Re: Logic family compatibility



On Mon, 11 Jul 2005 21:45:43 GMT, Dave Boland
<NODARNSPAMdboland9@xxxxxxxxxxx> wrote:

>I spent a few days doing homework on what logic family and
>voltage was compatible with what. This was a lot of work
>because each vendor has their own spin on the specifications
>(which makes me wonder why standards even exist, but that is
>another story).
>
>Below is my take on things, and I would appreciate your
>comments if I have it wrong, or something is incomplete.
>
>I refer to CMOS, NMOS, etc. as xMOS.
>
> From To
>---- -----------------------------------------------
>TTL: TTL; LVTTL*; 3.3V xMOS*, LCX*, LVC*; LVT*, LVX*
>
>LVTTL: TTL**; LVTTL; 3.3V xMOS, LCX, LVC; LVT, LVX
>
>xMOS, 5V: xMOS 5V; 3.3V xMOS*, LCX*, LVC*; LVT*, LVX*
>
>xMOS, 3.3V: TTL; LVTTL; 3.3V xMOS, LCX, LVC; LVT, LVX
>
>'L's, 3.3V: TTL**; LVTTL**; 3.3V xMOS, LCX, LVC; LVT, LVX
>
>*5 volt tolerance required, which they all seem to have today.
>
>**Make sure that the Ioh is high enough to drive the loads.
>
>Driving 5 volt xMOS is difficult because the Vih is .7Vcc,
>or about 3.5 volts. Thus, a 3.3 volt part can't do this
>without a device like a 74LVC4245 or 74LVX4245.


When a 5-volt family drives a lower-voltage CMOS part, you run the
risk of forward-biasing esd diodes and poking current into the Vcc
rail of the victim chip. This is especially bad for 5V CMOS driving
3.3 or lower CMOS, as the curents can get high. Lots of lvcmos parts,
including FPGAs, are *not* 5v tolerant!

TTL doesn't pull up as hard as 5v CMOS, but enough to poke some
current anyhow; the schottky families are worse than classic old TTL.
And some TinyLogic cmos parts and older FPGAs can be pulled up to +5
or so without drawing input current.

My test guys came to me last week with a bunch of boards that had the
3.3 volt FPGA supply running 3.7 or something. I was driving the
Xilinx FPGAs from a 5-volt CPU, with 100 ohm current-limit resistors,
and it still pulled the supply up. I scaled down the sense resistors
on the 3.3 volt reg to dump more current, and that fixed it.

Right now, we're doing a board where we'll run the 3.3 volt rail at an
actual 3.5 to guarantee we can legally drive some 5v cmos stuff.

All these different supplies are a real pain. They should have stopped
at 5.0. The Spartan3 chips need 3.3 (or 3.5!), 2.5, and 1.2.

John

.