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Accumulated minor design issues: USB resistors, AVCC ferrite, C42 mismatch, TODO flag, SDA_OB loading

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Valutazione

Difficoltà
5/5
Tempo stimato
Più di una settimana
Idoneità per principianti
25/100
Tipo di issue
Bug
Chiarezza
Da chiarire
Stato di attività
Tranquilla
Ambito
embedded-iot

Direzione di ricerca

Start with the Okapi Eagle schematic/layout and BOM, reviewing the USB resistor values, AVCC connection, C42 attributes, the “Check current rating!!” annotation, SDA_OB device list and pull-ups, and the TLV3691 variant. Trace each finding to its affected component or net and verify the relevant electrical values. Done means the design discrepancies, TODO, bus loading, and part variant are resolved or explicitly documented before CAM output.

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

Descrizione

bug

Summary

This issue collects minor design findings from the Okapi schematic/layout review that are individually low-priority but collectively worth tracking and addressing before a production build.

1. USB resistors: 27 Ω placed vs. 22 Ω USB 2.0 spec

USB 2.0 full-speed specification requires 22 Ω series resistors on D+ and D−. The schematic places 27 Ω. The 5 Ω difference shifts the impedance of the USB line from 45 Ω (22 Ω + 28 Ω trace/source) toward 50 Ω, which slightly increases reflections. In practice, most USB 2.0 full-speed devices work with 27 Ω due to the relaxed timing requirements at 12 Mbps, but strict compliance requires 22 Ω.

Recommended fix: Change USB series resistors to 22 Ω.

2. AVCC ferrite bead missing

The ATmega1284P application circuit (AVR042) and datasheet recommend a ferrite bead (e.g., 600 Ω at 100 MHz, BLM21P/BLM31P series) between VCC and AVCC, with a 10 nF capacitor from AVCC to GND, to isolate the ADC analog supply from digital switching noise. Currently AVCC is tied directly to 5V_AUX (same as VCC) with no filtering.

Without this filter, fast switching on IC1's digital pins injects noise directly into the ADC supply reference, degrading ADC accuracy. For sensor measurements (pressure, voltage monitoring, etc.) this degrades resolution.

Recommended fix: Add a ferrite bead (e.g., Murata BLM21PG600SN1D) between 5V_AUX and AVCC, with a 100 nF + 10 nF capacitor combination from AVCC to GND.

3. C42 value field mismatch (schematic vs. layout)

Capacitor C42 appears to have a discrepancy between the value shown in the schematic symbol and the value in the BOM or layout attribute. Verify that the placed value matches the intended design. Document the correct value and reconcile all instances in the Eagle file before generating CAM outputs.

4. Unresolved "Check current rating!!" TODO comment in schematic

There is at least one instance of a "Check current rating!!" annotation in the Eagle schematic. This type of TODO flag in a schematic submitted for fabrication is a risk — it indicates a design parameter was not verified at the time of layout. Identify the specific net/component flagged, calculate the actual maximum current, and either confirm the existing component rating is adequate or replace it with a correctly rated part.

5. SDA_OB bus may be heavily loaded

SDA_OB appears to be the "on-board" I²C bus that serves multiple sensors (BME280, BMA456, MLX90393, VEML6030, ADS1115, DS3231M RTC, PAC1934, MCP23018, and possibly the B-Series I²C connection).

I²C capacitance budget per specification: 400 pF maximum at 100 kHz (standard mode), 400 pF at 400 kHz (fast mode). With 8+ devices each contributing 5–15 pF package capacitance, plus PCB trace capacitance (~1 pF/cm on typical FR4), the bus may approach or exceed this budget. Exceeding 400 pF total causes degraded signal rise times and can cause data errors at fast-mode speeds.

Recommended action:

  • Count all devices on SDA_OB and estimate total bus capacitance
  • If total exceeds ~350 pF, consider splitting the bus (e.g., move some devices to SDA_CTRL or a dedicated bus segment via an I²C bus expander)
  • Alternatively, lower the bus pull-up resistance (currently check the value) to shorten rise time — but this increases quiescent current
6. TLV3691 comparator: specific variant dependency

The TLV3691 is used as a comparator (likely for LBI/low-battery indicator or similar). The TLV3691 comes in multiple threshold variants (A, B, C suffixes with different fixed thresholds). Verify that the BOM specifies the correct variant, and that the threshold matches the intended detection voltage for the specific rail being monitored. A generic "TLV3691" in the BOM without the suffix will result in wrong-threshold parts being sourced.

Priority

These issues are individually minor but should all be resolved before a production run. The USB resistor and AVCC ferrite changes are most impactful for measurement quality. The TODO flag must be resolved before any fabrication order.

🤖 Generated with Claude Code

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