Android USB Host + Arduino

The ultimate goal is to setup a communication interface between an Arduino Uno/Mega board and an Android tablet over USB. Everything will be as user-friendly as possible, i.e. no root will be required on your Android tablet. The following posts Arduino USB transfers and Android USB Host + Arduino: How to communicate without rooting your Android Tablet or Phone conclude the series.

The Arduino Uno is a popular and affordable hardware platform which comes with its own IDE and core libraries for programming. If you like the Arduino IDE you may skip this post, however, if you feel more comfortable developing in Eclipse this might help you out.
We are using Ubuntu 11.10 (oneiric ocelot) and Eclipse 3.6.1 (helios), although this should not play an important role (as it turns out, Eclipse Indigo might need a few tweaks, check the end of this post). Also, setting up Eclipse for programming the Arduino Uno Atmega328P is pretty straightforward. Nevertheless, here is a quick how-to for the Arduino Uno (Arduino Mega 2560 below):

1. Install avrdude and AVR libraries:
   sudo apt-get install gcc-avr avr-libc avrdude
2. Start Eclipse and install CDT Plugin (C/C++ Development Tools):
   • Help -> Install New Software…
   • Work with: (your current Eclipse version)
     i.e. “Helios – http://download.eclipse.org/releases/helios”
   • Download the “pending” software package (don’t worry, download starts automatically  )
   • Choose “Programming Languages” and select “C/C++ Development Tools”
   • Accept and continue by restarting Eclipse
3. Install AVR Eclipse Plugin:
   • Help -> Install New Software…
   • Add new repository: http://avr-eclipse.sourceforge.net/updatesite/
   • Re-download the “pending” software package, download will be faster since it is probably cached 
   • Download AVR Eclipse Plugin and restart Eclipse
4. Create new C project named “BlinkBlink”:
   • Project Type AVR Cross Target Application (Empty Project, AVR-GCC Toolchain)
   • Click next…
   • Untick “Debug” (in Debug mode, no hex-files are generated and avrdude can’t flash the device)
   • Click Advanced settings…
   • AVR -> AVRDude -> Programmer configuration…
   • Create a new programmer and name it “Arduino Uno”. Make sure this newly created programmer configuration is selected for the current project.
     • Programmer Hardware: Arduino
     • Override default port: /dev/ttyACM0 or similar
     • Override default baudrate: 115200
   • AVR -> Target Hardware:
     • MCU Type: ATmega328P (or load from MCU)
     • MCU Clock Frequency: 16000000 (default external clock source of Arduino Uno)
5. Click Apply and OK to leave the properties window and click Finish to create the new project in the workspace.
6. Create a new source file main.c from the source code

7. #include <stdio.h>
   #include <avr/io.h>
   #include <util/delay.h>
   
   
   #define LED PB5 // LED is on Pin 13 or Pin 5 of Port B
   
   void initIO(void)
   {
   DDRB |= (1<<LED);
   }
   
   
   int main(void)
   {
     initIO();
   
     while (1)
     {
      PORTB |= (1<<LED); // set
      _delay_ms(500);
      PORTB &= ~(1<<LED); // clear
      _delay_ms(500);
     }
     return 0; // never reached
   }
   

    
8. Make sure to save main.c before proceeding (File -> Save).
9. Project -> Build Project
10. Click on the AVR Button within Eclipse to upload the generated hex file from BlinkBlink/Release/BlinkBlink.hex.
    Your Arduino Uno’s LED should be blinking on and off repeatedly. If somehow it doesn’t work, right-click your Project and select Properties. Make sure all AVR and Programmer settings are active as mentioned above.

For the Arduino Mega 2560 you should choose Atmel STK500 Version 2.x firmware as Programmer Hardware, and ATMega2560 as target hardware, the rest is the same as with the Arduino Uno. Also, if you are using the above source file for testing, you should change the definition of LED from PB5 to PB7, since the LED on the Arduino Mega is on Pin7 of Port B.

Update: if you are using Eclipse Indigo, some specific AVR symbols such as DDRB (data direction register of port b) may not be recognized.
To solve this problem go to preferences…

• C/C++
• Language Mappings
• Add the following mappings:
  • Content Type: C Header File / Language: GNU C
  • Content Type: C Source File / Language: GNU C

Your Language Mapping preference window should look like the following screenshot:

If it still does not work, also try adding this line at the beginning of your main.c source file:
#include <avr/iom128.h>

ARDUINO BOARDS

Arduino Uno

Arduino Uno R3 Front		Arduino Uno R3 Back
Arduino Uno R2 Front	Arduino Uno SMD	Arduino Uno Front	Arduino Uno Back

Overview

The Arduino Uno is a microcontroller board based on the ATmega328 (datasheet). It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz ceramic resonator, a USB connection, a power jack, an ICSP header, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started.

