ST72340 datasheet(30/191 Pages) STMICROELECTRONICS

30 / 191 page

ST72340, ST72344, ST72345

30/191

MULTI-OSCILLATOR (Cont’d)

Internal RC Oscillator

The device contains a high-precision internal RC

oscillator. It must be calibrated to obtain the fre-

quency required in the application. This is done by

software writing a calibration value in the RCCRH

and RCCRL Registers.

Whenever the microcontroller is reset, the RCCR

returns to its default value (FF 03h), i.e. each time

the device is reset, the calibration value must be

loaded in the RCCRH and RCCRL registers. Pre-

defined calibration values are stored in XFLASH

for 3 and 5V VDD supply voltages at 25°C, as

shown in the following table.

Note:

– To improve clock stability, it is recommended to

place a decoupling capacitor between the VDD

and VSS pins.

– These two 10-bit values are systematically pro-

grammed by ST, including on FASTROM devic-

es. Consequently, customers intending to use

FASTROM service must not use these address-

es.

– RCCR0 and RCCR1 calibration values will be

erased if the read-out protection bit is reset after

it has been set. See “Memory Protection” on

page 17.

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 ex-

ternal reference signal.

7.3 REGISTER DESCRIPTION

RC CONTROL REGISTER (RCCRH)

Read / Write

Reset Value: 1111 1111 (FFh)

Bits 7:0 = CR[9:2] RC Oscillator Frequency Ad-

justment Bits

RC CONTROL REGISTER (RCCRL)

Read / Write

Reset Value: 0000 0011 (03h)

Bits 7:2 = Reserved, must be kept cleared.

Bits 1:0 = CR[1:0] RC Oscillator Frequency Ad-

justment Bits

This 10-bit value must be written immediately after

reset to adjust the RC oscillator frequency in order

to obtain the specified accuracy. The application

can store the correct value for each voltage range

in EEPROM and write it to this register at start-up.

0000h = maximum available frequency

03FFh = lowest available frequency

Note: To tune the oscillator, write a series of differ-

ent values in the register until the correct frequen-

cy is reached. The fastest method is to use a di-

chotomy starting with 200h.

RCCR

Conditions

Address

RCCR0

VDD=5V

TA=25°C

fRC=1MHz

BEE0, BEE1

RCCR1

VDD=3V

TA=25°C

fRC=1MHz

BEE4, BEE5

CR9

CR8

CR7

CR6

CR5

CR4

CR3

CR2

CR1

CR0

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