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IC1 (ATmega1284P) VCC bypass capacitor missing

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#17 0 comentarios 0 reacciones 0 asignados Ver en GitHub

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Evaluación

Dificultad
3/5
Tiempo estimado
1-2 días
Aptitud para principiantes
65/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Tranquilo
Área
embedded-iot

Línea de trabajo

Start with the ATmega1284P datasheet Table 1-1 and AVR042 to identify every VCC pin on IC1 and confirm the required local bypassing. Inspect the IC1 schematic and board layout, then verify that each VCC pin has its own nearby 100 nF ceramic capacitor and that the separate 10 µF 5V_AUX bulk capacitor remains complementary.

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

Descripción

bug

Summary

IC1 (ATmega1284P, WQFN-44) has no dedicated bypass capacitor on its VCC pin (pin 10 or equivalent — verify against package datasheet). VCC (5V_AUX) bulk decoupling exists elsewhere on the board, but the ATmega application note (AVR042) and the ATmega1284P datasheet both require a 100 nF ceramic capacitor placed as close as possible to the VCC pin of the device. Without this, high-frequency transients from fast I/O switching can appear on the supply rail at the die, causing false resets, clock jitter, or intermittent ADC errors.

Why this matters for Okapi specifically

IC1 drives a large number of digital peripherals (SD card, FT231X UART, MCP23018 I/O expander, multiple SPI/I²C devices). Fast switching on these lines induces dI/dt spikes on the supply. Shared supply decoupling placed remotely on the board cannot suppress these transients at the package level.

Affected components

  • IC1: ATmega1284P-MUR (WQFN-44)
  • Net: 5V_AUX

Recommended fix

Add a 100 nF 0402 X5R/X7R ceramic capacitor from IC1 VCC to GND immediately adjacent to the VCC pin in the layout. A 10 µF bulk capacitor on 5V_AUX near the regulator output (U3) is complementary but does not substitute for a local bypass at the IC.

Also check: the ATmega1284P WQFN package has multiple VCC pins (see datasheet Table 1-1). Each VCC pin should have its own local 100 nF bypass cap.

🤖 Generated with Claude Code

Lenguaje dominante
G-code
Estrellas
8
Forks
2
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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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