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/** \mainpage AVRISP MKII Programmer Project
 *
 *  \section SSec_Compat Project Compatibility:
 *
 *  The following list indicates what microcontrollers are compatible with this project.
 *
 *  - Series 7 USB AVRs
 *  - Series 6 USB AVRs
 *  - Series 4 USB AVRs
 *  - Series 2 USB AVRs (8KB versions with reduced features only)
 *
 *  \section SSec_Info USB Information:
 *
 *  The following table gives a rundown of the USB utilization of this project.
 *
 * <table>
 *  <tr>
 *   <td><b>USB Mode:</b></td>
 *   <td>Device</td>
 *  </tr>
 *  <tr>
 *   <td><b>USB Class:</b></td>
 *   <td>Vendor Specific Class</td>
 *  </tr>
 *  <tr> 
 *   <td><b>USB Subclass:</b></td>
 *   <td>N/A</td>
 *  </tr>
 *  <tr>
 *   <td><b>Relevant Standards:</b></td>
 *   <td>Atmel AVRISP MKII Protocol Specification</td>
 *  </tr>
 *  <tr>
 *   <td><b>Usable Speeds:</b></td>
 *   <td>Full Speed Mode</td>
 *  </tr>
 * </table>
 *
 *  \section SSec_Description Project Description: 
 *
 *  Firmware for an AVRStudio compatible AVRISP-MKII clone programmer. This project will enable the USB AVR series of
 *  microcontrollers to act as a clone of the official Atmel AVRISP-MKII programmer, usable within AVRStudio. In its
 *  most basic form, it allows for the programming of 5V AVRs from within AVRStudio with no special hardware other than
 *  the USB AVR and the parts needed for the USB interface. If the user desires, more advanced circuits incorporating
 *  level conversion can be made to allow for the programming of 3.3V AVR designs.
 *
 *  This device spoofs Atmel's official AVRISP-MKII device PID so that it remains compatible with Atmel's AVRISP-MKII
 *  drivers. When prompted, direct your OS to install Atmel's AVRISP-MKII drivers provided with AVRStudio.
 *
 *  Note that this design currently has the following limitations:
 *    - Minimum ISP target clock speed of 500KHz due to hardware SPI module prescaler limitations
 *    - No reversed/shorted target connector detection and notification
 *    - A seperate header is required for each of the ISP, PDI and TPI programming protocols that the user wishes to use
 *    - XMEGA EEPROM erase section command does not work (but EEPROM read/write and chip erase does)
 *
 *  On AVR models with an ADC converter, AVCC should be tied to 5V (e.g. VBUS) and the VTARGET_ADC_CHANNEL token should be
 *  set to an appropriate ADC channel number in the project makefile for VTARGET detection to operate correctly. On models
 *  without an ADC converter, VTARGET will report a fixed 5V level at all times.
 *
 *  While this application can be compiled for USB AVRs with as little as 8KB of FLASH, for full functionality 16KB or more
 *  of FLASH is required. On 8KB devices, ISP or PDI/TPI programming support can be disabled to reduce program size.
 *
 *  \section Sec_ISP ISP Connections
 *  Connections to the device for SPI programming (when enabled):
 *
 *  <table>
 *   <tr>
 *    <td><b>Programmer Pin:</b></td>
 *    <td><b>Target Device Pin:</b></td>
 *    <td><b>ISP 6 Pin Layout:</b></td>
 *   </tr>
 *   <tr>
 *    <td>MISO</td>
 *    <td>PDO</td>
 *    <td>1</td>
 *   </tr>
 *   <tr>
 *    <td>ADCx <b><sup>1</sup></b></td>
 *    <td>VTARGET</td>
 *    <td>2</td>
 *   </tr>
 *   <tr>
 *    <td>SCLK</td>
 *    <td>SCLK</td>
 *    <td>3</td>
 *   </tr>
 *   <tr>
 *    <td>MOSI</td>
 *    <td>PDI</td>
 *    <td>4</td>
 *   </tr>
 *   <tr>
 *    <td>PORTx.y <b><sup>2</sup></b></td>
 *    <td>/RESET</td>
 *    <td>5</td>
 *   </tr>
 *   <tr>
 *    <td>GND</td>
 *    <td>GND</td>
 *    <td>6</td>
 *   </tr>
 *  </table>
 *
 *  In addition, the AVR's XCK pin will generate a .5MHz clock when SPI programming is used, to act as an external
 *  device clock if the fuses have been mis-set. To use the recovery clock, connect XCK to the target AVR's XTAL1
 *  pin, and set the ISP programming speed to 125KHz or below.
