It wakes up asleep

Registers

The chip is controlled the way most I²C sensors are: through registers, numbered one-byte boxes inside it. Some hold settings you write, some hold results you read. A sketch writes a register by sending its number and then the value; it reads one by sending the number and then asking for bytes back.

Three exchanges are all the first sketch does. Press play:

First, its name

Register 0x0F, WHO_AM_I, is read-only and always holds 0x11 on this chip. Reading it first costs a few hundred microseconds and settles two questions at once: that something answered at 0x19, and that it is the part you think it is. A sketch that skips it and goes straight to readings fails later, less clearly.

It is also why libraries written for similar chips refuse this one: each checks WHO_AM_I for the value its own chip returns, and 0x11 is not it.

Then, wake it

Register 0x20, CTRL_REG1, is the one that matters. Its top four bits set the data rate: how many readings a second the chip takes. From the factory they are 0000, which the data sheet calls power-down. The chip draws 0.5 µA, measures nothing, and nothing new ever reaches its output registers.

The sketches write 0x57. In bits that is 0101 0111: 0101 in the top four selects 100 readings a second; the next bit, 0, keeps normal rather than low-power mode; the bottom three switch on Z, Y and X. The chip is taking readings within about a millisecond of that write, and draws 16 µA doing it.

The other write, 0x80 to CTRL_REG4 at 0x23, leaves the range at ±2 g and sets one safety bit: the two bytes of an axis always come from the same reading, never half from one and half from the next.

Then, read six at once

Each axis is two registers, low byte first, and the three axes sit in a row from 0x28 to 0x2D. Asking for them one at a time is six exchanges. Asking once with the top bit of the register number set, 0x28 + 0x80 = 0xA8, tells the chip to step to the next register after each byte, and all six come back in one exchange. That is the reg | 0x80 in the sketch's readRegs.

At the sketches' 100 kHz, one six-byte read is nine bytes on the wire in all — the address and register out, the address again, six bytes back — at nine clocks a byte: 81 clocks, about 0.8 ms. Plenty, at a hundred readings a second.