This tutorial is an advanced topic and is not recommend for beginners. If you want to write software in your design we recommend using the microcontroller on the Mojo before trying to embed a processor as the tools and setup can be very messy. Checkout this tutorial for more information.

This tutorial will outline how to add a MicroBlaze MCS to your project. The MicroBlazeMCS is a 32bit microprocessor core provided by Xilinx. It will allow you to write C/C++ code that will interact with the rest of your design. While this can be very helpful in many projects, setting it up can be a bit confusing. This tutorial will hopefully demystify the process.

Like most tutorials here, the first step is downloading the Mojo Base Project.

Generating the core

Go ahead and open up your fresh project in ISE.

Right click on somewhere in the Hierarchy panel on the left and select New Source...

Select IP (CORE Generator & Architecture Wizard) and set the File Name to microblaze_mcs and click Next.

Expand Embedded Processing and Processor to find MicroBlaze MCS. Choose that and click Next.

Click Finish.

The configuration wizard should now show up.


In the first tab, MCS, change the Input Clock Frequency to 50 since we will be running the core at 50MHz.

Take note of the Instance Hierarchical Design Name, it is very important later on. For now we'll just leave it at the default mcs_0.

Set Memory Size to 16KB. The absolute minimum required for this project is 8KB, but if you add any extra code, you will want a little more.

The Memory Size option is important because it dictates how much memory the processors has to store your code and variables. You will want to use a little memory as possible that will still fit your program since it uses valuable block RAM inside the FPGA.

If you look at the top of the window you will see tabs for all the different peripherals available to you.

In this tutorial, we will be using a timer to control how fast some LEDs blink. That means we will need a FIT (Fixed Interval Timer) and some GPO (General Purpose Outputs).

In the FIT tab, check the Use Timer box.

We want our timer to count the number of milliseconds that elapse which will allow us to create a flexible delay function. Since the Mojo uses a 50MHz clock, we want the Number of Clocks Between Strobes to be 50,000,000 / 1,000 = 50,000.

We will also want the timer to generate an interrupt, so check the Generate Interrupt box.


Head over to the GPO tab since we need some outputs for the LEDs.

In the General Purpose Output 1 box check the box Use GPO.

We only have 8 LEDs, so change the Number of Bits to 8.

The Initial Value of GPO is fine as the default.


Those are the only peripherals we need for this project, but feel free to check out the other ones as they may come in handy for your own projects. If you really want to get your feet wet, head over to Xilinx's documentation.

Click Generate to generate the core!

Adding the core to your project

After the core is done generating (it can take a while), you should see microblaze_mcs as a file in your project.

We need to now instantiate this in our design. Open up mojo_top.v in your editor of choice.

Click on the microblaze_mcs source file and expand CORE Generator in the Processes panel. Double click on View HDL Instantiation Template. This file contains an example instantiation of the core. You can refer to this file to make sure you get all the port names right.


It's usually a good idea to just copy/paste it into your design and make the edits you need.

In our case, we need to add it to mojo_top.v. The file should look as follows.

module mojo_top(
    // 50MHz clock input
    input clk,
    // Input from reset button (active low)
    input rst_n,
    // cclk input from AVR, high when AVR is ready
    input cclk,
    // Outputs to the 8 onboard LEDs
    // AVR SPI connections
    output spi_miso,
    input spi_ss,
    input spi_mosi,
    input spi_sck,
    // AVR ADC channel select
    output [3:0] spi_channel,
    // Serial connections
    input avr_tx, // AVR Tx => FPGA Rx
    output avr_rx, // AVR Rx => FPGA Tx
    input avr_rx_busy // AVR Rx buffer full
  wire rst = ~rst_n; // make reset active high
  // these signals should be high-z when not used
  assign spi_miso = 1'bz;
  assign avr_rx = 1'bz;
  assign spi_channel = 4'bzzzz;
  microblaze_mcs mcs_0 (
    .Clk(clk), // input Clk
    .Reset(rst), // input Reset
    .FIT1_Interrupt(), // output FIT1_Interrupt
    .FIT1_Toggle(), // output FIT1_Toggle
    .GPO1(led), // output [7 : 0] GPO1
    .INTC_IRQ() // output INTC_IRQ

Notice I made the instance name mcs_0. This MUST match the name you use when you generate the core. If you don't instantiate the core at the top of your design, you must include the path in the name in CORE Gen. For example, if you have your core in a module called magic_sauce then the path would be magic_sauce/mcs_0.

In our design, we don't care about any of the outputs except GPO1, which is hooked up to the LEDs.

The last step is to run the setup script that was generated. Open the Tcl Console by clicking View->Panels->Tcl Console. In the Tcl Console tab near the bottom of ISE, enter the following line and hit enter.

source ipcore_dir/microblaze_mcs_setup.tcl

It should print the following.

microblaze_mcs_setup: Found 1 MicroBlaze MCS core.
microblaze_mcs_setup: Added "-bm" option for "microblaze_mcs.bmm" to ngdbuild command...
microblaze_mcs_setup: Done.

