Not to be confused with the real time clock (RTC), which is clocked independently.
In cartridge systems
The main system clock (often called mclk) is either 24MHz (MVS) or 24.167829MHz (AES). It is generated and divided by 2, 4 and 8 by NEO-D0 from a crystal oscillator, to provide the 12MHz clock for NEO-ZMC2, the inverted 6MHz one for video output and also the 3MHz one for NEO-B1.
The cartridge connector provides the 24MHz, 12MHz, 8MHz and 4MHz inverted clock signals.
A second quartz oscillator is used for generating the color burst needed by composite video, for the video encoders.
A third 32768Hz quartz oscillator is used for the RTC.
To do: 8M is 33high/66low, not 50/50
In CD systems