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How game board components work


channelmaniac
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This will be a series of posts on how specific components work on a game board. Its purpose is to help you understand how the circuitry works. Feel free to add to the series if there's a topic you want to cover.

 

First up: Reset!

 

You'll typically see the signal defined as RESET with a bar over the top. In normal text like this where we don't have a bar over the top we write it with a slash in the front like this: /RESET. This bar signifies it's an Active Low signal. Normally the reset line is high (for most CPUs...) and you take it to logic low to reset the CPU and have it restart. Some CPUs will start up in a random state while others, like the Z80 will appear to be dead until you give it a /RESET pulse so without it the game will never start.

 

CPUs do different things when the /RESET is applied. They set the internal registers to a known state, clear interrupts, and start executing the program code. Z80s and many others start at 0000h (h = hexadecimal address) while the 6809 looks to address FFFE and FFFF for the 16bit address of where to start executing code.

 

So the first thing you check is for physical damage... the second, the clock, and the third is /RESET when a game doesn't want to boot.

 

If you have a logic probe with audio, it's easy to hear the logic level change of low to high when the game is powered on. That lets you know the reset circuit is working.

 

Some boards have a watchdog - if the CPU doesn't write to a location in memory, or do something else like read it to periodically reset the watchdog, the watchdog will reset the CPU. This is normal and signifies something else is wrong. More on that later in this series.

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Got through this post and went back again and again, great stuff and thanks for the effort to help the noobs like myself to take on more challenges in the world of digital logic's! @channelmaniac
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