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

From: Tom Walsh (tom@mytoys.com)
Date: Thu Feb 03 2000 - 23:26:49 EST


james wilson wrote:

> 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.

You cannot guarentee that the TTL output will be at 5 volts, what you achieve with
the circuit is a voltage loop (Kirchof 's law) in which you will have a voltage
that is 'near' the VCC supply, remember the the TTL output is a totem pole
transistor device, where if you feed a higher voltage onto the emiter of the output
pullup totem transistor, you will effectively turn it off. This is a problem with
not being able to drive a standard TTL output to the full 5 volt rail as you have a
semiconductor device "pulling" it to +5, not a resistor.

Look at one of the older TTL data books, mine is copyrighted 1982, and you will see
the transistor / resistor construction of the output circuit totem. On the HCmos,
they use a FET, this has a lower voltage drop due to the 'open' channel of the FET
totem pullup device.

Regards,

TomW

>
>
> 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
> >
> >
> >
> This message resent by the ucsimm@uclinux.com list server http://www.uClinux.com/

--
Tom Walsh - WN3L - Embedded Systems Consultant - tom over_at mytoys(.)com
'www.mytoys.com', 'www.openhardware.net', 'www.cyberiansoftware.com'
"Windows? No thanks, I have work to do..."

This message resent by the ucsimm@uclinux.com list server http://www.uClinux.com/



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