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

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Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
45/100
Tipo de issue
Nueva funcionalidad
Claridad
Bastante claro
Estado de actividad
Tranquilo
Stack tecnológico
embedded-iot
Área
embedded-iot

Línea de trabajo

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.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

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

Lenguaje dominante
G-code
Estrellas
8
Forks
2
Sin PR fusionados en 30 d

Preparar el entorno

Primeros pasos

  1. Lee el issue completo y luego la guía de contribución del proyecto.
  2. Comenta en el issue que vas a ocuparte — evita que dos personas hagan lo mismo.
  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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