Clock
Not to be confused with the real time clock (RTC), which is a time-keeping system used in the MVS and clocked independently from the rest of the system.
In cartridge systems
The main system clock (often called mclk) is either 24MHz (MVS) or 24.167829MHz (AES). It is generated from a crystal oscillator and divided down by NEO-D0 (or other later chips), to provide the 12MHz clock for NEO-ZMC2, the 6MHz pixel clock for video output and many others.
LSPC2-A2 (or other later chips) divides it by 3 and 6 to provide the 8MHz clock for the YM2610 and the 4MHz one for the Z80.
The cartridge connector provides the 24MHz, 12MHz, 8MHz and 4MHz inverted clock signals.
A second quartz oscillator is used in the AES for generating the color burst needed by composite video, for the video encoders.
The value changes depending on the region of the system, 4.43361875Mhz for PAL systems and 3.579545Mhz for NTSC systems.
A third 32.768KHz quartz oscillator is present in the MVS used for the real time clock.
Phase relations
To do: 8M is 33high/66low, not 50/50
Signals
24M (~24MHz), 12M (12MHz), 68KCLK (12MHz), 68KCLKB (12MHz), 8M (8MHz), 6MB (6MHz), 4M (12MHz), 4MB (4MHz), 1MB (3MHz)
In CD systems
NEO-GRC ?