The Uno differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it features the Atmega16U2 (Atmega8U2 up to version R2) programmed as a USB-to-serial converter.
Revision 2 of the Uno board has a resistor pulling the 8U2 HWB line to ground, making it easier to put into DFU mode.
Revision 3 of the board has the following new features:

• 1.0 pinout: added SDA and SCL pins that are near to the AREF pin and two other new pins placed near to the RESET pin, the IOREF that allow the shields to adapt to the voltage provided from the board. In future, shields will be compatible both with the board that use the AVR, which operate with 5V and with the Arduino Due that operate with 3.3V. The second one is a not connected pin, that is reserved for future purposes.
• Stronger RESET circuit.
• Atmega 16U2 replace the 8U2.

"Uno" means one in Italian and is named to mark the upcoming release of Arduino 1.0. The Uno and version 1.0 will be the reference versions of Arduino, moving forward. The Uno is the latest in a series of USB Arduino boards, and the reference model for the Arduino platform; for a comparison with previous versions, see the index of Arduino boards.

Summary

Microcontroller	ATmega328
Operating Voltage	5V
Input Voltage (recommended)	7-12V
Input Voltage (limits)	6-20V
Digital I/O Pins	14 (of which 6 provide PWM output)
Analog Input Pins	6
DC Current per I/O Pin	40 mA
DC Current for 3.3V Pin	50 mA
Flash Memory	32 KB (ATmega328) of which 0.5 KB used by bootloader
SRAM	2 KB (ATmega328)
EEPROM	1 KB (ATmega328)
Clock Speed	16 MHz

Schematic & Reference Design

EAGLE files: arduino-uno-Rev3-reference-design.zip (NOTE: works with Eagle 6.0 and newer)

Schematic: arduino-uno-Rev3-schematic.pdf

Note: The Arduino reference design can use an Atmega8, 168, or 328, Current models use an ATmega328, but an Atmega8 is shown in the schematic for reference. The pin configuration is identical on all three processors.

Power

The Arduino Uno can be powered via the USB connection or with an external power supply. The power source is selected automatically.

External (non-USB) power can come either from an AC-to-DC adapter (wall-wart) or battery. The adapter can be connected by plugging a 2.1mm center-positive plug into the board's power jack. Leads from a battery can be inserted in the Gnd and Vin pin headers of the POWER connector.

The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts.

The power pins are as follows:

• VIN. The input voltage to the Arduino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin.
• 5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
• 3V3. A 3.3 volt supply generated by the on-board regulator. Maximum current draw is 50 mA.
• GND. Ground pins.
• IOREF. This pin on the Arduino board provides the voltage reference with which the microcontroller operates. A properly configured shield can read the IOREF pin voltage and select the appropriate power source or enable voltage translators on the outputs for working with the 5V or 3.3V.

Memory

The ATmega328 has 32 KB (with 0.5 KB used for the bootloader). It also has 2 KB of SRAM and 1 KB of EEPROM (which can be read and written with the EEPROM library).

Input and Output

Each of the 14 digital pins on the Uno can be used as an input or output, using pinMode(), digitalWrite(), and digitalRead()functions. They operate at 5 volts. Each pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor (disconnected by default) of 20-50 kOhms. In addition, some pins have specialized functions:

• Serial: 0 (RX) and 1 (TX). Used to receive (RX) and transmit (TX) TTL serial data. These pins are connected to the corresponding pins of the ATmega8U2 USB-to-TTL Serial chip.
• External Interrupts: 2 and 3. These pins can be configured to trigger an interrupt on a low value, a rising or falling edge, or a change in value. See the attachInterrupt() function for details.
• PWM: 3, 5, 6, 9, 10, and 11. Provide 8-bit PWM output with the analogWrite() function.
• SPI: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK). These pins support SPI communication using the SPI library.
• LED: 13. There is a built-in LED connected to digital pin 13. When the pin is HIGH value, the LED is on, when the pin is LOW, it's off.

The Uno has 6 analog inputs, labeled A0 through A5, each of which provide 10 bits of resolution (i.e. 1024 different values). By default they measure from ground to 5 volts, though is it possible to change the upper end of their range using the AREF pin and the analogReference() function. Additionally, some pins have specialized functionality:

• TWI: A4 or SDA pin and A5 or SCL pin. Support TWI communication using the Wire library.

