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Sega Mega Drive Dual Frequency Oscillator Install Guide

Work in progress. Quick notes.

This guide will show you how to install a Dual Frequency Oscillator into a Sega Mega Drive.

When region modding a Mega Drive the console will run with an out of spec master clock frequency and video refresh rate when using the forced region. The Dual Frequency Oscillator corrects this by changing the master clock frequency to the correct one depending on what region is selected.

The Sega Mega Drive model 1 has a pair of jumpers called JP3/JP4 which are connected up to the PAL/NTSC selector pin of both the CXA1145P Video Encoder (Pin 7) and the SEGA 315-3513A VDP (Pin 52).

These jumpers are hard set to either +5V or Ground, which sets the VDP and the Video Encoder to the correct region. This makes the Mega Drive an almost universal console, it is only the master clock frequency that needs changing per region.

(DIAGRAM OF CLOCK MASTER CLOCK CONNECTIONS HERE)

PAL models use a 53.203Mhz master clock oscillator

  • Divided by 7 to generate a ~7.6004MHz CPU clock
  • Divided by 12 to get 4.433583Mhz for the PAL chroma subcarrier frequency

NTSC models use a 53.6931Mhz oscillator

  • Divided by 7 to generate a ~7.6704MHz CPU clock
  • Divided by 15 to get 3.57954Mhz for the NTSC chroma subcarrier frequency

Master Clock Oscillator

Pin 3 Clock Out to

  • VDP SEGA 315-5313A Pin 52 MCLK Master Clock in
  • SEGA 315-5364 Pin 51 MCLK Master Clock in

JP3 / JP4 (Video Jumpers) connected to

  • SONY CXA1145P Pin 7 (PAL/NTSC Switch In) - GND = PAL/ 5V = NTSC
    then to
  • SEGA 315-5313A Pin 46 (PAL/NTSC Switch In) - GND = 50Hz / 5v = 60Hz

VDP

  • VDP Pin 50 SCBR Subcarrier Out to CXA Pin 6 Subcarier In
  • VDP Pin 51 CLOCK0 Out to Zilog Z80 Pin 6 Master Clock in
  • VDP Pin 49 CLOCK1 Out to Yamaha Pin 24 Master Clock in

JP1/JP2 (Language Jumpers) connected to

  • SEGA 315-5309 Pin 25

In order to make the Mega Drive fully compatible with PAL and NTSC three things need changing together.

  • Master Clock Oscillator frequency (Using DFO)
  • The VDP PAL/NTSC selector pin so it knows to generate the correct Subcarrier frequency for PAL or NTSC depending on the region (Using a Switchless Region mod connected to JP3/JP4)
  • The Video Encoder CXA1145P PAL/NTSC selector pin so it knows to encode the correct video format for PAL or NTSC (Using a Switchless Region mod connected to JP3/JP4)