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Re: Should I use a resistor between an input pin of MCU/CPLD and VCC/GND?
Posted: Tue Jun 27, 2017 4:56 pm
by Giovanni
Hi,
You mean for unused pins? if so you can just enable the internal pullup/pulldown resistor and do not connect them externally.
Giovanni
Re: Should I use a resistor between an input pin of MCU/CPLD and VCC/GND?
Posted: Sun Jul 02, 2017 8:56 am
by rew
No, I think he needs pins tied to VCC or GND for configuration or something like that.
The "use a resistor" dates from very old (1960's or 1970's) technology where it was not good for a chip to be tied directly to the power rails. In TTL driving a signal high was difficult. So the high level signal was not even close to 5V (more like 2.4V). But checking the schematics of chips from those days, I do not see a reason to avoid tying an input to VCC without a resistor. So I'm deducing that this advice is even older than TTL.
IIRC ST advises to tie unused pins to ground and to program those pins as OUTPUT LOW. This will improve the noise margin on the GROUND trace inside the chip.
I once accidentally had a setup that allowed me to measure the resistance of the ground trace inside the chip: I had 16 leds connected but due to a badly manufactured PCB, the 16th led output was shorted to ground. So when I enabled the 16th led all the others would dim slightly. (and more when they were closer to that 16th led).