Mega Drive Overview (Sega Mega-Tech)
The Mega Drive circuit is on the left side of the board, and is mostly made up of the following chips:
- 68000 CPU (Motorola MC68000, IC54)
- SRAM for 68000 (TC51832PL-12, IC55, IC43)
- Mega Drive VDP (315-5313, IC64)
- Video DRAM for VDP (MB81461, IC59, IC60)
- Co-Processor Z80 CPU (Zilog Z80, IC56)
- Bus Arbiter (315-5308, IC65)
- IO Controller (315-5309, IC66)
- Clock Generator (315-5345, IC61)
Slightly different than an actual Mega Drive, but still useful to see is the architecture of the Mega Drive.
The 68000 CPU is responsible for most of the work on the console including running Mega Drive games.
The Z80 was the CPU from the Master System and was included in the Mega Drive primarily to support backwards compatibility.
It also handles audio and controller input.
This custom Sega IC 315-5313 controls video output generation, reading data from both the CPU and Co-Processor, utilizing the video DRAM
The custom Sega IC 315-5308 holds the Z80's bank register used to select the appropriate 68k bank, whose address this chip puts on the bus during free cycles. It basically interfaces communication between the 68000 CPU and the Z80 Co-Processor.
The main job of the Sega IC 315-5308 is as an IO Controller for translating data between the Z80 (8 bit) and 68000 (16 bit) CPUs.
A bad one of these can cause black screen output.
The custom Sega IC 315-5345 is responsible of EDCLK generation (external dot clock in H40 mode, based on HSYNC and MCLK inputs) and 68k RAM refresh.
