I am an electrical engineer and feel I should know this, but I for the life of me still cant quite get this concept.
I understand conceptually the idea behind a pull up or a pull down, in that we want the IC to read a high state, by default, in the case of a pull up and low by default in case of pull down. But earlier to day an engineer was saying that we needed a lower pull down resistor to drive a pwm from 3 to 5 volts. I still dont get/understand the connection between a pull up/pull down and pwm signal? I get that its ideal to drive a motor using a pwm signal and a resistor could be used as an impedance to drive the pwm, but i still dont get the use of resistor in pwm to pull it “up” or “down”? an example, links or clarifications would be much appreciated.
He might mean that the current resistor used has too high resistance and creates a voltage divider with the already present PWM load (resistance). The result of it is that the voltage ratio is too close to 3 volt rather than the max 5 volt. (which make you wonder what it is!) If the load impedance stays the same but the pull-up resistor gets a lower resistance then the “pull up” pulls to a higher voltage. But it probably requires more knowledge of the circuit in question to understand what he has in mind exactly.
Pull ups or pull downs are not just associated with PWM, they form part of the topology of an output driver circuit.
Generally, digital outputs are formed by a current sink (usually to gnd), and a current source (usually to supply).
Yes, if the output is open collector, or open drain, a pull up of one sort or another is usually required. In this case, the transistor (usually) provides the current sink, while the current source is the pull-up.
If the output stage is totem-pole (Push–pull) then output current sink and source are provided already.