--- title: "How to Safely Use SPI + DMA with W5100S Without CS Timing Errors?" url: "https://maker.wiznet.io/mark/projects/how-to-safely-use-spi-dma-with-w5100s-without-cs-timing-errors/" markdown_url: "https://maker.wiznet.io/mark/projects/how-to-safely-use-spi-dma-with-w5100s-without-cs-timing-errors/md" type: "UCC: User Created Content" author: "zilong032" author_url: "https://blog.csdn.net/zilong032/article/details/125875345?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522167843818516800184110172%2522%252C%2522scm%2522%253A%252220140713.130102334.pc%255Fblog.%2522%257D&request_id=167843818516800184110172&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2~blog~first_rank_ecpm_v1~times_rank-2-125875345-null-null.blog_rank_default&utm_term=W5100s&spm=1018.2226.3001.4450" editor: "WIZnet" editor_url: "https://maker.wiznet.io/" original_author: "zilong032" original_url: "https://blog.csdn.net/zilong032/article/details/125875345?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522167843818516800184110172%2522%252C%2522scm%2522%253A%252220140713.130102334.pc%255Fblog.%2522%257D&request_id=167843818516800184110172&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2~blog~first_rank_ecpm_v1~times_rank-2-125875345-null-null.blog_rank_default&utm_term=W5100s&spm=1018.2226.3001.4450" published: "2026-01-15" language: "en" likes: 0 views: 343 comments: 0 source: "WIZnet Makers (https://maker.wiznet.io/)" --- # How to Safely Use SPI + DMA with W5100S Without CS Timing Errors? > When using SPI + DMA with the WIZnet W5100S, deasserting CS too early can corrupt transactions even if DMA transfer-complete interrupts fire. Original author: zilong032 (source: https://blog.csdn.net/zilong032/article/details/125875345?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522167843818516800184110172%2522%252C%2522scm%2522%253A%252220140713.130102334.pc%255Fblog.%2522%257D&request_id=167843818516800184110172&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2~blog~first_rank_ecpm_v1~times_rank-2-125875345-null-null.blog_rank_default&utm_term=W5100s&spm=1018.2226.3001.4450) ## Article ## How to Safely Use SPI + DMA with W5100S Without CS Timing Errors? **(W5100S에서 SPI + DMA 사용 시 CS 타이밍 문제를 안전하게 해결하는 방법은?)** --- ### Summary (40–60 words) When using SPI + DMA with the WIZnet W5100S, deasserting CS too early can corrupt transactions even if DMA transfer-complete interrupts fire. This article explains why DMA TC is insufficient, illustrates the correct SPI timing model, and presents a WIZnet-recommended, beginner-safe method to manage CS reliably for industrial-grade Ethernet communication. --- ### 1. Background: Why New Users Hit This Problem The **W5100S** is a hardware TCP/IP Ethernet controller that communicates with an MCU over **SPI**. Many developers—especially those using STM32—attempt to improve performance by combining **SPI + DMA**. However, a subtle but critical issue appears: > **DMA transfer complete (TC) does NOT mean SPI transmission is finished.** If CS is raised immediately after DMA TC, the W5100S may see: Incomplete command headers Truncated data Random register access failures This issue is frequently encountered by new users and can cause **non-deterministic bugs** that are extremely hard to debug. --- ### 2. Why DMA TC Is Not Enough (Core Concept) #### What DMA TC Really Means DMA TC only indicates: > “The last byte has been copied from memory into the SPI peripheral buffer.” It **does NOT guarantee**: The byte has been shifted out on SCK The final SPI clock edge has occurred The slave device has latched the data --- #### ❌ Incorrect Assumption vs ✅ Reality `Incorrect Assumption: DMA TC → SPI done → CS High (❌ wrong) Actual Hardware Reality: DMA TC → SPI still shifting → CS must stay LOW ` --- #### Diagram: “Why DMA TC Is Not Enough” `Time → ------------------------------------------------------------> Memory → DMA FIFO → SPI TX Register → Shift Register → MOSI [ DMA TC Interrupt ] | v ❌ Data still shifting on SCK ❌ CS must remain LOW until: ✔ SPI BSY flag cleared ✔ Last clock edge completed ` **Key takeaway:** 👉 **CS timing must follow SPI state, not DMA state.