 *
 *  <b><sup>1</sup></b> <i>Optional, see \ref SSec_Options section - for USB AVRs with ADC modules only</i> \n
 *  <b><sup>2</sup></b> <i>See AUX line related tokens in the \ref SSec_Options section</i>
 *
 *  \section Sec_PDI PDI Connections
 *  Connections to the device for PDI programming (when enabled):
 *
 *  <table>
 *   <tr>
 *    <td><b>Programmer Pin:</b></td>
 *    <td><b>Target Device Pin:</b></td>
 *    <td><b>PDI 6 Pin Layout:</b></td>
 *   </tr>
 *   <tr>
 *    <td>Tx/Rx <b><sup>2</sup></b></td>
 *    <td>DATA</td>
 *    <td>1</td>
 *   </tr>
 *   <tr>
 *    <td>ADCx <b><sup>1</sup></b></td>
 *    <td>VTARGET</td>
 *    <td>2</td>
 *   </tr>
 *   <tr>
 *    <td>N/A</td>
 *    <td>N/A</td>
 *    <td>3</td>
 *   </tr>
 *   <tr>
 *    <td>N/A</td>
 *    <td>N/A</td>
 *    <td>4</td>
 *   </tr>
 *   <tr>
 *    <td>XCK</td>
 *    <td>CLOCK</td>
 *    <td>5</td>
 *   </tr>
 *   <tr>
 *    <td>GND</td>
 *    <td>GND</td>
 *    <td>6</td>
 *   </tr>
 *  </table>
 *
 *  <b><sup>1</sup></b> <i>Optional, see \ref SSec_Options section - for USB AVRs with ADC modules only</i> \n
 *  <b><sup>2</sup></b> <i>The AVR's Tx and Rx become the DATA line when connected together via a pair of 220 ohm resistors</i> \n
 *
 *  \section Sec_TPI TPI Connections
 *  Connections to the device for TPI programming (when enabled):
 *
 *  <table>
 *   <tr>
 *    <td><b>Programmer Pin:</b></td>
 *    <td><b>Target Device Pin:</b></td>
 *    <td><b>TPI 6 Pin Layout:</b></td>
 *   </tr>
 *   <tr>
 *    <td>Tx/Rx <b><sup>2</sup></b></td>
 *    <td>DATA</td>
 *    <td>1</td>
 *   </tr>
 *   <tr>
 *    <td>ADCx <b><sup>1</sup></b></td>
 *    <td>VTARGET</td>
 *    <td>2</td>
 *   </tr>
 *   <tr>
 *    <td>XCK <b><sup>2</sup></b></td>
 *    <td>CLOCK</td>
 *    <td>3</td>
 *   </tr>
 *   <tr>
 *    <td>N/A</td>
 *    <td>N/A</td>
 *    <td>4</td>
 *   </tr>
 *   <tr>
 *    <td>PORTx.y <b><sup>3</sup></b></td>
 *    <td>/RESET</td>
 *    <td>5</td>
 *   </tr>
 *   <tr>
 *    <td>GND</td>
 *    <td>GND</td>
 *    <td>6</td>
 *   </tr>
 *  </table>
 *
 *  <b><sup>1</sup></b> <i>Optional, see \ref SSec_Options section - for USB AVRs with ADC modules only</i> \n
 *  <b><sup>2</sup></b> <i>The AVR's Tx and Rx become the DATA line when connected together via a pair of 220 ohm resistors</i> \n
 *  <b><sup>3</sup></b> <i>See AUX line related tokens in the \ref SSec_Options section</i>
 *
 *  \section SSec_Options Project Options
 *
 *  The following defines can be found in this project, which can control the project behaviour when defined, or changed in value.
