2 Create an AVR project in MPLAB X

In this video and hands-on training: After demonstrating MPLAB X's new active kit detection, the kit schematic is located and a new standalone project for the ATmega324PB is created in MPLAB X. The datasheet is used to understand how to set PC7 as OUTPUT, LOW. Then debugging is used to verify functionality.

Overview and objectives

We will build a basic program for the ATmega324PB AVR MCU from scratch. The objective is to turn on the kit's USER_LED, as shown in the following image, then to toggle this LED every second.
  • For existing AVR users used to working in Atmel Studio 7, some of the similarities and differences with MPLAB X are emphasized.
  • For existing PIC MCU users of MPLAB X, an overview of AVR bare-metal programming references is given (listed below). For each of these references, an attempt is made to show how each one is used in the typical process of writing AVR code.
  • ATmega324PB device datasheet
  • ATmega324PB device header file
  • ATmega324PB Xplained Pro schematic
  • AVR Libc: Utility delay function header file

Kit used in this example:

The kit used will be the ATmega324PB Xplained Pro (ATMEGA324PB-XPRO), as well as the Atmel ICE (ATATMEL-ICE) debugger.
  1. Creating a new MPLAB X project for AVR
    When the ATmega324PB Xplained Prokit is plugged into MPLAB X, a welcome page for the kit opens, with key links to find related technical information.
  2. New Project Wizard
    The new project wizard can be opened from File > New Project, or from the New Project icon .
    1. Select Standalone Project
      Info: This is equivalent to a GCC C Executable Project in Atmel Studio 7.
    2. Select AVR from the MCU Families, or search directly for the part number in the Device field.
    3. Select the hardware tool to associate with the project.
      Info: In Atmel Studio 7, this is done after creating the project; but in MPLAB X IDE, this is part of the project creation process.
    4. Select Toolchains.
      Info: GCC projects are the default when new Projects are created in Atmel Studio 7.

      The project is given a name and location.

      Attention: The Set as main project checkbox is selected by default. The main project is the project associated with the compile and debug options.
  3. Writing some lines of AVR code
    The first thing to do is add the AVR GCC main.c template to the project.
    1. This is done by right clicking on Source Files, then selecting main.c.
      Name the file main, and click Finish.
    2. The project opens with the standard AVR GCC main.c format, with an include to the AVR io.h header file.
      Info: AVR io.h will add the relevant device header file to the project, which in this case is a file called iom324pb.h. To open this file, hover your cursor over a device register name, e.g. DDRC (Data Direction Register C), and from the right-click menu select Navigate > Go to Declaration/Definition.
    3. Which pin do we need to configure and how should it be configured, in order to turn on the LED?
      In order to answer this question, we need to check the ATmega324PB Xplained Pro Schematic (as shown in an extract of the following schematic).
      Info: USER_LED is connected to PC7: To turn on the LED, PC7 should be set as an OUTPUT, LOW.
      Info: USER_BUTTON is connected to PC6 and there is no pull-up resistor. To read the switch status, PC6 should be set as an input and internal pull-ups should be enabled.
    4. How can PC7 be configured as an OUTPUT, LOW?
      The datasheet is consulted to find an answer.
      Info: If DDxn is written to ‘1’, Pxn is configured as an output pin.
      Info: If PORTxn is written logic zero, when the pin is an output, the port pin is driven low.
    5. DDRC7 should be written to ‘1’, while PORTC7 is set low.
      The PORTC register is consulted below:
      Tip: PORTC7 has a default value of ‘0’. So only DDRC7 needs to be set to ‘1’ to turn on the LED.
    6. How are AVR register bits set and cleared?
      The code extract from the datasheet gives an example of how to configure PORTx and DDRx register bits.
      DDRC = (1<<DDRC7); will turn on the USER_LED.
    7. What help does the MPLAB X editor give to write AVR code?
      Although the section above works through a first-principle bare-metal approach to writing AVR code, there is also some help that the MPLAB X Editor can give. The user can take advantage of the MPLAB X editor's auto-complete functionality by typing Ctrl+SPACE, as shown in the example below. The MPLAB X editor references the header file for the ATmega324PB (iom324pb.h -> i/o map 324pb). Picking up the correct name from the suggested options can prevent the user from typing misspelled texts or undefined variable names, which is a typical cause of compilation errors.
  4. Verifying Functionality by Debugging
    The functionality can simply be verified by debugging.
    1. Click on the Debug Main Projecticon.
      This starts a debug session and runs the program. The LED should turn on.
      Info: This is equivalent to Start Debugging, rather than Start Debugging and Break.
      Tip: Debugging should be stopped at this point before proceeding to the next step. Single stepping will not work when debug hits while(1). Also, a breakpoint should be added to demonstrate single stepping. Without a breakpoint, Debug will run continuously. You can Pause and then single step, but a breakpoint will stop where you want.
    2. By single stepping, the LED turns on as we step over line 14: DDRC = (1<<DDRC7).