Coming from an Arduino / PICAXE environment I didn’t give adding a 20mA LED to an IO pin a second thought. This might have cost me my $75 FEZ Domino.
It was not before I read this thread [url]http://www.tinyclr.com/forum/3/467/[/url] (way down) I got the information on the 4mA limit. I feel that this information should be easily available for the new users.
I’s not a great selling argument that the FEZ only handles 1/10 of the current pr IO compared to Arduino, but still I feel it should be noted on the FEZ Domino brochure [url]http://www.tinyclr.com/downloads/Domino/Broch_FEZ_Domino.pdf[/url] in the Arduino Compability section. Users with homemade shields might not be aware of this issue and might plug in something that works fine on an Arduino but fries their FEZ.
Note that almost all new 3.3v 32bit processors have much less current limit that older 5v 8bit micros.
Like I said in other post. This is highly unlikely what damaged your board but we agree that this info should be made more clear. The LEDs on all boards we ship are 20mA I think so we know this is not causing problems
Edit: Like explained in other post, note that the 20mA LED doesn’t necessary drawing 20mA. Do the math, 3.3V minus 1V drop for LED = 2.3V…put that in 330 ohm resistor will give you 7mA but knowing that voltage will slightly drop due to going over 4mA will give you about 5mA to 6mA on the pin…which is just fine. The LED will not be at its brightest but I doubt anyone will notice.
Before you hook anything to a circuit board it behooves you to understand the voltage/current requirements/limitations of both devices. All micro-controllers have different specifications so it is never safe to assume anything.
Typically you may kill (or just maim, ie loosing a pull-up resistor or some capability) a pin, but pulling a bit more current through an I/O is HIGHLY unlikely to kill “the whole board.”
A transistor with a high HFE can help you restore the current capability, as well as using an I2C IO expander. Using the expander gives you a level of protection as instead of potentially destroying a $35-$150 (essentially) CPU, you’re destroying a $1.50 expander
MCP23017 is $1.44 (DigiKey), comes in PDIP package for easy prototyping, and has 16 I/O pins, each capable of sourcing OR sinking 25mA per pin.
When I purchased the IO40 expander from the site, I had to read the TINY print on the chip to get the manufacturer’s datasheet to see what the source/sink currents were, it may be a good idea to have this in the brochure.
Correct guys but this website is about making everything freakin’ easy so we do not mind doing the extra work and provide more info to users who do not know much about electronics.
I totally agree, but very few (including me) takes the time to study the datasheet for the microprocessor or even have the knowledge to understand it. Thats why I feel that a line or two in the manual (or the brochure) on the current limits of the IO pins would be good.
@ Geir I apologise…You must remember Some of us have been doing this too long!!
I downloaded the spec sheet from NXP directly I have also downloaded the specs from GHI site for the EMX module… I keep forgetting that they want to keep it “FEZ”
I’m made some projects with Arduino.
For my new project I consider to choose a Panda II instead.
Reason: I’m an experienced C# developer, like Visual Studio and for my new project I would like to have more RAM and speed.
About my project:
I want to controll 200 LEDs by the help of 30 shift registers (74HC595).
The hardware is mostly finished.
Now I focus to the software.
I want to control the LEDs bright by PWM.
For this the shift registers status should be updated rapidly.
In general the Panda II should work fine with my shift registers.
The 74HC595 datasheet tells, that 3.3 V inputs should be supported.
My worry:
I added 5 status LEDs to my board.
Now I want to calculate the ohm values for the resistors, but for this I need the know the maximum current of each IO pin.
I fully agree!!
It’s really annoying that I searched more than one hour to find information about this important feature.
Now I found this forum topic, but even now no clear information.
Yes, I already looked to the datasheet of the LPC2387 (chip on Panda II), but couldn’t found the needed value.
So, what’s the maximum (continuous) current for an IO pin of Panda II?
I suppose that there is an additionally maximum for the total current of all IO pins.
Is it 4 mA per pin? Or more?
I would be happy about a clear answer.
Additionally many users would be happy to have a notice about this feature on the Panda II datasheet.
I searched for similar key words on Google and also by the search box of this forum, but found no clear anwer.
In one post was a current value, but related to an other chip.
Thank you - good boy (-:
There is a historic building in Weimar, Germany. (The city is famous because of the poets Goethe and Schiller).
Now the building will be restored. In front of the main entrance fountain with a pedestal in the middle.
On top of the pedestal will added a ramble plant (e.g. vine). Inside of this plant I will install the 200 LEDs with green and yellow colour and should symbolize fireflies.
Well, it’s difficult to explain it by words.
Unfortunately I can’t show you photos, yet.
When my project is finished, I can publish some photos and maybe a video.
Good notice, but it’s not usable for in my case.
But this LED type remembers me to a very interesting project of somebody else, who embedded many LEDs to his clothes.
