BigEd wrote:
There are other more complex circuits - try searching or image searching for
[parallax propeller 5v interface]
Thanks, Ed. Actually I was hoping cbmeeks would define the problem. (How many 3V->5V level conversions? How many 5V->3V? How many bi-directional?) With your link it's more a case of defining
solutions...
... regarding which I have some misgivings. I'll explain.
Quote:
There are parasitic catch diodes on every pin of the device which will clamp incoming out-of-range voltages to within ~0.4V of the VSS and VDD rails. This will force the resistor to drop 1.3V (5.0V-(3.3+0.4)) for an incoming high signal.
Okay, this is the 5V->3V case. I see this "just add a series resistor" technique mentioned often, but it overlooks the possibility that the 3V supply will actually be driven higher than 3V due to the current passing through the resistors and the parasitic catch diodes. It's not unlikely if the 3V supply is lightly loaded (eg: only driving some CMOS chips). That's why when lacking full details I recommend not just a series resistor but also a resistor from the 3V node to ground, resulting in a voltage divider. (Garth's suggestion of a cap will reduce delay.) The voltage divider means no current through the diodes and no chance of the 3V supply rising above 3V (which I admit may be harmless, but it's a violation of the specs).
Quote:
To output to a 5V system might be a simple matter if the threshold voltage is TTL (~1.4V) or CMOS (2.5V). You can connect straight in.
This is the 3V->5V case. I agree about the ~1.4V TTL input threshold -- that's the "free" conversion we mentioned earlier. 3V is well above 1.4V, and the datasheet will show the input can accept 3V as a logic high. But as for CMOS levels, 3V (or 3.3V) is only marginally above the 2.5V half-way point, and it's likely the datasheet will
not show 3V as an acceptable logic high. But it will probably work.
BigEd wrote:
That quote is from Chip Gracey, founder of Parallax and designer of the Propeller
I accept that Mr Gracey is a very well-qualified person. But it would be a mistake to suppose such a person always provides comprehensive answers. In this case he has "dumbed it down" quite a bit, and that's worth noting.
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