# Implementing a dual core processor in FPGA.

Few years ago, microprocessors manufacturers competed to get the faster processor. High speed means more tasks can be executed in the same amount of time, but also, means higher power consumption. To improve this, the voltages of the core starts decreasing to levels below 1V, and according the equation P=VxI, the power will be decreased as well, but decreasing the voltage is not the solution to all their problems, because as the voltage decrease, the transistors in the processors turns slower, so what we solve on the one hand, we make it worse on the other. At this point the manufacturers arrived a compromise between speed and voltage, and I am not sure if there was any processor above 5GHz. In order to keep improving the performance of the processors, manufacturers starts to integrate in the same part more than one processor, at the beginning with many shared parts, but now we can find several complete processors in the same part that can work together or separately executing different tasks or even different operating systems. Multi-core fever is not only for the high-end processors, there are many “little” microprocessors or even microcontroller families that offer 2 or more cores with a very competitive prices.

At this point, we have seen the advantages of multi-core systems, and in this blog we already use them when we use Zynq7000 family (Arm A9 dual-core processor), and Zynq MPSOC family (Arm A53 Quad-core processor + Arm R5 dual-core processor). These two examples are perfect to explain the big difference between them. Both are multi-core systems, but Zynq7000 has 2 similar cores, with the same characteristics, benefits and limitations, on the other side, Zynq MPSOC has 2 different kind of processors, and each one is complemented with the other one since they have different capabilities. Now, what about FPGAs? well, FPGA are a blank canvas, and we can implement a customizable multi-core system, even in little FPGA. In this post I will show you how we can implement in the Digilent’s Arty A35 board a dual core system, with one core processor dedicated to communications, run a realtime Operating System (RTOS), and manage the main IOs. This core will act as main core. The other one will be used as mathematical co-processor, with floating point capabilities, and fast memory access.

First, we will open Vivado and we have to create a new project for the Digilent Arty A35T board (board file available here). This board is based on Artix 7 A35T, an FPGA with 33k logic cells. For this kind of projects we also need to verify the amount of Block RAM available, since this memory will be used as local memory for the co-processor. In this part we have a total of 1.8 Mb of BRAM, what is enough for this project. Also, in the Arty board we can find 256Mb of DDR3 RAM that we will use to store the application of the main core, several leds and buttons, and many IOs. Once the project is created we will create a block design. Now on the left of the block design we can found a tab named Board. In this tab we can found the hw elements that we can find in the board. First we will add the clock. Now by double clicking in the IP we can configure it. The first tab is the interface with the board, and we can see how the input CLK_IN1 is connected to the signal sys_clock, that is configured by default.

In the Output Clocks tab we will configure the needed clocks for the design. Since we will use the external DDR3 RAM, we need to include in our design a MIG. This IP needs a 200MHz clock, and a 166Mhz clock for the Arty board. Also we will need a 25Mhz clock for the Ethernet phytter. IN this tab, we also need to configure the Reset Type as Active Low.

Now we can add the Memory Interface Generator. inputs clk_ref_i and sys_clk_i are connected to an external ports by default. We need to delete the external ports and connect the input clk_ref_i to 200MHz clock, and the input sys_clk_i to the 166Mhz clock.

Now we can add the first Microblaze processor. Clicking in Run Block Automation, we will select the Microcontroller preset, and the rest of the option will remain by default.

The button Run block automation will be enabled again, this time to configure the ports and memories of the Microblaze. Local memory size for the main processor has no importance since the code will run in the external memory, but is interesting keep enabled the local memory because it is faster than the external, and maybe we will need execute some task faster than others, so a 8kB of memory should be enough. Regarding the clock, has to be connected to the output clock of the MIG module, since the processor needs to be synchronized with this clock to enable the communication to read the instructions and write data.

At this point we will have our Microblaze configured with the basic connections. Now we have to connect the AXI interface to enable the RAM memory. reset ports will be connected to the external reset automatically. AXI connection of the MIG, can be connected to the Cached port or to the peripherals port. cached port includes a cache memory that improves the performance, and is very interesting when we are using external memories.

Now we will add the rest of the peripherals starting by the Ethernet MII, that will need to make external the 25Mhz clock. Also we will add from the board tab the USB UART, Leds and Push Buttons. All of them connected to the Peripheral AXI and the ui_clk. Also we will add an AXI Timer that will be used for the RTOS. The entire block design for the main core is shown in the next image.

Now is the time of the co-processor. We need to add a second Microblaze to the block design. This time we will change the default presets. First we need to apply the Real-time preset. Also, since this processor have no access to the external RAM memory, weare going to increase the local memory up to 32kB. Remember that this processor is only to perform some mathematical operations, so the amount of the code will be low.Also we will disable the interrupt controller, and the clock will be connected to the 166MHz to improve the performance.

