ST7LITE49M datasheet(35/188 Pages) STMICROELECTRONICS

35 / 188 page

ST7LITE49M

Supply, reset and clock management

35/188

In 38-pulse ICC mode, the internal RC oscillator is forced as a clock source, regardless of

the selection in the option byte.

Section 13: Electrical characteristics on page 142 for more information on the frequency

and accuracy of the RC oscillator.

To improve clock stability and frequency accuracy, it is recommended to place a decoupling

capacitor, typically 100nF, between the VDD and VSS pins and also between the VDDA and

VSSA pins as close as possible to the ST7 device.

These bytes are systematically programmed by ST, including on FASTROM devices.

Caution:

If the voltage or temperature conditions change in the application, the frequency may need

to be recalibrated. Refer to application note AN1324 for information on how to calibrate the

RC frequency using an external reference signal.

7.1.2

Auto Wake Up RC oscillator

The ST7LITE49M 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 (AWU) RC oscillator can also be configured as the startup clock through

the CKSEL[1:0] option bits (see Section 14.1: Option bytes on page 175).

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 12):

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 till 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 till the AWU_FLAG is cleared (1AWU 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.

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