Work in progress.
This guide will show you how to install a Dual Frequency Oscillator (DFO) 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 non-native, forced region. For a 50Hz PAL console the 60Hz NTSC setting will be out of spec, and vice versa. 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. GND for 50Hz PAL consoles and 5V for 60Hz NTSC consoles. This makes the Mega Drive an almost universal console, it is only the master clock frequency that needs changing per region.
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
- SEGA 315-5313A Pin 46 (PAL/NTSC Switch In) – GND = 50Hz / 5v = 60Hz
- 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
- The VDP PAL/NTSC selector pin so it knows to generate the correct Subcarrier frequency for PAL or NTSC depending on the region
- The Video Encoder CXA1145P PAL/NTSC selector pin so it knows to encode the correct video format for PAL or NTSC