In the configuration of the second processor we will disable the Data and Instructions cache and we will enable the Floating Point Unit. For now we don’t open the Run Connection Automation. First we will add the peripherals that this processor will be access. We need to include in the block design the RGB leds of the board and the switches. Also we will add a second Uart Lite IP to debug this second processor. Now we can click over Run connection automation and connect all of this peripherals to the AXI port of the Microblaze 1 and the 166MHz clock. We will keep unselect the AXI port of the Microblaze_1 since the only options are available are the peripherals of the microblaze_0. This port will be automatically connected when we connect the rest of the peripherals to the microblaze_1 AXI peripheral.

Now we have a block design with 2 microblaze processors that can run independently, but we need a way to communicate them. There are some methods to communicate both peripherals, and we will use a shared memory with separate regions for write and read. To do that we will add a AXI Bram Controller, and configure it with only one BRAM interface. Now we can connect the AXI port to the microblaze_0 AXI port by clicking on Run connection automation. BRAM block created has only one interface, we need to change this in the configuration of the BRAM block by configuring the Memory Type as True Dual Port RAM. Now we will add a second AXI BRAM controller, again with only 1 BRAM interface, and we will connect this one to the AXI interface of the microblaze_1, and the BRAM port to the second port of the BRAM added before. This way both processors have access to this memory.

The entire block design is shown in the next figure.

Finally, we can generate the wrapper of the block design.

Now we have to define the constraints for the ports that are not configured by default, that are the 25 MHz ethernet reference clock, and the second uart for the co-processor connected to the PMOD A.

set_property PACKAGE_PIN G18 [get_ports eth_ref_clk]
set_property IOSTANDARD LVCMOS33 [get_ports eth_ref_clk]

set_property PACKAGE_PIN G13 [get_ports uart_rtl_txd]
set_property IOSTANDARD LVCMOS33 [get_ports uart_rtl_txd]

set_property PACKAGE_PIN B11 [get_ports uart_rtl_rxd]
set_property IOSTANDARD LVCMOS33 [get_ports uart_rtl_rxd]
~                                                         

Now we can generate the bitstream, and then export the hardware including the bitstream.

Once the hardware is exported, we have to open Vitis from Tools > Launch Vitis IDE.

Inside Vitis we will create a new Platform Project to from the .xsa file created before. In the platform creation wizard, only one processor can be included, so we will include the microblaze_0.

Once the platform is created, we can add a new domain for the microblaze_1 processor.

Now we have a platform with 2 different processors, The next is create an application project for the first Microblaze, using the template Hello World. The name will be hello_world_1, and also we will create a system named hello_world_system. In this system we will create the application for both processors, and then we will debug the entire system.

One the project for the microblaze_0 is created, we will do the same for the microblaze_1, using the same system. The final project tree is the next.

When both projects are created we are going to modify the template. The goal will be that both processors send a message through uart, and then the microblaze 1 writes a word in the first half of the shared memory, and read a word in the second half of the shared memory. On the other side, the microblaze_1 will send a message through its uart, that is connected to PMOD_A, and then write a word in the second half of the shared memory and read a word in the first half of the shared memory.

The code for the microblaze_0 is the next.

#include <stdio.h>
#include "platform.h"
#include "xil_printf.h"
#include "xil_io.h"

int main()
{
init_platform();

print("Hello World from Microblaze 0!!\n\r");
print("Successfully ran Hello World application \n\r");

Xil_Out32(SHARE_WRITE_MB0_BASE, 0x00003434);

cleanup_platform();
return 0;
}


And the code for the microblaze_1, with read and write addresses flipped is the next.

#include <stdio.h>
#include "platform.h"
#include "xil_printf.h"
#include "xil_io.h"

#define SHARE_WRITE_MB1_BASE (0xC0000000 + 0x00000800U)
#define SHARE_WRITE_MB1_END 0xC0000fff

int main()
{
init_platform();

print("Hello World from Microblaze 1\n\r");
print("Successfully ran Hello World application \n\r");

Xil_Out32(SHARE_WRITE_MB1_BASE, 0x00004545);

cleanup_platform();
return 0;
}


The outputs in two putty terminales are shown in the next capture.

At this point we have a heterogeneous dual core system with a main processor that will be in charge of manage the tasks and the communications, and a second fastest processor with floating point capabilities. besides this 2 processors, remember that both Microblaze have AXI interface, so we can add hardware accelerators for specific tasks like HASH computing or xFFT.