Mega Drive Z80 (Sega Mega-Tech)
If you are suspecting issues with the Mega Drive system and want to check the Z80 operation, this guide will take you through the expected behaviour and bring-up.
Make sure to have at least one game in the slot when doing any of these tests.
The Mega Drive Z80 reset can be controlled by both the 315-5308 or the SMS Z80 (via the I/O Expander, OR gate and Inverter).
- The !Reset (pin 18) goes to the output of an OR Gate IC21 HD74LS32P.
- Input 1 (pin 4) comes from the Inverter output (IC23 pin 6), with the input pin 5 going to the I/O Expander IC24 pin 24 (Port D5), which is controlled by the SMS Z80.
- Input 2 (pin 5) goes to the 315-5308 !ZRESET (pin 45).
Meaning if the 315-5308 !ZRESET pin 45 goes high it will bring it out of reset, or if the Z80 sets Port D5 to low, it will also bring it out of reset.
The first thing in the 315-5313 does is come out of reset so long as the 315-5308, 315-5309 are ok and don't pull it low.
Then the Mega Drive system loads the game and once successful, the 315-53 sends !ZRESET (pin 45) high to allow the Z80 to come out of reset.
Observe the Z80 reset pin during power up on pin 26.
The Mega Drive Z80 comes out of reset if the 315-5308 brings it out.
This only happens once a game has successfully loaded, or a game loaded but failed after it was brough out of reset.
Make sure at least 1 game is inserted, otherwise the Mega Drive will never come out of reset.
Apply power to the system. During the first 1 to 2 seconds of boot, the Mega Drive is held in reset (low), while the SMS BIOS loads up the top menu.
Right after this it releases the Mega Drive from reset, instructs it to read the game headers (titles and game details), then holds the Mega Drive in reset for a short period before releasing it from reset and instructing it to play the selected game.
The more games you have the longer the initial out of reset lasts. It is approximately 70 milliseconds per game, so if you have all 8 games inserted it takes just over half a second.
A good way to detect the difference between a faulty Mega Drive system and a faulty SMS system is if the reset pin comes out of reset. If the SMS is fully working, and there is at least one game inserted, the SMS should attempt to bring the Mega Drive out of reset on the 68k pin, and if a game has loaded the Z80 will also be brought out of reset.
So, if you see the reset pin go high, even if just for a short period, the SMS side is most likely working, and your issue lies with the Mega Drive system.
A common symptom of a faulty 315-5308 is the Z80 will be held in reset for a second or so, and then released from reset. There is no initial short out of reset and back into reset before coming out again, it is just low for a second and then comes out of reset, but the game never loads.
If you see the first pulse of out of reset when it attempts to read the game metadata, but then it stays in reset after that, and the Top Screen displays the 6 empty game slots, then something has gone wrong.
Either the Mega Drive system isn't working, or part of it, or the glue logic that connects the Mega Drive and SMS together is faulty (bad ICs, broken traces and so on) meaning the data could not be read.
If the PLAs (315-5348) at IC30 or IC31 are bad, this often leads to the Mega Drive coming out of reset and staying out of reset, regardless of the games inserted. This is usually accompanied by the SMS Top menu showing a solid black screen, and the Mega Drive showing a solid white screen.
If the 315-5349 IC49 is also bad, it would give the Mega Drive a solid black screen.
To do this test, you still require a game inserted, otherwise the Mega Drive will show no screen (no valid CSYNC).
The next thing to check is you have a valid 3.58MHz clock on pin 6.
This clock should be there all the time regardless of reset or state of the system.
