STM32W108CB datasheet(50/278 Pages) STMICROELECTRONICS

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System modules

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Doc ID 16252 Rev 14

6.3.1

High-frequency internal RC oscillator (HSI)

The high-frequency RC oscillator (HSI) is used as the default system clock source when

power is applied to the core domain. The nominal frequency coming out of reset is 12 MHz.

Most peripherals, excluding the radio peripheral, are fully functional using the HSI clock

source. Application software must be aware that peripherals are clocked at different speeds

depending on whether HSI or HSE OSC is being used. Since the frequency step of HSI is

0.5 MHz and the high-frequency crystal oscillator is used for calibration, the calibrated

accuracy of HSI is ±250 kHz ±40 ppm. The UART and ADC peripherals may not be usable

due to the lower accuracy of the HSI frequency.

See also Section 14.6.1: High frequency internal clock characteristics on page 252.

6.3.2

High-frequency crystal oscillator (HSE OSC)

The high-frequency crystal oscillator (HSE OSC) requires an external 24 MHz crystal with

an accuracy of ±40 ppm. Based upon the application's bill of materials and current

consumption requirements, the external crystal may cover a range of ESR requirements.

The crystal oscillator has a software-programmable bias circuit to minimize current

consumption. ST software configures the bias circuit for minimum current consumption.

All peripherals including the radio peripheral are fully functional using the HSE OSC clock

source. Application software must be aware that peripherals are clocked at different speeds

depending on whether HSI or HSE OSC is being used.

If the 24 MHz crystal fails, a hardware failover mechanism forces the system to switch back

to the high-frequency RC oscillator as the main clock source, and a non-maskable interrupt

(NMI) is signaled to the ARM® Cortex-M3 NVIC.

See also Section 14.6.2: High frequency external clock characteristics on page 252.

6.3.3

Low-frequency internal RC oscillator (LSI10K)

A low-frequency RC oscillator (LSI10K) is provided as an internal timing reference. The

nominal frequency coming out of reset is 10 kHz, and ST software calibrates this clock to

10 kHz. From the tuned 10 kHz oscillator (LSI10K) ST software calibrates a fractional-N

divider to produce a 1 kHz reference clock, LSI1K.

See also Section 14.6.3: Low frequency internal clock characteristics on page 253.

6.3.4

Low-frequency crystal oscillator (LSE OSC)

A low-frequency 32.768 kHz crystal oscillator (LSE OSC) is provided as an optional timing

reference for on-chip timers. This oscillator is designed for use with an external watch

crystal.

See also Section 14.6.4: Low frequency external clock characteristics on page 253.

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