Re: 6502/6510 ROM selection daughterboard

From: Gábor Lénárt <>
Date: Thu, 7 Jun 2012 15:41:38 +0200
Message-ID: <>
On Thu, Jun 07, 2012 at 02:32:18PM +0200, Gerrit Heitsch wrote:
> On 06/07/2012 02:09 PM, wrote:
> >Well, you can always use a brute force method, like cutting the +5V pin
> >- than anyone can do ;)
> That would be a bad idea. Most chips don't like it if the voltage on
> the signal pins is above the supply voltage (look under 'maximum
> ratings' in the data sheet). That can have all kinds of undesirable
> effects including destroying the chip.

Btw, I had an odd problem similar to this one, I can't understand: ok, it's
an MCU (Atmel ATmega8A): the SPI programmer gave some power to its
MOSI/MISO/SCK pins, while I switched off the power itself (Vcc).  The odd
thing: the circuit still worked (!!) just the LEDs (I controlled with the
MCU) was quite dim, bit it was clear it still blinking morse code (I
programmed the MCU to do that, and the LED got power only from the output
pin of the MCU, the other pin of the LED was soldered onto GND through a
resistor).  In forums people say it's called "parasite power" that it can
work even without Vcc connected, if the MCU can get power from other pins. 
However it sounds very strange for me, and honestly I can't understand this.

However this clearly shows, that not every IC will "keep in quiet") without
the Vcc pin either ...

Maybe that's a bit off-topic (the MCU topic, but to protect myself: I want
to build something for C64, just I started with simple examples like LED
blinking), but if someone knows what is this "parasite power thing" I would
be happy to get to know (even in private mail, not so much the topic of this
mailing list, I guess).

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Received on 2012-06-07 14:00:19

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