Please, help me understand this Atari 2600 cartridge switch

Discussion of development of software for any "obsolete" computer or video game system. See the WSdev wiki and ObscureDev wiki for more information on certain platforms.
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Fisher
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Please, help me understand this Atari 2600 cartridge switch

Post by Fisher »

Recently I've been "hunting" some old Atari 2600 cartridges.
I disassembled a multigame cartridge and was a bit confused about the game-switching circuit.
This circuit switches the game when the console is powered off and then on again:
Power game switch.png
Power game switch.png (10.32 KiB) Viewed 1228 times
Although I successfully rebuilt the same circuit, so I can confirm I draw it correctly, I just can't understand how it works.
Is the IC receiving current through the clock pin, how? :roll:


Another interesting thing I've seen in this cartridge:
WhatsApp Image 2022-09-26 at 07.59.56.jpeg
This is the game's PCB.
WhatsApp Image 2022-09-26 at 07.59.56 (1).jpeg
Many games use a NOT gate to generate the enable pin for the ROM, but interestingly this one uses the /PGM pin connected to A12.
This seems to work as a +CE pin. Cool!!

Can someone please help me to understand the game switching circuit?
Thanks in advance.
lidnariq
Posts: 11432
Joined: Sun Apr 13, 2008 11:12 am

Re: Please, help me understand this Atari 2600 cartridge switch

Post by lidnariq »

Fisher wrote: Mon Sep 26, 2022 8:37 am Although I successfully rebuilt the same circuit, so I can confirm I draw it correctly, I just can't understand how it works.
Is the IC receiving current through the clock pin, how? :roll:
Yup!

Because CMOS parts are so static sensitive, they often will have diodes as possible to reduce the possibility of static discharge damaging something.
TI-CD4020B-CD4024B-CD4040B-input-protection-network.png
TI-CD4020B-CD4024B-CD4040B-input-protection-network.png (5.11 KiB) Viewed 1207 times
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