Features of the SN76489 are:
The frequency of the square waves produced at each channel is derived by two factors: the speed of the external clock and a value provided in a control register for that channel (called N). The eventual frequency is arrived at by dividing the external clock by 32 and then dividing the result by N for each channel. The SN76489 is included 'on-chip' in Sega's Video Display Processors (VDP) for their Master System, Game Gear, and Mega Drive game consoles. The Sega VDP is in fact based on the Texas Instruments TMS9918A VDP.