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BOM: settle RTC battery on CR2032, mark optional

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The backup pin doesn't charge, so a rechargeable ML2032 would just
drain with a third of the capacity. Adds the expected life (~1.5-2 yr
typical), polarity and post-fit test notes, and marks the item low
priority since NTP covers almost every online boot.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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mikkeli
2026-10-08 09:43:47 +09:00
parent 9e5e41f071
commit 787c414c0a
1 changed files with 6 additions and 2 deletions
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docs/BOM.md
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@@ -56,14 +56,15 @@ type: reference
## RTC backup battery ## RTC backup battery
**Status: research pending.** Part choice and footprint not yet verified against NVIDIA's schematic. **Status: optional, low priority.** Battery choice settled (CR2032); footprint/R560 still to verify against NVIDIA's schematic.
| Field | Value | | Field | Value |
|-------|-------| |-------|-------|
| **Purpose** | Keep the RTC (`rtc0`, `nvvrs-pseq-rtc`) running while the board is unpowered, so the clock is correct at boot without network or GPS | | **Purpose** | Keep the RTC (`rtc0`, `nvvrs-pseq-rtc`) running while the board is unpowered, so the clock is correct at boot without network or GPS |
| **Carrier board** | P3768 (Orin Nano Developer Kit): pads present, **connector not populated** (needs soldering) | | **Carrier board** | P3768 (Orin Nano Developer Kit): pads present, **connector not populated** (needs soldering) |
| **Connector (per NVIDIA docs, as quoted on the forum)** | Wieson AC2651-0011-003-HH, 2-pin, 1.25 mm pitch. A community-used substitute is Molex 53398-0271 header + 51021-0200 housing | | **Connector (per NVIDIA docs, as quoted on the forum)** | Wieson AC2651-0011-003-HH, 2-pin, 1.25 mm pitch. A community-used substitute is Molex 53398-0271 header + 51021-0200 housing |
| **Battery** | TBD: coin cell with leads; confirm chemistry/voltage against the carrier board spec (SP-11324-001) | | **Battery** | **CR2032** (primary 3 V lithium) in a holder with leads. **Not ML2032 or other rechargeables:** the module datasheet says the backup pin (PMIC_BBAT) does not support charging, so a rechargeable just drains, and it has ~⅓ of the capacity |
| **Expected life** | RTC draws 12–50 µA while unpowered (~15 µA measured on the forum) → CR2032 (~220 mAh) lasts ~1.5–2 years typical, ~6 months worst case. Drains only while the board has no power |
| **Extra parts** | Forum reports also populating R560 (0.56 Ω, e.g. Panasonic ERJ2BQFR56X); confirm in the P3768 schematic | | **Extra parts** | Forum reports also populating R560 (0.56 Ω, e.g. Panasonic ERJ2BQFR56X); confirm in the P3768 schematic |
### Notes ### Notes
@@ -71,3 +72,6 @@ type: reference
- Without it the clock boots at a stale date (seen 2026-10-08: June 4) until NTP syncs. See the [time sync report](impl/reports/TIME-SYNC.md). - Without it the clock boots at a stale date (seen 2026-10-08: June 4) until NTP syncs. See the [time sync report](impl/reports/TIME-SYNC.md).
- Forum sources: [RTC battery on Orin Nano DevKit](https://forums.developer.nvidia.com/t/rtc-battery-on-jetson-orin-nano-developer-kit/296732), [How to add RTC battery](https://forums.developer.nvidia.com/t/how-to-add-rtc-battery-in-jetson-orin-nano/255792). - Forum sources: [RTC battery on Orin Nano DevKit](https://forums.developer.nvidia.com/t/rtc-battery-on-jetson-orin-nano-developer-kit/296732), [How to add RTC battery](https://forums.developer.nvidia.com/t/how-to-add-rtc-battery-in-jetson-orin-nano/255792).
- A reported side effect: the battery lead and socket make the case cover sit unevenly. - A reported side effect: the battery lead and socket make the case cover sit unevenly.
- Polarity: one forum post says the positive pad is marked with a white triangle. Confirm with a meter before connecting.
- Test after fitting: power off fully, wait a few minutes, boot with no network, and check `cat /sys/class/rtc/rtc0/date` (should be today, not a reset date).
- Low priority: online almost always means NTP within a minute; the gap only matters for offline drives (see the [time sync report](impl/reports/TIME-SYNC.md)).