driving speakers and other pseudoanalog functions such as using a PWM to implement a DAC.

... on this web page: http://www.circuitcellar.com/pastissues/articles/Tom100/text.htm

I was curious, too, so I Googled: "symmetrical drive" +SX

Phil Pilgrim (PhiPi)

Symmetrical drive is considered a "feature" because it's more difficult/expensive to fabricate PMOS transistors on-chip with drive characteristics equivalent to their NMOS counterparts. (I'm not sure why, though. It might be a size issue or something to do with symmetrical gate thresholds and/or doping issues.) Consequently, budget microcontrollers often have more pull-down capability than pull-up capability on their output pins. It also explains why some MOSFET H-bridges use n-channel devices in both their lower and upper legs, as it's easier to include a gate-voltage-boosting charge pump than to incorporate high-current p-channel output drivers.

(I'm sure Beau or Chip could explain this much better than I just did!)

-Phil

Dennis Ferron

Phil: The reason that it is harder to make P-channel mosfets carry current as well as N-channel has to do with the fact that N-channel relies on electron current, in which individual electrons move charge, but P-channel has to use "hole current" in which many electrons get out of the way to move a hole between them. To understand why hole current is harder to push than electron current, think about this analogy:

Imagine a person walking down the aisle in an empty movie theater - he can walk right down the aisle with little resistance. That's an N-channel mosfet.

Now imagine a nearly full movie theater, and an usher asks a crowded row of people to free up the seat on the end. There's a empty seat on the other side, but to "move" this "hole" out to the far end, everyone has to get up and move one seat to the right. It would take a while and you would encounter some resistance. That's a p-channel mosfet.

terryzz

As you can see the data sheet shows the symetrical nature of the A port· compared to the others