S2-LPS datasheet(34/73 Pages) STMICROELECTRONICS

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The full / empty status of the TX / RX FIFO is readable on the bits [9:8] of the MC_STATE registers, and at the

same time, the related IRQs are generated.

In the SLEEP state, the FIFO content is retained only if the SLEEP_B mode is selected (bit

SLEEP_MODE_SEL=1 in the register 0x79).

4.4.7

Integrated RCO

The S2-LPS contains an ultra-low power RC oscillator with accuracy better than 1%. The RC oscillator frequency

is calibrated using as a reference the XO frequency. It depends on two values: raw (4 bits) and fine (5 bits). The

raw value is obtained by a linear search algorithm in which for each value a counting of half clock reference inside

the period of RCO is done. When the correction is near to the final value, a dichotomy search algorithm starts.

The RCO calibration starts as soon as the RCO_CALIBRATION bit is set to 1. When it finishes, the RC_CAL_OK

bit is set and the ERROR_LOCK bit is reset.

Moreover, after a sleep or standby state, if the RCO_CALIBRATION bit is kept to 1, when the device returns to the

ready state, an RCO calibration automatically runs to compensate some drift.

It is possible to perform an offline calibration of the RCO using the following procedure:

Enable the RCO CALIB setting the bit to 1

Wait until the RC_CAL_OK becomes 1

Copy the RWT_OUT and RFB_OUT (registers 0x94 and 0x95) out values in the RWT_IN and RFB_IN fields

(registers 0x6E and 0x6F)

Disable the RCO CALIB setting the bit to 0

In this way, the RCO works with these values. It is advisable to repeat the RCO calibration to reject effects related

to the variation of temperature. It is recommended to use this procedure if the following SLEEP time (i.e. when

using LDC mode) is shorter or comparable to the calibration time.

By default, the calibration is disabled at reset to avoid using an out-of-range reference frequency, after the internal

clock divider is correctly configured, the user can enable the RCO calibration by register.

Once calibrated, the RCO generates a clock frequency that depends on the XO frequency used:

Table 40. RCO Frequency

Ref. frequency [MHz]

RCO frequency [kHz]

24 or 48

25 or 50

33.33

26 or 52

34.66

4.4.8

Low battery indicator

The battery indicator can provide the user with an indication of the battery voltage level.

There are two blocks to detect battery level:

•

Brownout with a fixed threshold

•

Battery level detector with a programmable threshold

The MCU enables optionally these blocks to provide an early warning of impending power failure. It does not

reset the system, but gives the MCU time to prepare for an orderly power-down and provides hardware protection

of data stored in the program memory.

The low battery indicator function is available in any of the S2-LPS operation modes. As this function requires the

internal bias circuit operation, the overall current consumption in STANDBY, SLEEP, and READY modes increase

by 400 µA.

4.4.9

Voltage reference

This block provides the precise reference voltage needed by the internal circuit.

S2-LPS

Digital modulator

DS13831 - Rev 1

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