Deep sleep isn't off
Raspberry Pi documents a typical ~180 µA for RP2040 even in deep sleep, varying from chip to chip, with supply voltage and (non-linearly) with temperature. For a coin cell or a sensor meant to run for years, that is too much.
Recommendation from Raspberry Pi: for minimal standby current, power the RP2040 (or the whole system) off completely and wake it with external hardware, e.g. a real-time clock or a power switch controlled by a low-power timer. Raspberry Pi has a white paper, "Power switching RP2040 for low standby current applications", linked from the RP2040 documentation page.
On a Pico board, pulling 3V3_EN (pin 37) to ground turns the board off, a simple hook for an external timer or power-latch circuit.
Note that the board's regulator and any attached sensors add their own current; measure the whole device.
The internal temperature sensor
- It is low-resolution and needs calibration. Uncalibrated readings are unlikely to be accurate.
- The conversion is very sensitive to the ADC reference voltage (VREF), and RP2040 has no internal fixed reference. Either measure VREF (it can drift) or use an external precision reference.
- The sensor voltage falls as temperature rises.
Use it for rough chip-temperature monitoring. For room or process temperature, use an external sensor (for example a DS18B20 via the PIO 1-Wire example, or an I2C sensor).
RP2350?
These figures are documented for RP2040. For RP2350-based designs, check the RP2350 datasheet for its low-power modes instead of assuming the same numbers.