I have my exam tomorrow , i need to know , what is the maximum fanout of an arduino pin ?
That's a question that cannot be answered - fanout is a property of both the Arduino output
capabilities and the input impedance of the gates/chips you drive with it. For instance
it has a fanout of 1 for driving 20mA LED's, but at least 20 for driving 74HCxx series logic
gates (probably more as its a high current output).
Usually fanout is defined within a particular logic family and quoted for a particular timing
specification (such as no more than 10ns increase in propagation delay).
For CMOS the input capacitance and stray capacitance on the PCB are usually the
limiting factor for fanout if speed is an issue. With TTL the input current load used to be
a limiting factor, but CMOS (usually) draws no static current so its not relevant.
You probably already know that the word, "fanout," doesn't appear in the datasheet. The phrase "fan out," doesn't either. It looks like your professor wants you to calculate it, using the definitions and techniques that you've learned in class.
So, how would you calculate the fanout?
murtaza13:
I have my exam tomorrow , i need to know , what is the maximum fanout of an arduino pin ?
There's no such thing as an "average fanout". Fanout means how many "things" requiring X amount of input current can be connected to one source of Y output current.
You need to look up the current that one Arduino pin can provide, then look up what input current your "things" require.
The "things" could be LED's, TTL inputs, CMOS inputs... anything.
Then divide "Arduino output" by "thing's required input" and you have fanout.
Make sense?
1 Like
fanout is an easy question to answer. actually a pretty well defined characteristic for all digital devices. using typical Atmel output drive current and input leakage values it appears fanout is in the order of 12 million.
Capacitance matters don't forget, the result will not be in the millions!
As the OPs exam was today then I suspect he has lost all interest in the answer, and Arduinos altogether until maybe the day before his next exam.
Grumpy_Mike:
As the OPs exam was today then I suspect he has lost all interest in the answer, and Arduinos altogether until maybe the day before his next exam.
We answer not only for the OP, but the OPs to come who will find our collected wisdom via the great and wise Google search engine. 
MarkT:
Capacitance matters don't forget, the result will not be in the millions!
capacitance would only matter in a minority of the highest speed applications. but im willing to compromise so lets say you can only hook up a few hundred thousand other mcus to an output pin. practically speaking.
JoeN:
We answer not only for the OP, but the OPs to come who will find our collected wisdom via the great and wise Google search engine.
I think not.
The fact that this fellow came here indicates an inherent inability to use "the great and wise Google search engine". :
As well as a few other cognitive matters.
{Not to say of course, that this is anything but an excellent repository of technical know-how. 
}
Paul__B:
I think not.The fact that this fellow came here indicates an inherent inability to use "the great and wise Google search engine". :
As well as a few other cognitive matters.
{Not to say of course, that this is anything but an excellent repository of technical know-how.
I've been directed here more than once from Google when doing a general search on not only Arduino, but other electronics subjects, especially ICs that are commonly used in the hobby community.
Hi all,i was going to ask about the fan out into the A4988 stepper driver,i would like to use two stepper motors per axis on a cnc xy bed,only need one stepper for the z axis,it's to keep the cutting forces as balanced as possible with regards to the drive mechanism.
I can't see any good info on the stepper drivers,probably due to them not used like this,i'm making my own control board,using the ch340G usb to serial chip and a atmega 328p chip,i'll add as mant headers and connectors as needed for the final build.
Any help with this would be most helpfull,thanks.
Since the steppers are current-controlled, the fan-out is always one. 
This input currents for the A4998 chip are to be found in the chip’s data sheet. A link to which is here https://www.allegromicro.com/en/Products/Motor-Driver-And-Interface-ICs/Bipolar-Stepper-Motor-Drivers/A4988.aspx
I suppose with four-wire, "bipolar" steppers, you could put the coils of two steppers in series. 
Yes, its how its done if you want two motors running in lockstep, for instance.
I have put the coils in parallel instead of series because they are capable of running faster. The only down side is your driver has to handle twice the current.
But then you get circulating currents between the motors, and torque imbalance.
In theory yes but there was no consequence in practice.