On 18/06/2011 22:45, David wrote:
> I am not particularly tight on space nor constrained by a few extra
> pennies, but it would be good to know which combination of (A|B)+(Y|Z)
> is correct and for what reasons.
>

I would say there is no "correct" way, as it depends on the situation.=20
You might gather this from the differing replies here (and on the net).
If you are dealing with a noisy environment, or need to ensure the FET=20
is always off unless the PIC instructs otherwise, then a resistor from=20
gate to ground would be useful. Leaving a FET gate floating is never a=20
good idea (unless you are making a static detector* or similar), so if=20
the FET does something important you would want to make sure nothing bad=20
happens at any point when the PIC may not have control of the gate (e.g.=20
turn on/off).
If you want to protect outputs, or reduce current surge at turn on=20
(caused by charging of gate capacitance) then you might want to use a=20
series resistor. This will slow turn on, which might be an issue in some=20
cases.
For example in a high speed environment you might have to balance the=20
series value somewhere between current surge causing noise, and=20
switching speed (and noise pickup if you go too high).
With a low end PIC the speed/current surge is rarely an issue but in the=20
case of e.g. an ADC switching at 200MHz, then it has the capability to=20
disturb it's own ground (and sensitive analog input) when switching many=20
digital outputs simultaneously.
As long as you know *why* each option might be used, then you can make=20
the decision according to your particular design.

* http://www.zen22142.zen.co.uk/Circuits/Misc/staticdet.htm

--=20
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