Tag Designs
Six fabricated tag boards, each documented with its major components, a block diagram, what it records, and a link to its design files.
| Senses | Processor | Memory | Switched rail | Cell | |
|---|---|---|---|---|---|
| BitTagv7 | Movement | STM32L432KC | Internal only | — | Coin |
| BitTagNG | Movement | STM32L432KC | 4 MB serial | — | Coin |
| PresTag-v6 | Pressure | STM32L431KC | 4 MB serial | Pressure sensor | Coin |
| CompassTag | Movement, magnetic field | STM32L432KC | 4 MB serial | Magnetometer (+1V8) | Coin |
| UIUC Tag | Movement, pressure | STM32L432KC | 4 MB serial | Pressure sensor | Coin |
| IMUTag | Movement, rotation, magnetic field, pressure | STM32U375KG | NAND | — | 12 mAh LiPo |
The project home describes the tag families from a user's point of view — what each one is for and which questions it answers. These pages are about the boards.
Measured power
Every tag but BitTagNG has been measured against firmware release
fw-v0.6, on a
Joulescope, with nothing attached during the measurement window.
| Supply | Resting | Running | Configuration | |
|---|---|---|---|---|
| BitTagv7 | 2.4961 V | 0.12 µA | 0.49 µA | 1 bit / 5 min (default) |
| BitTagNG | — | — | — | not measured |
| PresTag-v6 | 2.4961 V | 0.11 µA | 0.39 µA | 60 s sample period |
| CompassTag | 2.4960 V | 0.21 µA | 1.95 µA | 30 s compass period |
| UIUC Tag | 2.4960 V | 0.16 µA | 0.56 µA | 300 s sample period; measured with the ADXL367 |
| IMUTag | 3.6932 V | 6.43 µA | 665 µA | 400 Hz |
Three orders of magnitude separate IMUTag from the rest, and that is the whole story of the two architectures. The coin-cell tags sleep between events and wake briefly; IMUTag stays awake through a run, streaming an IMU into NAND. A deployment's battery life is decided by IMUTag if there is one in it, and barely affected by the others.
These are firmware qualification measurements, one board per tag, taken with a Joulescope at the supply and configuration shown. A different cell, sample period or room temperature moves them — CompassTag's idle shifts about 9% for 3.5 °C. Battery lives on each page are capacity over current and ignore self-discharge and derating, so treat them as upper bounds.
How to read the block diagrams
Every diagram on these pages follows the generic structure set out in the project's Hardware Architecture page. A tag is divided into three regions:
Always on holds the processor, the RTC, and any sensor or memory whose quiescent current is low enough to leave powered for the life of the deployment. The ADXL362 and ADXL367 accelerometers sit here at around 10 nA idle, as does the AT25 serial flash — chosen specifically because it is a low quiescent current part, which most serial flash is not.
Switched holds anything fed from its own supply line rather than from the main rail. In practice this means sensors a tag cannot afford to leave idling: the LPS27 pressure sensor draws about 0.9 µA doing nothing, which on a budget measured in microamps is not negligible. A switched part is powered either directly from a processor pin or through a regulator whose enable pin a processor pin controls.
Power holds the cell and any regulator. Most of these boards have no regulator at all — parts run at battery voltage less one Schottky diode drop. IMUTag is the exception.
A tag with no switched region is not an oversight. It means every part on the board was chosen to be cheap enough at idle that gating it would not pay for itself — and in IMUTag's case, that conclusion came from measuring a load switch that was already there and then removing it.
Shared conventions
All six boards are programmed and read through an ARM serial wire debug (SWD) interface, reached by spring-loaded pogo pins contacting test pads on the board. They connect to a base board, which emulates the ST-LINK protocol so that standard open-source tools work against them unmodified.
All six carry a Micro Crystal RV-3028 real-time clock, in the C7 or C8 variant. At roughly 40 nA and 1 ppm it is what lets a tag sleep for months and still timestamp what it recorded.
Cells
The five boards without a core regulator take one of two Seiko rechargeable coin cells, and the choice between them is mass against endurance:
| Cell | Capacity | Mass |
|---|---|---|
| MS621FE-FL11E | 5.5 mAh | 230 mg |
| MS920SE-FL27E | 11 mAh | 450 mg |
IMUTag is the exception. Its switching converter regulates whatever the cell gives down to 1.8 V, which frees it from the coin-cell voltage window; it runs from a 12 mAh LiPo, with the specific cell not yet settled.
Battery chemistry is not interchangeable
On the boards without regulators the rail follows the cell, less one Schottky diode drop. A 3 V coin cell lands near 2.8 V, comfortably within range. A 3.7 V LiPo at full charge would reach roughly 4.0 V — the absolute maximum rating of several of the parts involved, not merely outside their operating range. That substitution is safe only on IMUTag, and only because the converter stands between the cell and everything else.