Electronic – Atmega8 input impedance

atmegacmos

What is the Atmega8 input impedance. I want to add a push button to an Atmega8 and my configuration would look something like this:

A +5V source, resistor, push button not pressed goes to Atmega8, pushed goes to ground.

From my understanding Atmega8 is a CMOS and therefor the input impedance would be very large, most likely in the 100MOhm. For a logical high the Amtega8 need 0.6Vcc, my Vcc is 5V, therefore a high will be considered for anything above 3V. Assuming that my assumption of very high input impedance is right, the resistor can be:

3V = 5*100M/(100M+R) ==>R = (500-300)/3 = 66.66MOhm.

Would that be the correct theoretical highest resistor value that I can use?

**The actual resistor used will be a 47kOhm but I really want to know how to find the maximum theoretical value.

Best Answer

Look in the datasheet and find the pin leakage current. CMOS digital inputs are usually specified as having a maximum leakage current as apposed to a minimum resistance. Now look at the minimum guaranteed logic high input voltage at your operating conditions. The supply voltage minus the minimum high voltage tells you the maximum voltage the pullup can drop and the input to still be interpreted as high. That drop voltage divided by the total leakage current is the maximum theoretical allowed pullup resistor.

Note that I said "total" leakage current. This is not only the pin input leakage but leakage from all other sources, like other pins of other devices connected to that node, whatever you think the leakage is accross the PC board at the worst case level of dirt and humidity, etc. In practise you can't know all these things. Do all the calculations you want, but I'd want a pullup at least a order of magnitude lower resistance than the maximum theoretical you calculate. When current consumption matters, I usually figure 100 kΩ is the maximum safe and reliable value. When it doesn't, I'll use 10 kΩ unless there is a good reason not to.

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Electronic – arduino – digitalRead sampling rate for Arduino Uno

You have two problems right now that are separate from each other.

1) Button bounce

2) Improper button wiring

First, you need to introduce a delay into your interrupt function so that it won't keep registering each rise and fall of the bounce as a separate button press. Use "delayMicroseconds()" because apparently you can't use regular "delay()" inside interrupts. Id suggest a 20,000 microsecond delay.

Second, you need to put a pull down resistor onto the digital pin that you are sampling.

The problem you are having is that the pin is in input mode and thus at high impedance (very high resistance) and thus has a tenancy to "float". If you put a voltage to it, it charges up like a capacitor and since the pin itself has very high resistance to ground, it takes a while for the charge to dissipate to the point where the Arduino will register a LOW again. A 10k resistor from the pin to ground will force the "capacitor" to discharge quickly after current from the button has stopped flowing.

Finally, the Arduino is pretty quick at it's read/write operations (I'm a mechanical engineer, ALL electronics happen "quickly" in comparison to mechanical systems, so it's relative I guess). Here's a link to some test results.

TL;DR verson:

What these values tell me, is that we can do about:

10 analog 10-bit readings per millisecond with analogRead()
128 pwm settings per millisecond with analogWrite()
220 pin reads per millisecond with digitalRead()
224 pin writes per millisecond with digitalWrite()
1056 pin reads per millisecond with direct port reads
1059 pin writes per millisecond with direct port writes

Electronic – Cabling simple sensors 5-20 ft from a uC – tips and gotchas; UTP vs STP cables

This is essentially what commercial alarm systems do: run power, ground, and a couple of signal wires to sensors and keypads. They don't even run twisted pair, because they're not shipping data (except for the keypad, of course, and even then they rarely bother).

Alarm system sensors signal with a simple voltage divider. When the sensor trips the voltage changes from its idle state, and the panel raises the alarm.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

I see no reason why you can't do the same with your switches and PIR (which, if it is a "normal" commercial PIR, should have an NO relay in it that it holds closed until the sensor trips (or loses power)). The LDR can work the same way, but if you want to measure the light level you'll need to pay more attention than just looking for voltage excursions outside the idle range.

Just feed the signals into the toyduino's analog inputs.

Best Answer

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Electronic – arduino – digitalRead sampling rate for Arduino Uno

Electronic – Cabling simple sensors 5-20 ft from a uC – tips and gotchas; UTP vs STP cables

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