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Published January 15, 2026 ©

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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.

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PROJECT DESCRIPTION

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 + DMA를 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 완료를 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

 

 

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