Servo signals straight into 3.3V microcontroller?

[EpicAdventure]

I'm working on reading servo signals with an AT91SAM7s microcontroller (3.3V). The docs on the AT91SAM7 say that the input pins can handle up to 5.5V.

Will a typical BEC keep the servo signals less than 5.5V or do I need to voltage limit the signals before they go into the micro? If I do need to limit them, I was just thinking of using unity gain follower amps with 0 and 3.3V on the supplies -- maybe something like a LMV344?

Then what about generating servo signals? From what I've found on the web, it sounds like a 3.3.V signal will be ok for most servos. (The servos will still be powered with the full BEC voltage.)

Just curious what other folks are doing for this...

[kd7ost]

I'm working on reading servo signals with an AT91SAM7s microcontroller (3.3V). The docs on the AT91SAM7 say that the input pins can handle up to 5.5V.

Will a typical BEC keep the servo signals less than 5.5V or do I need to voltage limit the signals before they go into the micro? If I do need to limit them, I was just thinking of using unity gain follower amps with 0 and 3.3V on the supplies -- maybe something like a LMV344?

Then what about generating servo signals? From what I've found on the web, it sounds like a 3.3.V signal will be ok for most servos. (The servos will still be powered with the full BEC voltage.)

Just curious what other folks are doing for this...

I don't know about the 5.5 vdc pulses going to the AT91SAM7 controller, but I do know that the servos can take 3.3 vdc pulses. Several manufacturers including the Futaba PCM radios put out 3.3 volt pulses and any servo will work with those.

Dan

[hg1]

I'll confirm what Dan says. We use 3.3V signals from a microprocessor to directly drive a variety of servos and ESC's that are powered at 5V, and this works 100%.

[matttay]

Ditto. I'm driving GWS servos from 3.3V uP with a series 1K just in case without any issues. I did have a 2.2K in series, and there was enough drop across the the resistor that there was an issue. So, the important poin there is that servos require a non-trivial amount of current (relative to, say, cmos) to drive.

Similarly, I'm connecting the RX (with ~4.5V output) to the 3.3V uP directly through a 47K resistor. The clamp diodes on the uP can handle do max of 2 mA, so I'm way below that.

Good to see other data here on other servos...

[EpicAdventure]

Thanks for the input everyone. I measured a few BECs I had laying around and they all seemed to be right at 5V. I think I will just go ahead and wire straight into the uC.

The series resistor is a good idea to limit current just in case....