ST7LUS5 datasheet(27/124 Pages) STMICROELECTRONICS

27 / 124 page

ST7LUS5, ST7LU05, ST7LU09

Supply, reset and clock management

27/124

Supply, reset and clock management

The device includes a range of utility features for securing the application in critical

situations (for example in case of a power brown-out), and reducing the number of external

components.

Main features

●

Clock Management

–

8 MHz internal RC oscillator (enabled by option byte)

–

External Clock Input (enabled by option byte)

●

Reset Sequence Manager (RSM)

●

System Integrity Management (SI) - Main supply Low voltage detection (LVD) with

reset generation (enabled by option byte)

6.1

Internal RC oscillator adjustment

The ST7 contains an internal RC oscillator with a specific accuracy for a given device,

temperature and voltage. It can be selected as the start up clock through the CKSEL[1:0]

option bits (see Section 14.1 on page 114). It must be calibrated to obtain the frequency

required in the application. This is done by software writing a 10-bit calibration value in the

RCCR (RC Control Register) and in the bits [6:5] in the SICSR (SI Control Status Register).

Whenever the microcontroller is reset, the RCCR returns to its default value (FFh), that is,

each time the device is reset, the calibration value must be loaded in the RCCR. Predefined

calibration values are stored in Flash memory for 3.3 and 5V VDD supply voltages at 25°C,

as shown in the following table.

Note:

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

selection in the option byte. Refer to note 5 in Section 4.4 on page 19 for further details.

2See Chapter 12: Electrical characteristics on page 90 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 as close as possible to the ST7

device.

Caution:

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

to be recalibrated.

Table 6.

RCCR calibration values

RCCR

Conditions

ST7LUxx address

RCCRH0

VDD =5V, TA = 25°C, fRC =8 MHz

DEE0h(1) (CR[9:2] bits)

DEE0h, DEE1h, DEE2h and DEE3h are located in a reserved area but are special bytes containing also

the RC calibration values which are read-accessible only in user mode. If all the Flash space (including the

RC calibration value locations) has been erased (after the readout protection removal), then the RC

calibration values can still be obtained through these two addresses.

RCCRL0

DEE1h(1) (CR[1:0] bits)

RCCRH1

VDD =3.3V, TA =25°C, fRC =8MHz

DEE2h(1) (CR[9:2] bits)

RCCRL1

DEE3h(1) (CR[1:0] bits)

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