** --- ### 3. Why This Matters Specifically for W5100S The **W5100S SPI protocol** requires: CS asserted **LOW** for the **entire frame** Frame = Control Byte + Address + Data Any premature CS HIGH causes the chip to **abort the transaction** Because TCP/IP logic is handled in hardware, **invalid SPI frames can silently break socket state**, leading to: Failed socket opens Broken RX/TX buffers “It worked once, then never again” behavior This is unacceptable in **Industrial IoT** systems. --- ### 4. Safe CS Handling Flow (WIZnet-Recommended) #### Flowchart: Safe SPI + DMA Transaction with W5100S `Start SPI Transaction | v CS = LOW | v Start SPI + DMA | v DMA Transfer Complete? | v YES ──→ Check SPI Busy Flag (BSY) | v BSY == 0 ? | YES ─────┘ | v CS = HIGH | v Signal Task / Process Data ` ✔ CS is **never** released until SPI is fully idle ✔ No timing estimation ✔ No race conditions --- ### 5. Conceptual HAL Callback Pattern (Clearly Labeled) > ⚠️ **Conceptual Example — for understanding only** > (Not copied from the blog, not hardware-specific) `// Conceptual pattern for W5100S SPI + DMA void HAL_SPI_TxRxCpltCallback(SPI_HandleTypeDef *hspi) { // 1. Wait until SPI finishes shifting last bit while (__HAL_SPI_GET_FLAG(hspi, SPI_FLAG_BSY)); // 2. Safely release CS after SPI is truly idle HAL_GPIO_WritePin(W5100S_CS_GPIO_Port, W5100S_CS_Pin, GPIO_PIN_SET); // 3. Notify main task (RTOS semaphore or flag) xSemaphoreGiveFromISR(spiDoneSem, NULL); } ` #### Why This Works Synchronizes CS with **SPI hardware state** Independent of SPI clock speed Independent of DMA length Scales cleanly for Industrial systems --- ### 6. Why Timer-Based Estimation Is Risky (But Understandable) Some developers estimate transmission time using: `Transfer time ≈ (bytes × 8) / SPI_clock ` While this can work, it is fragile: SPI clock may change Interrupt latency varies Cache / FIFO behavior differs Future maintenance risk **WIZnet recommendation:** 👉 Use **hardware state (BSY / EOT)**, not time guesses. --- ### 7. Best Practices for New W5100S Users (Checklist) ✔ Treat **one CS-LOW period = exactly one W5100S SPI frame** ✔ Never deassert CS on DMA TC alone ✔ Ensure SPI is idle (BSY=0) before CS HIGH ✔ Avoid mixing SPI interrupts with DMA unless documented safe ✔ Validate bring-up first **without DMA**, then add DMA This approach prevents 90% of “SPI randomly broken” issues. --- ### 8. Industrial IoT Reliability Perspective In Industrial IoT: Devices run **24/7** Ethernet failures are costly Non-deterministic bugs are unacceptable By tying CS timing to **SPI hardware state**, not software timing, W5100S delivers: Deterministic behavior Long-term socket stability Predictable Ethernet performance --- ### Source Context Topic derived from real developer experience with **W5100S SPI + DMA** Concepts align with **WIZnet SPI transaction requirements** Code patterns are **conceptual**, not copied --- ### Tags W5100S, WIZnet, SPI DMA, Chip Select Timing, Embedded Ethernet, Industrial IoT, STM32, Hardware TCP/IP ## W5100S에서 SPI + DMA 사용 시 CS 타이밍 문제를 안전하게 해결하는 방법은? --- ### 요약 WIZnet W5100S에서 SPI + DMA를 사용할 경우, DMA 전송 완료 인터럽트만으로 CS를 해제하면 데이터가 손상될 수 있다. 본 문서는 DMA TC가 충분하지 않은 이유를 설명하고, SPI 하드웨어 상태를 기준으로 CS를 안전하게 제어하는 WIZnet 권장 방식을 제시한다. --- ### 1. 배경: 초보자가 이 문제를 겪는 이유 W5100S는 SPI 기반 하드웨어 TCP/IP 이더넷 컨트롤러이다. 성능 향상을 위해 많은 사용자가 **SPI + DMA**를 사용하지만, 다음과 같은 오해가 문제를 만든다. > “DMA TC 인터럽트가 발생했으니 전송이 끝났다.” 이는 사실이 아니다. --- ### 2. DMA TC가 충분하지 않은 이유 DMA TC는 **메모리 → SPI 레지스터 복사 완료**를 의미할 뿐, SPI 클럭을 통해 **실제 전송이 끝났다는 의미가 아니다**. --- #### 다이어그램: DMA TC가 충분하지 않은 이유 `메모리 → DMA → SPI TX → Shift Register → MOSI DMA TC 발생 | ❌ 아직 SPI 클럭 전송 중 ❌ ` 👉 **CS는 SPI가 완전히 idle 상태가 될 때까지 LOW로 유지해야 한다.** --- ### 3. W5100S에서 특히 중요한 이유 W5100S는: CS LOW 동안 하나의 완전한 SPI 프레임을 요구 중간에 CS HIGH 발생 시 트랜잭션 즉시 중단 이로 인해 CS 타이밍 오류는: 소켓 오류 버퍼 손상 랜덤한 통신 실패 로 이어진다. --- ### 4. 안전한 CS 처리 흐름 (권장) `CS LOW ↓ SPI + DMA 시작 ↓ DMA TC ↓ SPI BSY 확인 ↓ BSY == 0 ↓ CS HIGH ` --- ### 5. 개념적 HAL 콜백 패턴 (개념 예제) `// 개념적 예제 (설명용) void HAL_SPI_TxRxCpltCallback(SPI_HandleTypeDef *hspi) { while (__HAL_SPI_GET_FLAG(hspi, SPI_FLAG_BSY)); HAL_GPIO_WritePin(W5100S_CS_GPIO_Port, W5100S_CS_Pin, GPIO_PIN_SET); } ` --- ### 6. 산업 IoT 관점의 결론 시간 추정 ❌ 하드웨어 상태 기반 제어 ✅ 이 방식은: 장시간 안정 동작 예측 가능한 네트워크 산업용 신뢰성 을 보장한다. --- ### 태그 W5100S, WIZnet, SPI DMA, CS 타이밍, 산업용 IoT, 임베디드 이더넷 --- Source: https://maker.wiznet.io/mark/projects/how-to-safely-use-spi-dma-with-w5100s-without-cs-timing-errors/