Clock System (CLK)
The internal network of STM8 clock system allows us to tune up operating speeds of peripherals and
CPU according to our needs. Software delays and power consumption depend on how the clock
system is set.
In STM8 micros, there are three main clock sources – High Speed Internal Clock (HSI), High Speed
External Clock (HSE) and Low Speed Internal Clock (LSI). The HSI has an oscillating frequency of 16MHz
and is an internal RC oscillator with good precision – about 1% tolerant over a wide temperature range.
HSE can be an external clock circuitry, temperature-compensated crystal oscillator (TCXO) or ordinary
crystal resonator. It accepts all frequencies from 1MHz to 24MHz. Lastly, LSI clock is also an
independent internal RC oscillator-based clock source that is mainly intended for idle or low power
operating modes and the independent watchdog timer (IWDG). It has a fixed factory calibrated
operating frequency of 128kHz and is not as accurate as HSI or HSE. There are also clock
dividers/prescalers at various points to scale clocks as per requirement. Mainly two prescalers are
what we need – the HSI prescaler and the CPU divider. Peripherals are directly feed by the main clock
source. Additionally, there is a clock output pin (CCO) that outputs a given clock frequency. It can be
used to clock another micro, generate clock for other devices like logic ICs. It can also be used as a free
oscillator or perform clock performance tests. There’s fail-safe clock security that makes HSI backup
of HSE. Should HSE fail, HSI takes over automatically. Check the internal block diagram below:
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