Atmega328 and Attiny85: Different Voltage Reading

I’m using an oscilloscope to compare to two signals coming from my microcontrollers: the attiny85 and atmega328

I’m using a blink led sketch example. For the attiny, it reads 3.3V when the pin and LED goes to HIGH and 0 when it is low. For the atmega, it reads 300mV when it is HIGH and 0 when it is low. They are both running on the same breadboard using the same power source 3.3V.

Why am I getting different readings and how can I get them to get to the same voltage (either 300mV or 3.3V)?

3V3 is a normal high condition and .3V is a normal low condition or a high condition shorted to ground (Way overloaded)

I don't understand what you mean. The two microcontrollers are setup pretty much the same way, why is there a voltage difference or what do you think I'm doing that caused the difference and how can I fix it to be the same value voltage?

On the atmega, the LED is connected to pin 8 with a resistor to ground. On the attiny, the LED is connected to pin 0 with the same value resistor connected to ground. They both used the same blinking led example sketch.

Where exactly are you measuring the voltage?

It's not a bad chip. I recently changed up a new chip and it's still outputting the same voltage level. You can see from the photos below.

The atmega's voltage on pin 8 and the attiny on pin 0 is where I measured my voltage. I even took out everything connected to pin 8 and just measured the pin alone and it's still at around 300mV. The attiny is displaying 3.3V which is correct I believe as it's taking power from the Duemilanove 3.3V power supply. The atmega is also taking power from the same source, but it's not displaying the right voltage for it's HIGH value.

Ah, I figured it out. On my oscilloscope, I have two probes: one to measure the attiny and one to measure the atmega. The measuring rate for the probe that was used on the attiny was set to 1X and the other probe was set to 10X.

I have set it both to 1X now. I just got my oscilloscope so this is a rookie's mistake. Thank you for even looking at this post and trying to help me.

It seems nobody asked whether the LED was lighting.

If the LED was flashing, there is no way that there was 300 mV on the output. :smiley:


Yep! When in doubt, look to the human for the error. :open_mouth: