Clocks

Clocks are square wave signals of certain frequencies
Selecting the right kind of clock for your application is important; there is a proportional correlation between power consumption and frequency
As clock frequency determines how fast the CPU can execute instructions, e.g. for a low-powered application we can use a lower frequency
The clock signal needs to be derived from clock sources. E.g. for STM32F407 Discovery Board, the three available clock sources are:
- Crystal Oscillator: this is an external component that needs to be connected to the microcontroller. This generates HSE (High Speed External signal)
- RC (Resistor/Capacitor) Oscillator: a moderate accuracy circuit already inside the microcontroller. This generates HSI (High Speed Internal signal)
- PLL (Phased Locked Loop): a high-accuracy circuit that generate higher frequency clock by multiplying the frequency of an input clock, also already inside microcontroller. This can be boosted to a higher speed than HSE or HSI.
  - PLL takes a source, either HSI or HSE, then multiplies the clock speed.
We can use one of these sources to drive the clock for the microcontroller
Some peripherals may need to run at a lower clock rate; we can use prescalers to divide HCLK from AHB bus

External clock does not require crystal oscillator, we connect the external clock by deriving an external source such as another MCU, circuit, or another part of the PCB, etc.
Often, crystal oscillators are already included on Development Boards, but not on microcontrollers.
On STM32-DISC boards, HSE is provided by the on-board crystal.
Nucleos use ST-LINK circuitry to supply HSE signals (Bypass). ST-Link has its own microcontroller from which we can pull the 8 MHz clock.

After power or system reset, HSI clock is used as default system clock. Note that if there is no external clock, the microcontroller will have to use HSI.
HSE and PLL have to be enabled in the code first, otherwise they are “dead”.

Clock configuration registers hold bit flags, e.g. clock source enabled/disabled, prescalers; clocks for various domains such as APB, AHB, memory domain
In modern MCUs, before using any peripheral, we must first enable its peripheral clock with peripheral clock registers
- First find out what bus the peripheral is connected to using the datasheet, and then find the corresponding clock enable register for that bus
Default behaviour is peripheral clocks of most peripherals are disabled to save power; a peripheral won’t respond to configuration values until its configuration clock is enabled
In STM32 MCUs, peripheral clocks are managed through RCC registers
- E.g. ADC register
- Add the address offset to the base address

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