Introduction

We have previously discussed the essence of ping-pong operations, but when it comes to practical implementation in engineering, an unavoidable question arises: How should the buffer size be determined?

Buffer design may seem like just “storing more data,” but if it is too small, it may lead to packet loss due to mismatched upstream and downstream rates; if it is too large, it can result in resource wastage and increased latency. The size of the ping-pong buffer actually depends on the characteristics of the system data flow, the rates of the upstream and downstream interfaces, and the delays in the processing chain.

The Essence of Ping-Pong Operations

Ping-pong operations refer to double buffering switching:

• • When the A buffer is being read, the B buffer is being written;
• • When B is full, switch to read B and write A.

Therefore, it is essential to ensure that each buffer is large enough to hold one frame/batch of data, otherwise switching cannot occur.

Thus, the most basic buffer size is:

SingleBufferSizeMaximumFrameDataVolumeTotalBufferPoolSizeSingleBufferSize

Data Flow Rate Matching

• • Upstream Write Rate: The rate at which data enters the FPGA (e.g., ADC, PCIe, DMA, Ethernet).
• • Downstream Read Rate: The rate at which the FPGA processes or outputs data.
• • The role of the ping-pong buffer is to ensure that data is not lost when upstream is fast/downstream is slow or upstream is bursty/downstream is steady.

Calculation Formula Concept

InputRateOutputRateMaximumRateMismatchedTime

If the rates at both ends are equal (theoretically), then it is sufficient to cover the processing delay.

Bus Burst Transfer

• • If your data is transported through burst transfer interfaces such as AXI, PCIe, DDR, you must consider the burst length.
• • The buffer pool must at least cover one maximum burst length to avoid efficiency loss due to splitting.

Processing Latency

• • If there is latency in the FPGA’s internal processing pipeline, the buffer must be able to “sustain” this latency period.
• • For example: If the upstream is continuously writing, and the FPGA needs to wait for a computation to complete before outputting, sufficient buffer must be prepared to store the data during this time.

System Design Practices

• • A safety margin (headroom) is usually added, generally taken as 1.2× to 2× the theoretical value.
• • If the peripheral clock domains are different, the buffer must also be able to absorb clock jitter/drift that causes rate differences.

EXAMPLE

Assuming a scenario:

• • Data source: ADC → FPGA, rate 200 MB/s
• • Downstream interface: PCIe DMA, average rate 180 MB/s
• • Rate difference: 20 MB/s
• • Possible maximum burst time: 10 ms

Calculation:

One buffer must be at least 0.2 MB, so the ping-pong requires 0.4 MB, plus a margin, approximately 1 MB is reasonable.

Final Thoughts

The size of the ping-pong buffer does not have a fixed answer; it depends more on the characteristics of the system itself. Generally, the design needs to comprehensively consider the following aspects:

1. 1. Single Frame Data Size — Ensure that a complete switch is not interrupted.
2. 2. Upstream and Downstream Rate Difference × Maximum Burst Time — Absorb data backlog caused by rate mismatches.
3. 3. Bus Burst Characteristics — Ensure the efficiency and integrity of burst transfers.
4. 4. Processing Latency — The buffer must be able to sustain internal pipeline or processing wait times.
5. 5. Safety Margin — Reserve some redundancy to avoid issues in extreme cases.

In summary, the design of the ping-pong buffer size is about finding a balance between performance, reliability, and resource utilization.

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#FPGA #Digital IC#Ping-Pong Operations#Buffer Design#Double Buffer Switching#Data Flow Rate Matching#Bus Burst Transfer#AXI#PCIe#DDR#Processing Latency#Latency#Safety Margin#Clock Domain#Clock Jitter#Rate Difference#Burst Time#Resource Utilization#Performance Optimization#Reliability#ADC#DMA#Ethernet#Calculation Example#Buffer Size#FPGA Design#System Design#Timing Constraints#Data Integrity#Pipeline#Margin Design#FPGA Engineering#Interface Rate#Data Loss#Resource Optimization#Latency Optimization#Burst Length#Efficiency Optimization #Ping-Pong Buffer