Arduino MEGA 1280 max input help requested

Forum 2005-2010 (read only) General Frequently-Asked Questions
1

I have two short questions:
I suspect I already know the answer to both of them but just want to be very sure.

  1. I have been reviewing the MEGA 1280 schematic and see both +5V and +3.3V. I only see the MEGA 1280 chip using +5V. What is the use of the 3.3V?

  2. GIven the +5V supply for the chip what is the max input voltage on any of the Analog or Digital inputs? I wouldnt think I should be putting +12V on these pins.

THanks in advance

Dillon

2

The 3.3v is coming from the FT232 (usb->serial) chip. It can only supply up to 50 mA. In most cases you should pretend it doesn't exist.

The 5v supply is what the 1280 is powered from. I don't have the datasheet in front of me, but if I remember correctly, you can only go a few tenths of a volt over the supply voltage. So I would say don't go over 5v on any of the inputs.

Head over to Atmel's website and lookup the 1280 datasheet, it will let you know the max voltages you can work with.

3

Well the arduino is being integrated (lock stock and barrel) into a repeater controller for amateur radio so there is also a high power (1.5A) regulator going to be on the board so I may not use the Arduino LDO regulator.

As far as the pin voltages.. all I can say is DUHHH! I should have thought of going to the data sheet..

Thanks..

Dillon

4

into a repeater controller for amateur radio

Would love to here more about this project.

5

Its actually a controller that will control several repeaters in a specialized linking system. We are not totally done with the design yet but will hopefully have a concept set to paper soon

6

Dear all, this has nothing to do with the data sheet but is most basic electronics and common sense ;D

But talking of data sheets - yes, do read them!!

7

but is most basic electronics and common sense

unfortunately that is not true in all cases. I am only going to give one (well several in a way) example. Find a chip that runs on 3.3 volts. There is a chance (50%? 10%? 90%?) that is is 5 volt tolerant. So common sense can not answer this question. Only the datasheet. So yes it has everything to do with the datasheet.

Although common sense may answer most questions... I have let the magic smoke out of many parts by following my "common sense"

8

This is an excellent example. I do not believe in "5 volt tolerance" because this is sometimes just marketing.

True 5 volt tolerance needs a quite complex circuitery at the input in place of the clamp diodes, which is expensive. To find out how the input looks can be in the datasheet. When you don't find it there and the datasheet says "5 volt tolerant" nonethelessly, you are lost....

I must admit, it's difficult to say what is to be expected and what is curious, without a lot of experience.... Reading through 300 pages datasheets in no fun...

Edit

Just the other way round. There are many people who do not shy away from connecting a 7805 to a 20V supply, because the datasheet says it's the limit.

Edit2

The logic families LVC, LVT, ALVT, LCX, LVX, LPT, FCT3 are considered to be 5 V tolerant, they have some kind of clever Zener-like FET arrangement in their input, saving the clamp diode to Vdd. However this will break down around 6 to 7 volts as well, but will hold 5 volts...

9

The logic families LVC, LVT, ALVT, LCX, LVX, LPT, FCT3 are considered to be 5 V tolerant, they have some kind of clever Zener-like FET arrangement in their input, saving the clamp diode to Vdd. However this will break down around 6 to 7 volts as well, but will hold 5 volts...

well, to be fair they don't say 6v tolerant now do they? The output of most 5v micro's is more like 4.5v anyway.