View Full Version : Pro Dev. Board Dip Switch question

08-17-2006, 08:56 PM
I have the pro dev. board

I measure 5 vdc on each of the pins.

But when I put even the slightest load on it like a chip select the voltage drops to 2.2 VDC. Is this behaviour normal?

If so, will the chip be enabled? it's not seeing a 5V signal.


08-18-2006, 03:31 AM
I meant the dip switch pins. They are connected to 5VDC through a 10K resistor when open. I'm wanting to pull up some pins on an IC to 5VDC.

Mike Green
08-18-2006, 05:04 AM
Any IC input should not put a significant drain on these signals. Any other drain could make a difference. Using Ohm's Law, a 1V drop occurs across a 10K resistor with a 100ua drain. To see a 2.2V drop, you'd need maybe 220ua current draw. What kind of voltmeter are you using? An old moving coil meter could draw that much. Any newer digital meter should have only a tiny draw.

08-18-2006, 05:27 AM

If you are seeing 5V from the DIP output with no load, and only 2V when the "chip select" is attached, the problem is not with your voltmeter (If·your voltmeter was drawing a lot of current, you would never see 5V due to the voltage divider formed by the in-line resister and the voltmeter itself).· This voltage drop can only be explained (as Mike does above) by more current going to the "chip select" pin than should be.

Check to make sure you really are attaching to a "chip select" pin.· Or IC you are using may be bad.·

You can do a test by using the "enable" pin in the L293D on the PDB·to test - This pin will not draw enough current to drop the voltage,·and·should be·similar to your·chip select pin.


08-18-2006, 09:10 PM

I'll give that a try.

08-18-2006, 09:16 PM
it stays up at 5V on the enable pins of the 293D.

That's what I thought it was supposed to do and thanks for the :duh: refresher on Ohm's Law.

Chris Savage
08-18-2006, 09:50 PM

·· Is the chip powered when you're connecting the /CS line and reading these voltages?

Chris Savage
Parallax Tech Support
csavage@parallax.com (mailto:csavage@parallax.com)