JRWatch

JRWatch

A smartwatch designed from scratch, board included. 36 mm across and 4 layers, with an nRF52840 for Bluetooth, an nPM1300 handling charging and the power rails, a BMI270 for motion, and a Sharp memory-in-pixel LCD that holds a static image on about 12 µW. Firmware runs on Zephyr.

Most of the design work went into idle current, meaning what it draws while sitting on a wrist doing nothing.

_{Front copper on the left, back on the right. 36 x 36 mm, 4 layers.}

These are calculated from datasheet typicals, not measured on hardware. The full itemization is in the verification report. Part choices are in the design rationale.

Architecture

flowchart LR
  USB["USB-C"]
  BAT["LiPo 150 mAh<br/>+ 10k NTC"]
  PMIC["nPM1300<br/>charger, 2 bucks,<br/>2 load switches"]
  MCU["nRF52840<br/>MDBT50Q-1MV2"]
  DISP["Sharp LS013B7DH03<br/>128 x 128 MIP"]
  IMU["Bosch BMI270"]

  USB -->|VBUS| PMIC
  BAT -->|VBAT| PMIC
  PMIC -->|3V0 always-on| MCU
  PMIC -->|VDD_DISP via LSW1| DISP
  PMIC -->|VDD_IMU via LSW2| IMU

  MCU -.->|I2C 0x6b| PMIC
  MCU -.->|SPI2| DISP
  MCU -.->|SPI1| IMU

Solid lines are power, dashed are data. The display and IMU sit behind load switches inside the PMIC. In armed sleep they draw about 4 µA and 6 µA. Ship mode disconnects the battery entirely and the whole watch drops to 370 nA, low enough that cell self-discharge dominates. Full net list is in hardware/skidl/jrwatch.py.

Status

Not yet fabricated. A review before ordering turned up a problem with the display connector, and both halves of it trace to one cause. The panel's ribbon exits the center and has to fold 180 degrees back underneath. That fold leaves roughly a 9 mm window where the connector can sit, and mine was outside it. The fold also inverts the contact face, which reverses the pin order, and I had the pins running straight through.

The pin order is corrected in the schematic. Repositioning the connector is a layout change and hasn't landed yet, so ordering is blocked. Written up as D-025 in the decision log.

Firmware

An out-of-tree Zephyr board port with devicetree, Kconfig, and board files. Three power modes cover active, armed sleep, and off. The display redraws only when its content changes, and the IMU wakes the SoC on motion. BLE reports battery level and step count (protocol).