ST7LUS5 datasheet(28/124 Pages) STMICROELECTRONICS

28 / 124 page

Supply, reset and clock management

ST7LUS5, ST7LU05, ST7LU09

28/124

Refer to application note AN2326 for information on how to calibrate the RC frequency using

an external reference signal.

The ST7LUxx also contains an Auto Wake-up RC oscillator. This RC oscillator should be

enabled to enter Auto Wake-up from Halt mode.

The Auto Wake-up RC oscillator can also be configured as the startup clock through the

CKSEL[1:0] option bits (see Section 14.1 on page 114).

This is recommended for applications where very low power consumption is required.

Switching from one startup clock to another can be done in run mode as follows (see

Figure 8):

Case 1: Switching from internal RC to AWU:

Set the RC/AWU bit in the CKCNTCSR register to enable the AWU RC oscillator.

The RC_FLAG is cleared and the clock output is at 1.

Wait 3 AWU RC cycles until the AWU_FLAG is set.

The switch to the AWU clock is made at the positive edge of the AWU clock signal.

Once the switch is made, the internal RC is stopped.

Case 2: Switching from AWU RC to internal RC:

Reset the RC/AWU bit to enable the internal RC oscillator.

Using a 4-bit counter, wait until 8 internal RC cycles have elapsed. The counter is

running on internal RC clock.

Wait until the AWU_FLAG is cleared (1 AWU RC cycle) and the RC_FLAG is set (2 RC

cycles).

The switch to the internal RC clock is made at the positive edge of the internal RC clock

signal.

Once the switch is made, the AWU RC is stopped.

Note:

When the internal RC is not selected, it is stopped so as to save power consumption.

When the internal RC is selected, the AWU RC is turned on by hardware when entering

Auto Wake-up from Halt mode.

When the external clock is selected, the AWU RC oscillator is always on.

Figure 8.

Clock switching

AWU RC

Set RC/AWU

Poll AWU_FLAG until set

Internal RC

Reset RC/AWU

Poll RC_FLAG until set

AWU RC

Internal RC

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