PresTag

PresTag trades the accelerometer for a barometric pressure sensor. Where a BitTag tells you when a bird was active, a PresTag tells you how high it was, and when it climbed or descended. The board documented here is PresTag-v6.

The board

Top

Bottom

The LPS27's circular pressure port is visible at the centre of the top side. These are rendered from the KiCad layout by kicad-cli; see Board Designs for how they are regenerated.

Block diagram

flowchart LR
  subgraph TAG[PresTag-v6]
    direction LR
    subgraph SW[Switched]
      PRS["LPS27HHTW<br/>pressure + temperature"]
    end
    subgraph ON[Always on]
      direction TB
      RTC["RV-3028-C7<br/>RTC"]
      MCU["STM32L431KCU6<br/>processor"]
      FLASH["AT25FF321A<br/>4 MB serial flash"]
      RTC --- MCU
      MCU --- FLASH
    end
    subgraph PWR[Power]
      BAT["MS621FE-FL11E or<br/>MS920SE-FL27E<br/>D401 Schottky"]
    end
    IF["Interface<br/>SWD + pogo pads"]
  end
  SW --- ON
  ON --- PWR
  ON --- IF

The pressure sensor sits in the switched section because it is fed from its own supply line, LPS_PWR, driven by a processor pin rather than by the main rail.

Major components

Role	Part	Domain	Notes
Processor	STM32L431KCU6	Always on
RTC	RV-3028-C7	Always on	32.768 kHz, 1 ppm
Sensor	LPS27HHTW	Switched	Barometric pressure and temperature
Memory	AT25FF321A-UUN	Always on	4 MB; unusually low quiescent current for serial flash
Power	D401 CDBQC0130L-HF	—	Schottky reverse-polarity protection; no regulator
Battery	MS621FE-FL11E or MS920SE-FL27E	—	5.5 mAh / 230 mg, or 11 mAh / 450 mg

Why the pressure sensor is gated. The LPS27 draws about 0.9 µA just sitting idle. On a tag whose whole budget is measured in microamps that is not a rounding error, so its supply is switched off between samples rather than left standing. The flash is the opposite case: the AT25FF321A was chosen precisely because its quiescent current is low enough to leave powered, which is why it sits in the always-on section next to the processor.

Driving a sensor rail from a GPIO has a firmware contract

While the sensor rail is off, its bus lines are still driven from the main domain. Firmware must not leave them high, or the sensor can be back-powered through its protection diodes.

What it records

Pressure and temperature, sampled at a configurable period. Barometric pressure converts to altitude, so a PresTag record is a flight-altitude profile: how high the bird flew, and the shape and timing of its climbs and descents.

The sample period sets the deployment length, which is why PresTag has no single headline endurance figure the way BitTag does — it is whatever the chosen period and the cell size imply.

Power

Measured on firmware fw-v0.6 at 2.4961 V from an unregulated 2.5 V cell, at the 60 s sample period.

State	Current
IDLE	0.11 µA
RUNNING, 60 s period	0.39 µA
FINISHED	0.11 µA

Sample period

The sample period is the main lever on a PresTag deployment, and it has been measured rather than modelled — six points on one board, 1200 s per window:

Period	RUNNING	Period	RUNNING
15 s	1.13 µA	45 s	0.45 µA
20 s	0.88 µA	60 s	0.37 µA
30 s	0.62 µA	90 s	0.28 µA

Fitting I(T) = I_rest + Q/T gives a resting term near 0.11 µA and 15.2 µC per sample, R² 0.99994. The per-sample energy is the durable number — an independent campaign a month earlier got 15.3 µC — while the resting floor it sits on varies from board to board, by more than any firmware change has.

What a deployment gets

Battery is not always what runs out first. The 4 MB flash holds 1,048,560 samples:

Period	5.5 mAh cell	11 mAh cell	Memory	Usable, 5.5 mAh	Usable, 11 mAh
15 s	204 d	407 d	182 d	182 d (memory)	182 d (memory)
20 s	262 d	523 d	243 d	243 d (memory)	243 d (memory)
30 s	371 d	742 d	364 d	364 d (memory)	364 d (memory)
45 s	505 d	1009 d	546 d	505 d (battery)	546 d (memory)
60 s	625 d	1251 d	728 d	625 d (battery)	728 d (memory)
90 s	822 d	1644 d	1092 d	822 d (battery)	1092 d (memory)

On a 5.5 mAh cell the constraint switches from memory to battery around 33 s; on 11 mAh, not until about 200 s. Below those, sampling faster spends current on data the flash cannot hold. At the 60 s default: about 625 days on 5.5 mAh, 728 on 11 mAh — the latter capped by flash rather than by the cell.

Full log: PresTag power results.

Design files

Design directory on GitHub Schematic (PDF)

No design review has been written for this board. An earlier revision, a v3 design, is in the repository under BoardDesigns/Tags/PresTag.