How to Arduino
                    ==============
                    
Arduino is an open-source hardware and software company, project
and user  community that designs and  manufactures  single-board
microcontrollers  and microcontroller  kits for building digital
devices at  www.arduino.cc.  Products are licensed under the GNU
Lesser  General  Public License (LGPL) or the GNU General Public
License  (GPL),permitting  the manufacture of Arduino boards and
software  distribution  by anyone.  Arduino boards are available
commercially  in preassembled  form or as  do-it-yourself  (DIY)
kits.

Arduino  board  designs  use a variety  of  microprocessors  and
controllers.  The boards are  equipped  with sets of digital and
analog input/output (I/O) pins that may be interfaced to various
expansion  boards or breadboards  (shields) and other  circuits.
The boards feature serial  communications  interfaces, including
Universal  Serial Bus (USB) on some models,  which are also used
for   loading    programs   from   personal    computers.    The
microcontrollers  can be programmed  using C and C++ programming
languages. In addition to using traditional compiler toolchains,
the  Arduino   project   provides  an   integrated   development
environment (IDE) based on the Processing language project.

Form  factor  is  similar  to RPIs,  programming  model  differs
considerably.   There  is  no  command  line,  no  terminal,  no
keyboard, no OS. CPUs  include tmel AVR (8-bit), ARM  Cortex-M0+
(32-bit), ARM  Cortex-M3  (32-bit),  Intel Quark (x86) (32-bit).
Memory  is tight,  typically  between  1kb and 8kb  static  RAM,
storage is flash memory (32-256kB) and EEPROM.

To install the IDE on a Pi, invoke:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install arduino

To  run  the  IDE,  an X11  server  is  required.  For  headless
configurations,   xvnc  does  just  fine.  The  Arduino  package
provided  by  apt-get is said to be  outdated.  Instructions  to
rebuild   latest  from   scratch  are   available,   reasaonably
straightforward to follow.

An Arduino talks to its host (Windows, Linux, BSD, Pi, Mac, ...)
through USB,  presents  itself as yet another  serial port named
/dev/ttyACM#, 0 <= #. Enumeration  across devices  appears to be
predictable  and constant.  Power draw might be an issue, if so,
drive Arduinos from a powered hub.

To control an  Arduino,  hook it up, invoke,  from  within  X11,
"arduino",  configure, via Tools->Board and  Tools->Serial  Port
what type of Arduino is listening on which port.

Arduinos are  programmed  in C/C++.  Unlike a C program on a Pi,
there is no main()  routine,  no stdin and no stdout. An Arduino
program  (called a sketch,  source code resides in an .ino file)
contains at least two functions:



    void setup(void)

and

    void loop(void)
    
Setup() gets  invoked  once at program  startup.  Initialization
code  goes  there,  while  loop()  gets   invoked   indefinitely
repeatedly.

Communication  with the  outside  world takes place via a serial
line  implemented on top of USB. Serial data has to be picked up
using the Serial object, described at

<https://www.arduino.cc/reference/en/language/functions/communication/serial/>

It might be a good  idea to check for  presence  of input on the
serial "port" ever so often, using  Serial.available(), then one
of the reader  functions  to pick up data, and/or implementing an input
handler named

void serialEvent(void)

which the  Arduino  environment  calls  between  invocations  of
loop().  Means  spending too much time in loop()  causes  serial
input to get handled delayed.

To handle input from within loop():

void identify(const char *theFile, int theLine)
{
    Serial.print("Uno-R3 : ");
    Serial.print(theFile);
    Serial.print(" @ ");
    Serial.print(theLine);
    Serial.print(" : ");
}

void inputHandler()
{
[...]
    while (Serial.available())
    {
        c = (char)Serial.read();
    }
[...]
}

void serialEvent()
{
    identify(__FILE__,__LINE__);
    Serial.println("have serial event.");
    inputHandler();
}
 

void Delay(int n)
{
    while (n > 0)
    {
        delay(100);
        n -= 100;
        inputHandler();
    }
}

void loop()
{
    digitalWrite(led, HIGH);   // turn the LED on (HIGH is the voltage level)
    Delay(1000);               // wait for a second
    digitalWrite(led, LOW);    // turn the LED off by making the voltage LOW
    Delay(1000);               // wait for a second
}

Check

<https://www.arduino.cc/reference/en/>

for Arduino-specific functions.