SparkFun Forums

[waltersobchak]

Hi all,

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.

Thanks.

[Valen]

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.

[markaren1]

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.

See http://en.wikipedia.org/wiki/Open_collector and http://en.wikipedia.org/wiki/Push%E2%80%93pull_output for a more exhaustive description.

-mark