Re: [uCsimm] Are the uCsimm I/O pins 5V Tolerant?

From: james wilson (jc.wilson@utoronto.ca)
Date: Thu Feb 03 2000 - 21:23:55 EST


        Why the 1K pull up to 5Volts? Since the TTL output would be high (=<
VCC) or low (=>Gnd) isn't it enought to have the voltage divider for the
ttl input.
         
Tom Walsh wrote:

> I would stay away from LS as when a device is new it can be
> 'healthy' enough to go to 4.2 volts VoH. But you are correct, if you
> are interfacing 3.3v logic to +5 logic, use LS (or about anything but
> HC). If you really need to kludge a +5 to 3.3v interface between LS
> and 3.3 logic, why not build a resistor totem and pick off at the
> lower resistor, example:
>
> +5
> |
> \
> /
> \ 1K
> /
> |
> |
> TTL --->O
> |
> |
> \
> /
> \ 1K
> /
> |
> |
> O---------> DragonBall input
> |
> |
> \
> / 2K
> \
> /
> |
> |
> -----
> / / /
>
> This would give you about 2.5 volts to 0v swing and won't damage the
> TTL gate as its output impedance is about 400 ohms?
>
>
> TomW
>
>
>
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