Re: 6502/6510 ROM selection daughterboard

From: Pasi 'Albert' Ojala <*a1bert_at_iki.fi*> Date: Thu, 07 Jun 2012 21:22:51 +0300 Message-ID: <4FD0F17B.9030600@iki.fi> · a1bert_at_iki.fi

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.

-Pasi

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