Re: 6502/6510 ROM selection daughterboard

From: Pasi 'Albert' Ojala <>
Date: Thu, 07 Jun 2012 21:22:51 +0300
Message-ID: <>
On 06/07/12 16:41, Gábor Lénárt wrote:
> 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.
There are protection/clamp diodes from each input/output pin to gnd and 
to vdd. If the voltage in the IO pin is above VDD+threshold (0.6V), the 
diode starts to divert the current to protect the pin. So, you can power 
a chip from its input pins, although it is not recommended and not good 
for the chip.

Yes, I have seen it plenty of times. Without power our processor runs 
the ROM code fine enough to send a character to UART after reset, but of 
course crashes a bit later.


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Received on 2012-06-07 19:00:05

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