Jot Rev A notes
Jot's own board, built around a pre-certified nRF52840 module. Hold the side button, speak, and release: the recording goes to the board's own flash first, then to the iPhone over Bluetooth Low Energy whenever the phone is in range. These are the engineering notes behind it, from the architecture document, draft 8 (4 Nov 2026). The power figures are datasheet estimates until the first board is measured.
Block diagram
Orange outline: power. Solid: microcontroller. Panels: peripherals. Scroll sideways to see all of it.
What it has to do
R1
Hold to record; recording starts at once, even from sleep
The button wakes the nRF52840 from System OFF, and audio goes straight to local flash with no wait for BLE.
R2
No lost recordings when the phone is out of range
32 MB flash buffer: roughly 4.5 hours of Opus-compressed audio (about 68 minutes at the prototype's simpler 8 KB/s ADPCM). Clips sync when the phone reconnects.
R3
Recording is always visible
The Rec LED is powered from the same switched rail as the microphones.
R4
Feedback without looking at the device
LRA haptic motor via a DRV2605L, an RGB status LED, and a buzzer.
R5
Days of standby and an accurate battery %
Low-Iq LDO, System OFF sleep, and a MAX17048 fuel gauge. Estimated standby draw is 10 to 17 µA, so the battery's own self-discharge sets the limit.
R6
Charge and update firmware over USB-C
USB data wired to the nRF52840, with a UF2 bootloader for drag-and-drop updates.
R7
Simple pairing
An NFC tag with a thin flex antenna inside the back: tap the back of Jot to the iPhone.
R8
Room to improve audio later
A footprint for a second PDM mic, left empty on Rev A.
R9
Safe to hand to someone else
A resettable fuse and reverse-battery switch at the battery, a surge diode on USB power, and a firmware charging cut-off based on board temperature (the cell itself isn't measured until Rev B) that still lets a flat battery charge. The limits that remain are written down with their Rev B fixes.
Power budget
Standby
10–17 µA
Haptic driver 4 µA, fuel gauge 3 µA, cell protection up to 7 µA, flash and accelerometer 1 µA each, nRF52840 0.4 µA
Recording
≈6.2 mA
CPU 3.3 mA, Rec LED about 1.3 mA, flash writes about 0.6 mA, PDM and mic about 0.9 mA
BLE sync
≈8 mA
Radio at 0 dBm, for about a minute a day
Ten one-minute notes and a sync use about 1.6 mAh a day: roughly 250 days on a 500 mAh cell, counting 80% of it as usable. Two firmware jobs matter most: put the flash into deep power-down before sleeping (left in standby it draws 10 µA typical, 60 µA worst case) and turn on the nRF52840's DC/DC regulator.
Bring-up plan
1
Bench supply at 3.7 V with a 50 mA limit on the battery holes. Check the 3.3 V rail, microamp idle current, nothing warm.
2
Flash a blinky over SWD, check reset, scope the 32.768 kHz crystal.
3
Read the flash ID over QSPI, switch it to 4-byte addressing, then write and read back.
4
Find the three I2C parts, switch on the mic rail, capture and play back PDM audio.
5
Pair over BLE and send a clip, then test USB charging and the UF2 bootloader.
6
Check the protection: charging stops on command, a reversed battery draws nothing, and the USB plug-in spike stays inside the chips' limits.
7
Measure every mode with a power profiler and replace these estimates with real numbers.
Key parts
Radio + MCU
Raytac MDBT50Q-1MV2 (nRF52840), pre-certified with chip antenna
Microphone
TDK T3902 PDM MEMS, 64 dBA SNR, bottom port
Storage
Winbond W25Q256JV, 32 MB QSPI NOR flash
Charger
Microchip MCP73831, about 200 mA into a 500 mAh LiPo
3.3 V rail
TI TPS7A0233 LDO, 25 nA quiescent current
Fuel gauge
Analog Devices MAX17048
Motion
ST LIS2DW12 accelerometer, wake on raise or tap
Haptics
TI DRV2605L driving a Vybronics 8 mm LRA
Protection
Resettable fuse (0.5 A hold, 1 A trip), reverse-battery P-FET, USB surge diode, firmware charge cut-off
Key decisions
Wireless link
BLE only, store-and-forward, so a recording never waits for a connection
Radio
A pre-certified nRF52840 module, so a first revision carries no RF layout risk
3.3 V supply
External 25 nA LDO, so the board is at 3.3 V from the first power-up
Microphone
TDK T3902: in stock at LCSC, 64 dBA SNR (the IM69D130 was first choice)
Storage
32 MB of soldered flash: smaller and lower power than microSD, and clips only stay there until they sync
Rec indicator
LED on the mic power rail, so recording can't happen with the light off
Haptics
DRV2605L + LRA for phone-like taps, with a built-in effect library
Second mic
Footprint only, leaving room for noise reduction later
Battery connection
Leads soldered into marked holes, no connector: a JST header was 0.8 mm too tall for the case
Protection
Fuse, reverse-battery switch and USB surge diode, so units can go to other people
Rev B
Temperature-sensing charger, USB over-voltage switch, full reverse protection; later a dock and display
Open items
12 done, 6 in progress
Done
Prototype One
Breadboard build streams audio from a button press into the app
Done
Pick the microphone
TDK T3902, in stock for assembly
Done
Pick the radio module
Raytac MDBT50Q-1MV2, every pin checked
Done
Pick the charger and 3.3 V regulator
Microchip MCP73831 and TI TPS7A0233
Done
Assign every signal to a pin
Pin list published
In progress
Pick the flash
Winbond W25Q256JV (32 MB, about 4.5 h of Opus audio), an eighth of the price of the 128 MB part; firmware support to check
In progress
Pick the vibration motor
8 mm Vybronics LRA, on order; its mount in the case is still to design
Done
Pick the battery
Adafruit 1578: 500 mAh, 5 mm thick, protected; fits the 12.8 mm case
Done
Check JLCPCB can assemble every part
Every part can be placed; the accelerometer needs JLCPCB's Standard line (X-ray inspection). The radio modules and haptic driver are in stock in the JLCPCB account
Done
Footprint for every part
Every part checked against its drawing, including the 9 added in the final review
Done
Confirm battery polarity
Leads soldered to holes marked + and − (no connector: a JST header was too tall for the case); check with a meter first, and reverse protection is on the board anyway
Done
PCB layout
4 layers, routed and reviewed; design-rule check clean and matches the schematic
Done
Order files
Gerbers, drill, parts list and placement files; JLCPCB quote about $200 for five assembled boards (Standard assembly for the accelerometer)
In progress
Order the boards
Every part is in stock; the order goes in after Raytac's final check
Done
Protect the battery and USB input
Fuse, reverse-battery switch, USB surge diode, and a firmware charge cut-off from board temperature (a cell sensor is a Rev B item)
In progress
Design the case
Arrived: five SLS nylon sets with numbered backs, light wells over the LEDs and sealed mic channels, ordered 27 Oct. Motor mount still to design
In progress
Antenna and layout review
Raytac reviewed the schematic and layout; the layout now follows their module guidelines (full-width antenna keep-out, stitched ground pours, ground under the module). Final check before ordering
In progress
Tune the NFC antenna
Flex antenna chosen and 82 pF capacitors calculated; final tuning on the first board, with spare pads