D
Dave Boland
- Jan 1, 1970
- 0
I need to design a card that has a 3.3 volt processor on it, a couple of
5 volt CMOS parts, and interface to TTL. This is turning into a
headache, so I'd appreciate some advice from experienced system/logic
designers. Let me start by telling you what I think I know, and you can
correct me.
The processor is called 5 volt tolerant, which seems to mean it will
handle a 5.1 volt input without problem. One reference I read said to
add a 100 ohm resistor in series to limit current for the protection
diodes in the processor. Right so far? What is the impact to
reliability of the processor? The reason for asking about reliability
is that I want a design that will work for years and years, not just
long enough for a show and tell.
Back to that processor. The Voh is at least 2.5 volts and the Vol is
about .5 volts, so the output looks like it would work with TTL. Right
so far?
The processor outputs can be either totempole, or open drain. With open
drain, I can pull it high so the output will be above the 3.5 volts
needed by 5 volt CMOS. I assume (yes, I know what that word means) that
the processor can pull the line to .6 volts or less for a low output.
Will this work well and reliably?
I'm told that doing things as described will work, but I sacrifice
propagation speed. Typical numbers seem to be 40 to 80 nS. This may be
a problem for use on a bus.
The alternative is an alphabet soup of logic families from ABT(?), HC,
HCT, and others. This is where I feel especially in the dark. Any good
rules of thumb for mixing logic families? I have some info from Philips
(AN240 I believe), but is is almost 10 years old and fails to mention
some of the logic families I see being used. Have also looked at other
web sites, but things still seem as clear as mud.
Finally, one particular troubling area is interfacing the I2C bus. It
will see a 3.3 volt device, two 5 volt CMOS devices, and one TTL. Since
this is a bidirectional bus things really get messy. Max has some
devices to do this, but I don't know how well they work, or if there is
a better alternative?
Sorry for the long append. Hope no one dozes off reading this. Thanks
for any helpful information.
Dave,
5 volt CMOS parts, and interface to TTL. This is turning into a
headache, so I'd appreciate some advice from experienced system/logic
designers. Let me start by telling you what I think I know, and you can
correct me.
The processor is called 5 volt tolerant, which seems to mean it will
handle a 5.1 volt input without problem. One reference I read said to
add a 100 ohm resistor in series to limit current for the protection
diodes in the processor. Right so far? What is the impact to
reliability of the processor? The reason for asking about reliability
is that I want a design that will work for years and years, not just
long enough for a show and tell.
Back to that processor. The Voh is at least 2.5 volts and the Vol is
about .5 volts, so the output looks like it would work with TTL. Right
so far?
The processor outputs can be either totempole, or open drain. With open
drain, I can pull it high so the output will be above the 3.5 volts
needed by 5 volt CMOS. I assume (yes, I know what that word means) that
the processor can pull the line to .6 volts or less for a low output.
Will this work well and reliably?
I'm told that doing things as described will work, but I sacrifice
propagation speed. Typical numbers seem to be 40 to 80 nS. This may be
a problem for use on a bus.
The alternative is an alphabet soup of logic families from ABT(?), HC,
HCT, and others. This is where I feel especially in the dark. Any good
rules of thumb for mixing logic families? I have some info from Philips
(AN240 I believe), but is is almost 10 years old and fails to mention
some of the logic families I see being used. Have also looked at other
web sites, but things still seem as clear as mud.
Finally, one particular troubling area is interfacing the I2C bus. It
will see a 3.3 volt device, two 5 volt CMOS devices, and one TTL. Since
this is a bidirectional bus things really get messy. Max has some
devices to do this, but I don't know how well they work, or if there is
a better alternative?
Sorry for the long append. Hope no one dozes off reading this. Thanks
for any helpful information.
Dave,