 *
 *  <table>
 *   <tr>
 *    <td><b>Define Name:</b></td>
 *    <td><b>Location:</b></td>
 *    <td><b>Description:</b></td>
 *   </tr>
 *   <tr>
 *    <td>AUX_LINE_PORT</td>
 *    <td>Makefile CDEFS</td>
 *    <td>PORT register for the programmer's AUX target line. The use of this line varies between the programming protocols,
 *        but is generally used for the target's /RESET line. <i>Ignored when compiled for the XPLAIN board.</i></td>
 *   </tr>
 *   <tr>
 *    <td>AUX_LINE_PIN</td>
 *    <td>Makefile CDEFS</td>
 *    <td>PIN register for the programmer's AUX target line. The use of this line varies between the programming protocols,
 *        but is generally used for the target's /RESET line. <i>Ignored when compiled for the XPLAIN board.</i></td>
 *   </tr>
 *   <tr>
 *    <td>AUX_LINE_DDR</td>
 *    <td>Makefile CDEFS</td>
 *    <td>DDR register for the programmer's AUX target line. The use of this line varies between the programming protocols,
 *        but is generally used for the target's /RESET line. <i>Ignored when compiled for the XPLAIN board.</i></td>
 *   </tr>
 *   <tr>
 *    <td>AUX_LINE_MASK</td>
 *    <td>Makefile CDEFS</td>
 *    <td>Mask for the programmer's AUX target line. The use of this line varies between the programming protocols,
 *        but is generally used for the target's /RESET line. <b>Must not be the AVR's /SS pin</b>. <i>Ignored when
 *        compiled for the XPLAIN board.</i></td>
 *   </tr>
 *   <tr>
 *    <td>VTARGET_ADC_CHANNEL</td>
 *    <td>Makefile CDEFS</td>
 *    <td>ADC channel number (on supported AVRs) to use for VTARGET level detection, if NO_VTARGET_DETECT is not defined.
 *        <i>Ignored when compiled for targets lacking an ADC.</i></td> 
 *   </tr>
 *   <tr>
 *    <td>ENABLE_ISP_PROTOCOL</td>
 *    <td>Makefile CDEFS</td>
 *    <td>Define to enable SPI programming protocol support. <i>Ignored when compiled for the XPLAIN board.</i></td>  
 *   </tr>
 *   <tr>
 *    <td>ENABLE_XPROG_PROTOCOL</td>
 *    <td>Makefile CDEFS</td>
 *    <td>Define to enable PDI and TPI programming protocol support. <i>Ignored when compiled for the XPLAIN board.</i></td>  
 *   </tr>
 *   <tr>
 *    <td>NO_VTARGET_DETECT</td>
 *    <td>Makefile CDEFS</td>
 *    <td>Define to disable VTARGET sampling and reporting on AVR models with an ADC converter. This will cause the programmer
 *        to report a fixed 5V target voltage to the host regardless of the real target voltage. <i>Ignored when compiled for
 *        targets lacking an ADC.</i></td>  
 *   </tr>
 *   <tr>
 *    <td>VTARGET_REF_VOLTS</td>
 *    <td>Makefile CDEFS</td>
 *    <td>Indicates the programmer AVR's AVCC reference voltage when measuring the target's supply voltage. Note that the supply
 *        voltage should never exceed the reference voltage on the programmer AVR without some form of protection to prevent damage
 *        to the ADC. <i>Ignored when compiled for targets lacking an ADC, or when NO_VTARGET_DETECT is defined.</i></td>  
 *   </tr>
 *   <tr>
 *    <td>VTARGET_SCALE_FACTOR</td>
 *    <td>Makefile CDEFS</td>
 *    <td>Indicates the target's supply voltage scale factor when applied to the ADC. A simple resistive divider can be used on the
 *        ADC pin for measuring the target's supply voltage, so that voltages above the programmer AVR's AVCC reference voltage can be
 *        measured. This should be the reciprocal of the division performed - e.g. if the VTARGET voltage is halved, this should be set
 *        to 2. <i>Ignored when compiled for targets lacking an ADC, or when NO_VTARGET_DETECT is defined.</i></td>  
 *   </tr> 
 *   <tr>
 *    <td>LIBUSB_DRIVER_COMPAT</td>
 *    <td>Makefile CDEFS</td>
 *    <td>Define to switch to a non-standard endpoint scheme, breaking compatibility with AVRStudio under Windows but making
 *        the code compatible with software such as avrdude (all platforms) that use the libUSB driver.
 *   </tr>
 *  </table>
 */