There are a couple of other pins on the board:

• AREF. Reference voltage for the analog inputs. Used with analogReference().
• Reset. Bring this line LOW to reset the microcontroller. Typically used to add a reset button to shields which block the one on the board.

See also the mapping between Arduino pins and ATmega328 ports. The mapping for the Atmega8, 168, and 328 is identical.

Communication

The Arduino Uno has a number of facilities for communicating with a computer, another Arduino, or other microcontrollers. The ATmega328 provides UART TTL (5V) serial communication, which is available on digital pins 0 (RX) and 1 (TX). An ATmega16U2 on the board channels this serial communication over USB and appears as a virtual com port to software on the computer. The '16U2 firmware uses the standard USB COM drivers, and no external driver is needed. However, on Windows, a .inf file is required. The Arduino software includes a serial monitor which allows simple textual data to be sent to and from the Arduino board. The RX and TX LEDs on the board will flash when data is being transmitted via the USB-to-serial chip and USB connection to the computer (but not for serial communication on pins 0 and 1).

A SoftwareSerial library allows for serial communication on any of the Uno's digital pins.

The ATmega328 also supports I2C (TWI) and SPI communication. The Arduino software includes a Wire library to simplify use of the I2C bus; see the documentation for details. For SPI communication, use the SPI library.

Programming

The Arduino Uno can be programmed with the Arduino software (download). Select "Arduino Uno from the Tools > Board menu (according to the microcontroller on your board). For details, see the reference and tutorials.

The ATmega328 on the Arduino Uno comes preburned with a bootloader that allows you to upload new code to it without the use of an external hardware programmer. It communicates using the original STK500 protocol (reference, C header files).

You can also bypass the bootloader and program the microcontroller through the ICSP (In-Circuit Serial Programming) header; see these instructions for details.

The ATmega16U2 (or 8U2 in the rev1 and rev2 boards) firmware source code is available . The ATmega16U2/8U2 is loaded with a DFU bootloader, which can be activated by:

• On Rev1 boards: connecting the solder jumper on the back of the board (near the map of Italy) and then resetting the 8U2.
• On Rev2 or later boards: there is a resistor that pulling the 8U2/16U2 HWB line to ground, making it easier to put into DFU mode.

You can then use Atmel's FLIP software (Windows) or the DFU programmer (Mac OS X and Linux) to load a new firmware. Or you can use the ISP header with an external programmer (overwriting the DFU bootloader). See this user-contributed tutorial for more information.

Automatic (Software) Reset

Rather than requiring a physical press of the reset button before an upload, the Arduino Uno is designed in a way that allows it to be reset by software running on a connected computer. One of the hardware flow control lines (DTR) of theATmega8U2/16U2 is connected to the reset line of the ATmega328 via a 100 nanofarad capacitor. When this line is asserted (taken low), the reset line drops long enough to reset the chip. The Arduino software uses this capability to allow you to upload code by simply pressing the upload button in the Arduino environment. This means that the bootloader can have a shorter timeout, as the lowering of DTR can be well-coordinated with the start of the upload.

This setup has other implications. When the Uno is connected to either a computer running Mac OS X or Linux, it resets each time a connection is made to it from software (via USB). For the following half-second or so, the bootloader is running on the Uno. While it is programmed to ignore malformed data (i.e. anything besides an upload of new code), it will intercept the first few bytes of data sent to the board after a connection is opened. If a sketch running on the board receives one-time configuration or other data when it first starts, make sure that the software with which it communicates waits a second after opening the connection and before sending this data.

The Uno contains a trace that can be cut to disable the auto-reset. The pads on either side of the trace can be soldered together to re-enable it. It's labeled "RESET-EN". You may also be able to disable the auto-reset by connecting a 110 ohm resistor from 5V to the reset line; see this forum thread for details.

USB Overcurrent Protection

The Arduino Uno has a resettable polyfuse that protects your computer's USB ports from shorts and overcurrent. Although most computers provide their own internal protection, the fuse provides an extra layer of protection. If more than 500 mA is applied to the USB port, the fuse will automatically break the connection until the short or overload is removed.

Physical Characteristics

The maximum length and width of the Uno PCB are 2.7 and 2.1 inches respectively, with the USB connector and power jack extending beyond the former dimension. Four screw holes allow the board to be attached to a surface or case. Note that the distance between digital pins 7 and 8 is 160 mil (0.16"), not an even multiple of the 100 mil spacing of the other pins.