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Cartridge reset


ralphb
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How does inserting a cartridge trigger a console reset, electrically?
I know that putting -5V to RESET will reset the console, but I thought that at some point those -5V need to go away again.
Looking at a TI cart I basically see this:
.
Vss      Vdd    RESET
 |        |       |
 |        |       |
 +---??---+---R---+

.

What's that mystery component -- looks like a (Zener?) diode? I assume that this will put RESET on Vss after a while, but how does it work?
And how do the Super Carts handle this? It seems the red board just permanently connects -5V to RESET?!
(BTW, I'm totally confused by negative voltages. Is -5V = Vdd still considered "high", with Vss being "low"?)
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There's a capacitor inside the console between the cartridge port reset input and the processor reset circuit. Applying -5V to the cartridge port reset input momentarily pulls /RESET within the console low, then /RESET returns high as the capacitor charges. So connecting the cartridge port reset to -5V permanently is not a problem - you get an initial reset pulse then nothing. When you remove the cartridge, the capacitor discharges through a parallel resistor.

 

-5V does not fit into the "logic low/high" scheme. Applying -5V to a normal logic gate normally results in something breaking ... ;-)

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Thanks for the answer, that makes sense.

 

But what's that diode on the cart board for then?

 

And regarding -5V and high/low: Yeah. :) But what if we have +5V, 0V, and -5V at the same time? The reason for asking was that Thierry calls RESET "active high", which would only make sense if -5V is considered "high" in this context. (I'm also not used to differentiating between Vss and GND, which again seems relevant for the cart port.)

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I suspect the "diode" is actually a smoothing cap on the -5V supply. Is it clear glass, with *no* band at one end to indicate orientation? Is there another identical one for smoothing the +5V supply?

 

Where exactly does Thierry say "active high"? Calling a signal that is essentially -5V "active anything" might perhaps be a little misleading.(I Googled for the pinout of the cartridge port connector and that said "active high" for RESET as well.)

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I remember playing with the innards of a broken TV set when I was a small kid. Of course the only meaningful thing we could do with something like a resistor or a capacitor was to cut it into two parts ... :) This probably contaminated me for life!

 

Well, this year I'm set to correct my hardware deficits ...

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