SoC Zynq - PS + PL Integration

Understanding the Zynq architecture: the processor (PS) and FPGA (PL) sides, their AXI communication, and designing a complete embedded system.

What is a SoC?

A SoC (System on Chip) integrates onto a single die:

• One or more processors (ARM, RISC-V…)
• Programmable logic (FPGA)
• Peripherals (memory, USB, Ethernet, UART…)

AMD/Xilinx's Zynq-7000 family is one of the most popular in the industry. It combines a dual-core ARM Cortex-A9 and an FPGA fabric on the same silicon.

The Two Sides of Zynq

Processing System (PS)

The PS side is the software world:

The PS can operate independently of the FPGA side - it boots from its Flash or SD card.

Programmable Logic (PL)

The PL side is the classic FPGA fabric:

• LUTs, flip-flops, BRAMs, DSPs
• All your VHDL runs here
• Can be reprogrammed at runtime (partially)
• Typical clock rates: 100–300 MHz

PS ↔ PL Communication

The PS and PL communicate via AXI interfaces:

Creating a SoC Project in Vivado

Step 1 - New Vivado Project

Create an RTL project and select your Zynq board (PYNQ-Z2, ZedBoard, Cora Z7…).

Step 2 - Create the Block Design

In the left panel: IP Integrator → Create Block Design.

Step 3 - Add the Processing System

Right-click in the canvas → Add IP → search for "Zynq7 Processing System".

Click "Run Block Automation" to automatically apply your board's constraints (DDR presets, clock…).

Step 4 - Configure the PS

Double-click the PS block to open its configuration:

• PS-PL Configuration: enable the required AXI interfaces (GP0, HP0…)
• Clock Configuration: set the PL clock frequency (100 MHz by default)
• MIO Configuration: choose which PS peripherals to enable (UART, USB…)

Step 5 - Add Your VHDL IP

To connect your VHDL to the PS, you wrap it in an IP with an AXI-Lite interface:

1. Tools → Create and Package New IP
2. Choose "AXI-Lite Slave" as the interface
3. Define the AXI registers (read/write)
4. Add your VHDL logic to the generated file

Step 6 - Connect the Blocks

After adding your IP to the block design:

1. Right-click → Add IP and search for your custom IP
2. Click "Run Connection Automation" → Vivado wires the AXI Interconnect automatically

Step 7 - Generate the Bitstream

1. Validate Design (F6) to check connections
2. Right-click the block design → Generate Output Products
3. Right-click → Create HDL Wrapper
4. Generate Bitstream

Step 8 - Export to Vitis / SDK

1. File → Export → Export Hardware (include bitstream)
2. Open Vitis (or SDK for versions < 2020.1)
3. Create a C/C++ bare-metal or Linux application

CPU ↔ FPGA Communication in Practice

Once deployed, the CPU accesses your VHDL registers via AXI-Lite:

// Bare-metal C - read/write PL registers
#include "xparameters.h"
#include "xil_io.h"
 
// Base address of your IP (defined by xparameters.h)
#define MY_IP_BASEADDR   XPAR_MY_IP_0_BASEADDR
 
// Write to register 0 (offset 0x00)
Xil_Out32(MY_IP_BASEADDR + 0x00, 0xDEADBEEF);
 
// Read register 1 (offset 0x04)
uint32_t val = Xil_In32(MY_IP_BASEADDR + 0x04);

On the VHDL side, your IP reads/writes these registers through the AXI-Lite signals generated by Vivado.

Comparison: Pure FPGA vs Zynq SoC

Key Points to Remember

• Zynq separates PS (ARM, software) and PL (FPGA, hardware)
• PS ↔ PL communication goes through AXI interfaces
• Vivado Block Design is the tool for visually assembling blocks
• Your VHDL code becomes a packaged IP accessible from the CPU
• Full flow: VHDL → Package IP → Block Design → Bitstream → Vitis/SDK

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