Hi,

Are current-limiting resistors required on the output of a TC74HC14 schmitt trigger towards the input pins on a PIC 16F MCU?

I googled, but don't seem to get any hits on that subject.

Thanks!

 

Not if they both run off the same power supply.

 

Not if they both run off the same power supply.

Yes, both will run on 5V from USB.

 

Yes, both will run on 5V from USB.

OK. No resistors required.

 

A single CMOS input, like the ones on a PIC 16F device, are normally very high impedance and consume very little current from the driving source. If on the other hand you try to drive a large number of CMOS inputs in parallel, then you have a much lower impedance from the driving circuit's point of view, which might require some added resistance. This would happen on the address lines for dynamic RAM chips (e.g. 16K x 1) where you might have 64 chips in parallel.

 

If on the other hand you try to drive a large number of CMOS inputs in parallel, then you have a much lower impedance from the driving circuit's point of view, which might require some added resistance.

What's the purpose of the added resistance?

 

What's the purpose of the added resistance?

It was to damp the ringing on the address lines of multiple DRAM chips connected in parallel. A typical resistance was 33Ω if I remember correctly, using LSTTL drivers (74LS244 perhaps).

 

I have accidentally destroyed quite a few of those devices about 20 years ago. So it seems that they do need some form of protection.

 

I never assume that the latest burn of my code is not going to accidentally set a pin intended for input as output, so always include limiting resistors when developing.