ST7LITEUSX datasheet(17/108 Pages) STMICROELECTRONICS

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ST7LITEUSx

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6 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 re-

ducing the number of external components.

Main features

■ Clock Management

– 8 MHz internal RC oscillator (enabled by op-

tion 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)

– Auxiliary Voltage detector (AVD) with interrupt

capability for monitoring the main supply

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 sec-

tion 14.1 on page 95). It must be calibrated to ob-

tain the frequency required in the application. This

is done by software writing a 10-bit calibration val-

ue in the RCCR (RC Control Register) and in the

bits [6:5] in the SICSR (SI Control Status Regis-

ter).

Whenever the microcontroller is reset, the RCCR

returns to its default value (FFh), i.e. each time the

device is reset, the calibration value must be load-

ed 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.

1. DEE0h, DEE1h, DEE2h and DEE3h are located

in a reserved area but are special bytes containing

also the RC calibration values which are read-ac-

cessible only in user mode. If all the Flash space

(including the RC calibration value locations) has

been erased (after the read-out protection remov-

al), then the RC calibration values can still be ob-

tained through these two address.

Notes:

– 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 12 for further details.

– See “ELECTRICAL CHARACTERISTICS” on

page 67. for more information on the frequency

and accuracy of the RC oscillator.

– To improve clock stability and frequency accura-

cy, it is recommended to place a decoupling ca-

pacitor, 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.

Refer to application note AN2326 for information

on how to calibrate the RC frequency using an ex-

ternal reference signal.

The ST7ULTRALITE also contains an Auto Wake

Up RC oscillator. This RC oscillator should be en-

abled to enter Auto Wake-up from Halt mode.

The Auto Wake Up RC oscillator can also be con-

figured as the startup clock through the CK-

SEL[1:0] option bits (see section 14.1 on page 95).

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 9):

Case 1: Switching from internal RC to AWU:

– 1. Set the RC/AWU bit in the CKCNTCSR regis-

ter to enable the AWU RC oscillator

– 2. The RC_FLAG is cleared and the clock output

is at 1.

– 3. Wait 3 AWU RC cycles till the AWU_FLAG is

set

– 4. The switch to the AWU clock is made at the

positive edge of the AWU clock signal

– 5. Once the switch is made, the internal RC is

stopped

Case 2: Switching from AWU RC to internal RC:

– 1. Reset the RC/AWU bit to enable the internal

RC oscillator

RCCR

Conditions

ST7LITEUS2/

ST7LITEUS5

Address

RCCRH0

VDD=5V

TA=25°C

fRC=8MHz

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

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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