Newbie questions: TTL or CMOS?
Tony Nixon email (remove spam text)
Nathan Burnside wrote:
> Are the PIC I/O pins TTL or CMOS? The data sheets say that the PICs are
> CMOS, but it also says the I/O pins have a TTL buffer. Does this mean that
> I need to treat them strictly as TTL? What kind of situations can damage
> the I/O pins on a PIC? (aside from the case of voltages over Vdd and under
> Vss) If I set an output pin high when it is tied to ground, will it simply
> source 20mA without damaging the chip, or will it cause a short to Vdd and
> fry the chip? Thanks for your help.
Depends which pin.
Most appear to be TTL in and CMOS out. Some have Schmitt Trigger input,
and there is an Open Collector output usually on RA4.
Because they (not all) are TTL in and CMOS out, I would assume that they
interface directly to TTL and CMOS.
Check out the back of the data sheets for specific IO pin information.
I've had shorts on pins and the chip seems to survive, but this may not
always be the case. Just avoid overloading or overdriving the pins, and
don't rely on them delivering a finite current source. Some appear to
survive reversed supply voltage as well.
The chip is more likely to get damaged by static discharge from you, or
by inductive spikes caused by insufficient protection.
See also: www.piclist.com/techref/microchip/begin.htm?key=pic
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