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SD card: missing signal series resistors and no power-down control

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
45/100
Tipo di issue
Funzionalità
Chiarezza
Abbastanza chiara
Stato di attività
Tranquilla
Stack tecnologico
embedded-iot
Ambito
embedded-iot

Direzione di ricerca

Start by locating the SD card slot, SD_CS, SCK/SCL, MOSI, MISO, and SD card VCC in the hardware design. Review the existing MCU GPIO assignments before assessing the proposed signal resistors and separate power-enable control. Done means the schematic and PCB reflect the agreed SD interface protection and card power-control changes.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

enhancement

Summary

The SD card interface has two related issues:

1. No series resistors on SD card signal lines

Standard practice for SD/SPI interfaces is to place 22–33 Ω series resistors on CLK, MOSI, CS, and MISO between the MCU and SD card. These resistors:

  • Limit edge rates, reducing EMI radiation from fast SPI clock edges
  • Protect the MCU GPIO pins if the card is accidentally hot-plugged or the card internal ESD clamp fires
  • Prevent overshoot/undershoot on long traces (a data logger PCB may have several centimeters of trace between the MCU and the card slot)

Without series resistors, the SPI edges are limited only by the MCU drive strength and PCB trace inductance, which on a 2-layer board can cause significant ringing on CLK especially.

Recommended fix: Add 22 Ω 0402 resistors on CLK, MOSI, and CS (MISO typically does not need one as it is driven by the card). Place them at the MCU end of the trace, before the signal runs to the card slot.

2. No power switch on SD card VCC

The SD card supply (typically 3.3 V) is permanently connected. This prevents a known-good recovery technique: power-cycling the SD card to clear a stuck state (infinite loop in card initialization, unresponsive after a failed write, etc.).

Long-duration field data loggers are especially dependent on SD card reliability. A card that enters an unresponsive state cannot be reset without cycling board power, which may cause other system disruptions.

Recommended fix: Add a load switch (e.g., AP2112K, TPS22810, or similar small PFET/load-switch IC) on the SD card 3.3 V supply, controlled by a MCU GPIO. The firmware can then:

  1. Power-cycle the card as part of a robust initialization sequence
  2. Power down the card during deep sleep to save ~100 µA quiescent from the card's standby current
  3. Recover from card errors without a full board reset

The existing MCU GPIO (e.g., SD_CS = IC1 pin) can coexist with a separate power-enable GPIO — these serve different purposes.

Affected components

  • SD card slot (verify designator)
  • SPI signals: SD_CS, SCK/SCL (shared), MOSI, MISO
  • SD card VCC net

🤖 Generated with Claude Code

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8
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