FPGA Practical Design - The Ultimate Guide to BRAM Initialization

When mapping a low-power AI semiconductor architecture onto an FPGA, one of the very first hurdles you encounter is 'Memory Design'. Particularly, efficiently loading massive weight data into BRAM for a MAC (Multiply-Accumulate) array dictates the development speed and flexibility of your entire system.

Usually, when first learning the Xilinx Vivado environment, you are taught to generate a Block Memory Generator IP and attach a .coe file. But what if your weights change every time you update your model, and you have to regenerate the IP and re-synthesize every single time? As your design scales, this method introduces massive inefficiencies.

In this article, I want to share 3 practical tips for BRAM initialization and foolproof hardware-level verificationthat I’ve learned the hard way while designing an AI NPU for edge devices.

1. Breaking Free from GUI and .coe: Leveraging $readmemh

Hardware design needs to be as agile as software. A vastly superior approach is to extract your trained weights in a 64-bit Hex format using Python, save them as a .mem file, and load them directly within your RTL (SystemVerilog) code.

// BRAM으로 추론될 메모리 배열 선언
(* ram_style="block" *) logic [63:0] ram [0:1023];

initial begin
    $readmemh("weights_data.mem", ram);
end

// Memory array targeted for BRAM inference
(* ram_style="block" *) logic [63:0] ram [0:1023];

initial begin
    $readmemh("weights_data.mem", ram);
end

Practical Troubleshooting Tip:

If you try to dynamically allocate the file path using a string type parameter, Vivado synthesis often halts or fails to recognize the path, throwing a [Synth 8-27] error.

Vivado is notoriously weak at handling dynamic string variables during synthesis. To resolve this, avoid passing the file name as a parameter. Instead, hardcode the local path inside the module, or pack the string into a wide logic array like logic [8*MAX_CHARS-1:0].

2. The Culprit Behind BRAM Inference Failure: Resetting the Memory Array

There are times when you write seemingly perfect RTL, only to open the synthesis results and find your design plastered with thousands of LUTs and Flip-flops instead of clean BRAM blocks. In most cases, this happens because you connected a reset signal to the memory array itself.

Physical BRAM primitives inside an FPGA (like RAMB36E2) do not possess the hardware capability to asynchronously clear the entire memory core to zero in a single clock cycle.

The Solution:

• Never apply an asynchronous or synchronous reset to the actual memory array variable (ram) where the data is stored.
• Instead, apply the reset logic only to the Output Register where the data exits the memory.
• Additionally, it is highly recommended to add the (* ram_style="block" *) attribute right before the array declaration to explicitly command Vivado to infer a BRAM.

3. Perfectly Verify Initialization in 1 Second via Cell Properties

Once you write the initialization logic, it’s natural to doubt whether the data actually made it into the bitstream. It is common for data to be dropped in the actual hardware due to file path issues. Many engineers run a Post-Implementation Functional Simulation to verify this.

However, on massive modern devices like the Versal series, this simulation is often poorly supported by the tool, throwing errors like Module not found and refusing to even run. In these cases, you don't need a simulation. After implementation is complete, directly open the physical netlist in the tool and visually verify the data.

The Ultimate 1-Second Verification Method:

1. Go to the Flow Navigator and open Implemented Design.
2. Press Ctrl + F to search for and click your BRAM cell (e.g., RAMB36E2).
3. In the Cell Properties window on the bottom left, open the Properties tab.
4. Check the values of parameters like INIT_00 to INIT_7F, and INITP_00.

"My data is 64-bit, why is the property filled with 256-bit data?" (The Secret of Data Packing)

You might panic when you first see this property window. You fed it a .mem file with 64-bit words per line, but Vivado shows a long 256-bit Hex string like 256'hB40C....

This is a phenomenon called Data Packing, bridging the gap between your logical design and the physical structure.

• A physical BRAM cell manages initialization data in 256-bit chunks per row (INIT_xx).
• To prevent wasted space, Vivado packs four of your 64-bit data words into a single row.
• Furthermore, because you utilized a wide 64-bit data width, Vivado even repurposed the INITP_xx space—originally reserved for Parity bits—to store your regular data.

References: AMD