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PostPosted: Wed May 16, 2018 6:29 am 
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A while ago, I had a pirate cartridge with problems, wich I asked for help here.
The game died for good, so, I'm trying to rebuilt it.
It uses an 1Mbit ROM for the PRG and a 256Kbit ROM for the CHR.
Since I only could get a 2Mbit and a 512Kbit flashes I decided to turn this 4 in 1 on a 8 in 1.

It uses a 74LS74 as a flip flop to change games.
I'll need one more bit to do 8 games switch.
I could only found an 74HC74, would it work fine if I simply duplicate the circuit an cascate it or do I need to do some slight modification with the HC part?

Thanks in advance.


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PostPosted: Wed May 16, 2018 9:32 am 
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It depends on how it's hooked up. You can use HC outputs on LS inputs, but not LS outputs on HC inputs. For that situation there is the HCT family, those are HC parts with TTL-level inputs. This site has a pretty good chart of the voltage levels.
http://www.interfacebus.com/voltage_threshold.html


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PostPosted: Thu May 17, 2018 7:57 am 
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Thanks!
Probably I'll do some bigger modification on this circuit, maybe even change teh IC for one with more FFs.

I see there's some differences between the circuit I've been using and this one, but I need to fully trace the last one to correctly list them.


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PostPosted: Sat May 19, 2018 6:47 am 
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To add the extra bit, first tried to switch to a 4520 and changing almost all connections without success.
Later, after crossing by some bad 74'74 ICs I finally got it working by duplicating the first block.
It's working like a charm!!

This is the circuit that was in the cartridge:
Attachment:
File comment: Cart
74LS74 Reset Cart.png
74LS74 Reset Cart.png [ 4.59 KiB | Viewed 99 times ]

And this is the circuit I use when I build my multicarts:
Attachment:
File comment: Mine
74LS74 Reset Cart 2.png
74LS74 Reset Cart 2.png [ 4.07 KiB | Viewed 99 times ]

Is anyone better than the other?


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PostPosted: Sat May 19, 2018 10:33 am 
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The former is "just" an asynchronous counter; the two outputs will count 0,1,2,3.
The latter is a twisted-ring counter; the two outputs will count 0,1,3,2.

They're both fine at this size. The former can scale arbitrarily—each extra latch doubles the number of states—but it's asynchronous: each latch adds more propagation time through the entire stack. The twisted-ring counter only adds two new states per extra latch, but is synchronous: all bits change (or fail to change) at the same time.


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PostPosted: Sat May 19, 2018 11:35 am 
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That's a very nice explanation. :)
Thank you!


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PostPosted: Sat May 19, 2018 4:56 pm 
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Another slightly off-topic question:
I know I can do a flip-flop with transistors, but would it be possible to implement a single bit FF with them to use in this case?
If so, could you please suggest a schematic?
Thanks in advance.


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PostPosted: Sat May 19, 2018 7:08 pm 
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A simple flip-flop is usually 2 (S-R) to 6 (D) NAND gates; it'll take an awful lot of bare BJTs to make one.

Although there is this one ridiculous way of making a single bit of DRAM... he also mentions a classic 2-BJT S-R latch.


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