Re: 6809 assembly knowledge needed

From: Nate Lawson <>
Date: Wed, 03 Feb 2010 12:50:46 -0800
Message-ID: <> wrote:
> - The PDF says: "I/Os are not 5V tolerant". I see a lot of resistors to
> handle this. But is it still capable of driving the C64 plus some
> extras? If not, what is needed to make it capable? For example, will
> 74HCT245's and 541's do?

They are using a level shifter or resistors for this. The level shifter
has both 5v and 3.3v supplies and is bidirectional. If configured for
just resistors, the board only provides current limiting and any
overvoltage will go through the FPGA's ESD diodes. This only works if
your input voltage is less than Vcc + 0.5v.

The output stage is usually simpler unless the FPGA has to source a lot
of current. 3.3V levels are compatible with 5V levels for Voh and Vol.

Designing mixed voltage circuits is tricky but can usually be done in a
straightforward way. Just be sure you test carefully. I've fried several
FPGAs using improper diode clamping approaches.


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Received on 2010-02-03 21:00:07

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