From 787c414c0a6b0e28c01fd32feed49e7b74485143 Mon Sep 17 00:00:00 2001 From: mikkeli Date: Thu, 8 Oct 2026 09:43:47 +0900 Subject: [PATCH] BOM: settle RTC battery on CR2032, mark optional 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 --- docs/BOM.md | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/docs/BOM.md b/docs/BOM.md index ab803ae..fcdb70e 100644 --- a/docs/BOM.md +++ b/docs/BOM.md @@ -56,14 +56,15 @@ type: reference ## 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 | |-------|-------| | **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) | | **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 | ### 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). - 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. +- 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)).