This changes your project settings to accommodate the new core. Note that these changes don't seem to be persistent and you will likely need to run the script each time you open ISE.

There seems to be a bug in ISE which causes one of these settings to be wrong.

Right click on Implement Design in the Processes panel with mojo_top selected in the Hierarchy panel. Choose Properties.

Under the Other Ngdbuild Command Line Options you should see -bm "ipcore_dir/microblaze_mcs.bmm". Replace that with -bm "../ipcore_dir/microblaze_mcs.bmm"

Setting up XPS

Now we need to actually setup XPS, the IDE Xilinx provides that will compile our code.

When you installed ISE, you actually installed XPS too!

If you are using Ubuntu then you need to issue the following command because Ubuntu doesn't have a program called gmake, which is really just make.

sudo ln -s /usr/bin/make /usr/bin/gmake

If you are using Linux you can create a snazzy launcher by running the following.

sudo gnome-desktop-item-edit /usr/share/applications/ --create-new

Under Name enter XPS SDK.

Under Command enter /opt/Xilinx/14.6/ISE_DS/EDK/bin/lin64/xsdk, note that your version of ISE may vary.

You can find an icon at /opt/Xilinx/14.6/ISE_DS/EDK/eclipse/lin64/eclipse/plugins/com.xilinx.sdk.product_1.0.0/icons/xps_sdk_32.png

Once you have XPS open, there are a few hoops to jump through before the coding can begin.

Creating the project

Click File->New->Other...

Under Xilinx choose Hardware Platform Specification and click Next.

Enter MojoDemo as the Project Name. Note that you can't have spaces in your project names.

Under Target Hardware Specification click Browse... and navigate to the ipcore_dir. Select microblaze_mcs_sdk.xml.


Click Finish.

Now you can create a project for your actual code. Click File->New->Other... and choose Application Project under Xilinx.

Under Project Name enter LED_Controller.

Make sure that Hardware Platform is set to MojoDemo and the processor is microblaze_mcs.

Under Target Software make sure the OS Platform is set to standalone, Language is set to C, and Board Support Package is set to Create New -> LED_Controller_bsp.


Click Next.

Choose Empty Application for a Template and click Finish.

Open the project folder on the left for your project, LED_Controller. Right click the src folder and choose New->Source File. Enter LEDBlinker.c as the file name and click Finish.

Open the new file and paste in the following code.

#include <xparameters.h>
#include <xiomodule.h>
XIOModule gpo;
volatile u32 ct = 0;
void timerTick(void* ref) {
  ct++; // increase ct every millisecond
void delay(u32 ms) {
  ct = 0; // set the counter to 0
  while (ct < ms) // wait for ms number of milliseconds
int main() {
  XIOModule_Initialize(&gpo, XPAR_IOMODULE_0_DEVICE_ID); // Initialize the GPO module
                              XPAR_IOMODULE_0_DEVICE_ID); // register the interrupt handler
  XIOModule_Start(&gpo); // start the GPO module
  XIOModule_Connect(&gpo, XIN_IOMODULE_FIT_1_INTERRUPT_INTR, timerTick,
                    NULL); // register timerTick() as our interrupt handler
  XIOModule_Enable(&gpo, XIN_IOMODULE_FIT_1_INTERRUPT_INTR); // enable the interrupt
  microblaze_enable_interrupts(); // enable global interrupts
  u8 leds = 0;
  while (1) {
    // write the LED value to port 1 (you can have up to 4 ports)
    XIOModule_DiscreteWrite(&gpo, 1, leds++);
    delay(1000); // delay one second

This code works by first initalizing the GPO module, fixed-interval timer, and interrupts. The fixed-interval timer is setup to fire an interrupt every millisecond which will call our interrupt handler, timerTick. Each time timerTick is called, it increments a global variable ct. The delay function uses ct to delay for specified number of milliseconds.

It is worth looking at the other templates when you create a project to get an idea of some other functions you can use.

Find the hammer icon on the top tool bar to build the project and generate an .elf file. This is the file that tells ISE how to program the processor.

Adding the .elf file to your project

There is one last command to run to finish the project setup.

Back in ISE, go to the Tcl Console again and enter the following command.

microblaze_mcs_data2mem /path/to/project/LED_Controller/Debug/LED_Controller.elf

Note that you will have to replace /path/to/project with your actual path.

It should output something similar to the following.

microblaze_mcs_data2mem: Found 1 MicroBlaze MCS core.
microblaze_mcs_data2mem: Using "LED_Controller.elf" for microblaze_mcs
microblaze_mcs_data2mem: Added "-bd" options to bitgen command line.
microblaze_mcs_data2mem: Running "data2mem" to create simulation files.
microblaze_mcs_data2mem: Bitstream does not exist. Not running "data2mem" to update...
microblaze_mcs_data2mem: Done.

You can now double click Generate Programming File to generate a .bin file that you can load onto your Mojo.

If all went well you should now see the LEDs counting the seconds.

Editing the code

If you want to edit the code in XPS all you have to do after building the project is re-run the Generate Programming File stage in ISE to update the .bin file. You don't need to re-run Synthesize nor Implement Design.