> This question has to do with interfacing the 3.3V DIO lines to
> external circuits. How do you folks usually do this? When using a
> line for output, I'm assuming it is okay to feed directly to TTL
or
> CMOS ICs (even if they have 5V Vcc) since 3.3V is logic HIGH for
> TTL. Are there any precautions (current requirements for TTL
inputs,
> etc.) I need to be aware of? I'm entertaining the use of buffers
as
> an extra precaution... The last thing I want to do is burn out my
> SBC.
3.3V is not a logic high in 5V CMOS. If you are using 5V CMOS, you
need at least 3.5V.
>
> When using DIO lines for inputs, what is the recommendation there?
> Should I use CMOS buffers (with TTL inputs) running at 3.3V Vcc?
Is
> it really the voltage that's the issue, or the current? In other
> words, if I place a resistor between a TTL output and the DIO
line,
> will that do the trick? I rather not over complicate things, but
I'm
> sure there are many good ways to interface a 3.3V IO with 5V or
even
> 12V IO.
If the DIO pin has a diode clamp, you only need to watch the
current. A series 470 - 1K ohm resistor is a good way to make a
3.3V w/diode clamp 5V tolerant. With a series resistor, the pin
becomes very weak if used as an output though.
If the DIO pin doesn't have a diode clamp, the above doesn't help,
and driving to 5V will blow out the I/O pin. You can make the I/O
5V tolerant by having an external diode clamp to 3.3V and series
resistor.
The pins on the LCD header are already 5V tolerant with a series
resistor. The DIO1 header is not, but has diode clamps so all you
need is a series resistor. The DIO2 header on the TS-7260 does not
have even a 3.3V diode clamp.
//Jesse Off
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