PWM Through an AND Gate?

Hey guys,

So I'm planning on using an AND gate to handle limit switches on a motor application. Sometimes I'll be using steppers, and other times I'll be using DC motors - and therefor using PWM.

I have no pins available on the arduino.

The AND gate I'm thinking of using is the TI sn74hct08 - a quad gate that has a switch time of 30 ns, and a capacitance of 20pF.

Would that impact PWM performance on those pins that go through the AND gate? If so, is there a way to handle non-software based switching of the state of the driving pins?

Thanks!

!c

Would that impact PWM performance on those pins that go through the AND gate?

Probably not so you'll notice. Probably: the HCT outputs will have lower current capacity than the Arduino I/O pins, so you might need to adjust the value of the base resistor and/or spend a few cents more for transistors with higher gain. If you're using FETs to drive the motors, you won't see any difference.

At the relatively-low frequencies the Arduino uses for PWM, you'd need a really fancy scope to see the infinitesimal difference the HCT08 will make in the waveform.

Typical limit switches for motors are often a safety item and it is best that they cut the source voltage to the H-drive directly rather then limit via logic level control. Of course in that case the limit switches would have to have current rating equal or higher then the max motor current drawn.

Just throwing out the idea.

Lefty

Would that impact PWM performance on those pins that go through the AND gate

Absolutely not in no way.

Thanks for the info guys.

In the case of both DC motors and Steppers, I'm using external driver cards, so the effect of breaking either the step pulse chain, or the pwm pulse is an engine brake - which is what I'm going for.

Lefty: FWIW, on my CNC setup, the E-Stop breaks power, and the limits are "soft". In the application I'm using however, breaking power is always a bad idea - as there are vertical rotational and vertical linear axes that must remain powered on to remain at position. Cutting power would cause a constant dropping and then re-engaging-- sliding away from the switch under its own weight, far enough to disengage the limit, and then moving right back into the switch.

!c