Step 29 of 56

Chip identity: die temperature and the eFuse MAC

Two small packages that answer questions about the silicon itself — how hot it is, and what addresses the factory gave it.

The on-chip temperature sensor

It reports die temperature, not ambient air. The chip self-heats under load, so an idle board typically reads a few degrees above room temperature and a busy one considerably more. Using it as a room thermometer will give you a plausible, wrong answer.

Accuracy is roughly ±1 °C after the part's factory trim, and is best near the middle of the selected measurement range — so pick the Measure_Range that brackets your expected readings rather than the widest one:

procedure Initialize (Span : Measure_Range := Range_Minus10_80);

Initialize is optional: the first Read brings the sensor up with the default range if you skip it. The sensor is owned by a protected object, so concurrent reads from different tasks serialise automatically — each busy-waits microseconds for its conversion under that lock.

Bring-up is more involved than it looks, which is why it is worth having a driver for: gate the SAR peripheral clock, pulse-reset it, open the analog REGI2C bus, program the sensor's DAC range over that bus through a ROM call, then power the sensor up. Every reading then drives a dump-out/ready handshake to latch a fresh conversion before sampling.

The factory MAC addresses

Espressif allocates each part a block of four universally-administered addresses. The eFuse base is the Wi-Fi station MAC, and the others are derived from it:

type MAC_Address is array (0 .. 5) of Interfaces.Unsigned_8;

function Base           return MAC_Address;   --  = Wi-Fi station
function Wi_Fi_Station  return MAC_Address;   --  base + 0
function Wi_Fi_SoftAP   return MAC_Address;   --  base + 1
--  Bluetooth  = base + 2
--  Ethernet   = base + 3

The practical use on a board with no Wi-Fi: give a W5500 Ethernet controller the Ethernet address instead of a hand-picked one. That is a real, manufacturer-assigned, globally unique MAC — which matters the moment two of your boards end up on the same network segment, and is free.

Every routine is a pure read of the eFuse shadow registers, which are latched at reset. No clock, no driver state, no initialisation — so these are safe to call from anywhere, at any time, under any profile.