Compilation of Advanced FPGA Development Interview Questions

Today, I will compile some interview questions related to FPGA development. These questions are derived from actual interview scenarios to help you stand out in your interviews!

📝 Detailed Explanation of Interview Questions

Question 1: Please introduce the two modes of Xilinx FIFO IP core and their differences.

Reference Answer: The Xilinx FIFO IP core provides two operating modes: Standard and FWFT. In Standard mode, data is output in the next clock cycle after the read enable is asserted; in FWFT mode, the first valid data is automatically output when the FIFO is not empty, and the next data is output immediately after the read enable is asserted.

Bonus Answer: In practical projects, FWFT mode is particularly suitable for pipeline processing scenarios. For example, in our image processing system, we used FWFT mode to achieve seamless connection of pixel data, avoiding pipeline bubbles. Standard mode is more suitable for situations requiring strict timing control, such as interfacing with external ADC chips to ensure the accuracy of data sampling moments.

Question 2: How to accurately generate a baud rate of 115200 with a 100MHz clock?

Reference Answer: There are mainly two solutions: integer division and fractional division. The integer division coefficient is 100M/(16×115200)≈54, but there is about a 0.2% error; fractional division can be implemented using an accumulator, achieving ppm-level accuracy.

Bonus Answer: In industrial communication projects, we found that integer division accumulates errors after long-term operation. After switching to the fractional division scheme, we ran continuously for 72 hours without any communication errors. The specific implementation is as follows:

parameter STEP = (115200 * 2**32) / 100_000_000;
reg [31:0] acc;
always @(posedge clk) begin
    acc <= acc + STEP;
    baud_tick <= acc[31];
end

Question 3: Describe the interaction mechanism between the PS and PL sides of the Zynq chip.

Reference Answer: The PS-PL interaction of Zynq is mainly realized through the AXI bus, including AXI_GP interface (general data transfer), AXI_HP interface (high-performance DMA transfer), and AXI_ACP interface (accelerator coherent port).

Bonus Answer: In the smart camera project, we achieved DMA transfer of image sensor data from PL to PS through the AXI_HP interface, reaching a bandwidth of 800MB/s. Key configurations include: burst length set to 256, FIFO depth set to 2048, effectively avoiding bandwidth bottlenecks. Additionally, we utilized the AXI_ACP interface to achieve coherent access between the hardware accelerator and CPU cache.

Question 4: What is the link establishment process after powering on the PHY chip?

Reference Answer: The link establishment of the PHY chip after power-on is divided into four stages: initialization (hardware reset, register configuration), auto-negotiation (detecting link partner capabilities), training (signal equalization, timing calibration), and stabilization (link ready, data transmission begins).

Bonus Answer: During the debugging of Gigabit Ethernet, we encountered a problem where the link downgraded to 100Mbps. By reading the PHY status register, we found that it was due to mismatched electrical parameters. After adjusting the pre-emphasis settings and optimizing the PCB layout, the problem was resolved. This experience made me realize the importance of collaborative optimization between hardware design and software configuration.

Question 5: What functions do the PCS and PMA of the GT transceiver perform?

Reference Answer: The PCS (Physical Coding Sublayer) is responsible for 8B/10B encoding, channel bonding, clock correction, and other digital processing; the PMA (Physical Medium Attachment Sublayer) is responsible for serial-to-parallel conversion, clock data recovery, equalization, and other analog functions.

Bonus Answer: In the 25G optical module project, we iteratively optimized the equalizer parameters in the PMA, reducing the bit error rate from 10^-6 to 10^-12. The key was to combine eye diagram testing with a systematic parameter scanning method to find the optimal equalizer configuration.

Question 6: What is the role of the XDMA IP core in the PCIe system?

Reference Answer: XDMA is a high-performance PCIe DMA solution provided by Xilinx, which encapsulates the complex PCIe protocol into a simple AXI interface. Users can perform data transfer through the AXI_Stream interface, and control signals are accessed via the M_AXI_LITE interface.

Bonus Answer: In the data center accelerator card project, we configured an 8-lane PCIe Gen3, combined with the XDMA DMA engine, achieving nearly 16Gbps of sustained data transfer. Key optimizations included: appropriately setting the DMA descriptor ring size, using multi-queue parallel transmission, and optimizing interrupt coalescing parameters to reduce CPU overhead.

Question 7: How is the clock architecture of the DDR IP core designed?

Reference Answer: The DDR IP core involves multiple clock domains: the reference clock provides a benchmark for the DDR PHY, the system clock controls user logic, and the read/write clock is used for data sampling and output.

Bonus Answer: In high-speed DDR4 design, we paid special attention to skew control of the clock network. By constraining to ensure that the delay difference of the clock to each DQ group is less than 10ps, we ensured the consistency of the sampling window. Additionally, we used write leveling techniques to compensate for phase discrepancies between the clock and data, significantly improving signal integrity.