STM32W108C8

System modules
STM32W108C8
calibrated accuracy of OSCHF is ±250 kHz ±40 ppm. The UART and ADC peripherals may
not be usable due to the lower accuracy of the OSCHF frequency.
See also Section 14.5.1: High frequency internal clock characteristics on page 199.
6.3.2 High-frequency crystal oscillator (OSC24M)
The high-frequency crystal oscillator (OSC24M) 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 OSC24M clock
source. Application software must be aware that peripherals are clocked at different speeds
depending on whether OSCHF or OSC24M 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 14.5.2: High frequency external clock characteristics on page 199.
6.3.3 Low-frequency internal RC oscillator (OSCRC)
A low-frequency RC oscillator (OSCRC) 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 (OSCRC) ST software calibrates a fractional-N
divider to produce a 1 kHz reference clock, CLK1K.
See also Section 14.5.3: Low frequency internal clock characteristics on page 199.
6.3.4 Low-frequency crystal oscillator (OSC32K)
A low-frequency 32.768 kHz crystal oscillator (OSC32K) 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 14.5.4: Low frequency external clock characteristics on page 200.
6.3.5 Clock switching
The STM32W108C8 has two switching mechanisms for the main system clock, providing
four clock modes.
The register bit OSC24M_SEL in the OSC24M_CTRL register switches between the highfrequency
RC oscillator (OSCHF) and the high-frequency crystal oscillator (OSC24M) as
the main system clock (SCLK). The peripheral clock (PCLK) is always half the frequency of
SCLK.
The register bit CPU_CLK_SEL in the CPU_CLKSEL register switches between PCLK and
SCLK to produce the ARM® Cortex-M3 CPU clock (FCLK). The default and preferred mode
of operation is to run the CPU at the lower PCLK frequency, 12 MHz, but the higher SCLK
frequency, 24 MHz, can be selected to give higher processing performance at the expense
of an increase in power consumption.
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