# It helps if you plug it in...

Posted by Tursi, 22 December 2014 · 119 views
I went ahead and put the logic analyzer on the flipflop, as mentioned, and found that it was neither flipping nor flopping. All the other signals looked as I hoped, everything seemed right. I checked timing on the datasheet, and while the TIME pulse was only 320ns, the flipflop only needed 40ns to do it's thing.
Finally I checked power, and found I'd wired the flipflop to ground, so no power. When I fixed that, the menu and TF2 correctly came up and selected. Since the second ROM select wasn't in place yet, TF3 correctly re-selected the menu, and TF4 selected TF2. All seemed good, and the logic signals on the bus were reassuring.
Next it was time to work out the chip selection. My original plan was to run the output of the flipflop directly to !G on one chip, and to invert it for the other.
I went to detach the !G pin on the PCB, and found it was not permanently tied low as I expected, rather, it was tied to the Genesis !CAS0, which is a general purpose read signal. I realized that its purpose was to ensure the ROM only responded to reads, not writes. I'd lightly tested the ROMs never write to ROM by accident (this is a pretty common bug, actually, and doesn't have any impact on real hardware in most cases, but if I subverted this signal so that the EPROM would respond to both reads and writes, a write would crash the hardware.) I decided not to chance it.
I went through a handful of circuits for merging in the !CAS0 select with my flipflop's output bit, remembering that it had to be split into two selects. Finally, I decided the best circuit was to adapt the circuit I had mapped out in the Lion King, using a quad NAND gate (using it as two inverters and two NANDs).
Final circuit is something like this:
I put all this together - the tricky part of this was lifting the !G pin of the EPROM so it was no longer attached to the PCB (I also had to cut the trace for the second chip later). Then I tested again, and again the cart behaved as expected - menu and TF2 worked, 3 and 4 crashed the system now (because the EPROM was disabled and no other EPROM was yet in place), but I checked the pins and they looked right. I took some logic captures showing selection of each menu option here.
To read these, start at the 'T' and look down. Each row is one signal from the circuit.
!RESET is the line from the Genesis for reset - it is active low, that is what the '!' means.
ROM 1MB is the output from the FlipFlop used to select the top or bottom 1MB in each EPROM.
A2 and A3 are the Genesis address lines. A2 represents multiples of 4, and A3 multiples of 8. These are the two lines we latch in the flipflop.
ROM Select is the output from the NAND Circuit that selects the first or second EPROM.
!TIME is the I/O select toggle from the Genesis that indicates an address in the range of $A13000 is accessed.
!CAS0 is the general read output line from the Genesis.
!G1 is the !G line going to the first EPROM
!G2 is the !G line going to the second EPROM
So here is TF2 being selected. !TIME goes low at the Trigger point (T), but it's the rising edge that matters to the flipflop. You can see at that point that A2 is high, and A3 is low. The flipflop's job is to control ROM 1MB and ROM Select - you can see that ROM 1MB goes high and ROM SELECT stays low - this selects the second bank in the first EPROM, which is correct. Nothing else matters until !CAS0 begins to toggle, at which point we can confirm that it is followed by !G1 and ignored by !G2.
Here is TF3. This time A2 is low and A3 is high, which leaves ROM 1MB low and raises ROM Select to select the second EPROM. And we see this reflected when !CAS0 starts toggling, that it is mirrored by !G2 and ignored by !G1.
And finally, TF4, where A2 and A3 are both high, and so both ROM 1MB and ROM Select go high. Again we see this map correctly when !CAS0 starts to toggle.
You can see, too, in the Reset case that everything goes back to low levels, reselecting the menu.
Finally, then, everything looked right, so I programmed and attached the second EPROM. This one I put on the bottom of the board, since there's not enough room in the shell for piggybacking on top. This isn't really trivial either, and I used resister legs to make longer legs (since you have to put it with the window against the PCB so the orientation matches the chip on top). Then attach !G2 to the correct pin, and plug it in!
Unfortunately, I had mixed results. TF3 does not start at all. I observed this in emulation, but, I had very little confidence in the emulation since hacking my switching in was so difficult. The emulator claims an address error early in startup, yet the ROM works fine in the emulator on its own, or if its swapped in during a breakpoint. But... it happens on hardware, so this must be real.
Thunder Force II and IV crash during play -- oddly I let the demos run for a long time and had no trouble, but about 5 minutes of play (or just starting TF2) locks it up (the Z80 keeps going, meaning the music doesn't stop, which suggests a 68000 exception). There could be a problem here, or it could just be the long wires picking up interference...? Could even be a cold solder joint. But given the read select signals are running through the wires, and that TF2 worked fine before the additional circuit was added, I'm willing to suspect interference.
All things considered, I think this board has reached the 'trash' state. There's only a little more work I can do to try and figure it out, but no matter what there is a problem with TF3. So this means that I will have to restart the project. (Something I sadly haven't time to do right now).
On the plus side, I've learned that the switching circuit seems to work, and I have a menu. I can fix the debounce issue (it's enough to add a 'sleep 15' after each move for a really cheap debounce). I need to spend more time in the emulator to understand the TF3 crash, and to make sure that TF2 and TF4 aren't crashing for software reasons (I don't think so). I suspect making a PCB for the next attempt would be a wise move. ;)
So, I have to leave this project at this point. But here's a video showing it partially working, at least! YOUTUBE