Digital Output does not have continuity to ground?Started by 7 years ago●12 replies●latest reply 7 years ago●261 views
What might the following statement mean?
A particular pin is configured as Digital Output, Logic Low, but it does not have continuity to ground?
It means don't use your DVM on the ohms setting to measure the resistance (or continuity) to ground.
I have seen it done with unfortunate results.
Reminds me of a story about a new technician who was given an old-style buzzer which worked at 300mA, and a box of 1/8th amp fuses to test.
Worse than that is watching someone put a DMM on continuity buzzer and declare these are defective. Heres another one;D
Another, rather enigmatic one. A factory producing AC/DC magnetic relays. Testing them involved connecting a small AC across them. One customer used them in a circuit that used close to the minimum DC coil voltage. Approximately 40% of the circuits failed ... due to the slight residual magnetism in the core that resulted from testing. The residual polarity depended on where in the AC cycle the connection was removed.. How long did it take the manufacturer to find the cause !
With power on, you need to measure the voltage if you are using a DVM. You should measure a logic low. If you want to check for continuity, be sure that it's powered off. There should be no short from output pin to ground, otherwise you have a damage output circuit
As a general advice, only pure resistive loads or resistive circuits should be measured with the DMM set to resistance mode (or continuity check, which is beeping if the resistance is lower then 100 or 200 Ohms, depends on the DMM's).
Measuring the semiconductors with DMM set to continuity mode, will polarize the intrinsic diodes forward or reverse (depending how you connect the test leads) based on that you will have the measurement results, not really useful, unless the port is blown out and it is shorted completely to ground. You can still deduct some information though on a digital output, because it has the free wheeling schottky diode in, you can measure with both polarization and the values should be different(DMM's have the diode check function which will tell you the forward voltage drop on the diode), one way the voltage drop of 0.3-0.5V and the other way a high resistance.
The better solution is to measure the output during run time, set the pin to high - measure voltage, set it to low - measure the voltage.
I had measured the output during run time. I set the pin high at beginning of a periodic task and set the pin low at the end of periodic task. And oscilloscope displayed train of ON and OFF pulses. This means the pin is correctly configured at Output? Am I correct?
The short answer is: yes.
The long answer is: check if the hardware has configurable pull-ups/pull-downs or other settings like slew control, current limit, etc.. Just to avoid any pitfall.
The Pin gets connected to ground through CMOS logic circuit. This means it does not get shorted to ground but provide the ground potential through the Drain-Source path of the CMOS. You can only measure the potential voltage by using a DVM and not a simple continuity meter.
Hope this helps.
This is very helpful. Thank you!
The same pin can be configured as Logic High as well as Logic Low so Definitely NO Continuity to Ground .