New to arduino - bootloader & pin voltage/current help.


I pulled my Arduino Mega 1280 out of the closet after a year (i purchased it and never used it) and have a few questions.

What is a bootloader? Do you need one to upload sketches using the arduino software? I have successfully uploaded sketches using the arduino software, does this mean i have a bootloader? Help!?

What voltage/current do the digital pins accept as input? Could i put 6V over them? What voltage/current do they output?

Also, what voltage/current do the analogue pins accept as input? Could i put 6V over them?

Many Thanks,


If you can upload sketches from the IDE using USB/Serial port, you have a bootloader - which is simply a program that loads another program (your sketch) into another part of memory, and then runs it after a reset when it determines nothing is trying to talk with the bootloader.

Digital pins - 0 t0 5V. 5.5V if you want to push it. Same with the analog/digital pins. All pins can source & sink 20mA safely, up to 40mA if you don't need them to last a long time. They need very little current when acting as input pins.

Thanks CrossRoads for the prompt reply!

I have noticed there are different bootloaders, what is the difference between them?

Out of curiosity, why would anyone want to use their board without a bootloader? Without a bootloader could you upload sketches and would your board run the most recent sketch when it was restarted?

Also, where can i find specifics on where in memory/what memory the arduino stores specific things (bootloader, sketches, etc)?

Thanks again

Different bootloaders may have different features, the most noticeable is how long it waits to see if the PC is trying to talk to if before starting the sketch.

Using a board without a bootloader leaves more room for the sketch - say you had a of strings to store, or patterns.

Without a bootloader, you can download sketches that will start on reset with no delay. Need something like this:

I'm off to bed ...

Thanks again for the reply.

Could someone explain to me why the digital pins require a pull up resistor (i know you can use the internal one) to properly detect input? What is meant by high impedance?

Thanks again.

The input pins need very little current to drive their internal transistors high or low. The input is not straight resistance, there is some capacitance there, maybe a little bit of inductance from the bond wires from the mechanical legs to the actual die inside, the effect of all that is lumped into the "impedance" of the pin. When the value of that impedance is very high, meaning it takes little external current to change the voltage level on the pin, it is called high impedance. But since they are high impedance, if nothing is connected the pin "floats" and can be randomly seen as high or low. Hence a pullup or pulldown resistor is needed to keep the input at a known state. If the pin is driven by another logic level part, no pullup/pulldown is needed.