driving Spartan-3 input from 74LS TTL

Can you guarantee it is always going to be a 74LS14 ? That's one important factor in skipping the limiter, and not always under your control.

Also, what is the 5V/3.3V supply tolerances - if you know that then you should be able to do a LS load line calc, to see the clamp currents.

- but the answer only applies to LS14 devices... If someone uses a HC/VHC/AHC/AC device, all that flies out the window :)

-jg

That was the part I wasn't sure about.

Yes. That would have to be taken into account.

Oooh, I hadn't considered that.

I wasn't trying to avoid the resistor, just trying to determine whether the value could be lower. I don't actually need it to be lower, but I wanted to understand the actual requirement.

Even with 300 ohms, won't there be a problem if the 5V comes up first, or there is a fault and the Vcco isn't present? If the 5V is really

5.5V and Vcco is at/near 0V, the TTL output could be as much as 4.1V above Vcco. That would allow 11.7 nA, while answer record 19146 suggests that the current through the protection diode should not be more than 5.51 mA. To limit the current to 5.51 mA, the resistor would need to be 635.2 ohms (680 ohm 5%). That seems pretty high, but since TTL is slow stuff anyhow, it looks like it won't introduce enough delay to be a problem.

Should answer record 19146 be revised to cover startup and Vcco fault conditions? Or is it safe to allow the I/O pin to power Vcco through the clamp diodes provided that it doesn't rise above Vcco(max)?

Thanks, Eric

Given the LS series is slow compared to modern logic, a resistor with a higher value may even be a better choice. Less emitted radiation and lower power consumption.

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Yes, but beware going too high, or the edges slew so slowly, that noise immunity suffers. You can use a parallel R/ small C for lowest power consumption, and little speed penalty. Same principle as Scope Probes.

-jg

OK. I suppose the official Xilinx documents (including answer records) have to be written conservatively.

I was already taking that into account in my calculation.

Thanks for the advice! Eric

I have found that I can get 74LVC244AD 3.3V octal buffers with 5V-tolerant inputs and 6 ns max prop. delay for less than the cost of 330 ohm 16-pin SMT resistor networks!

part Digikey quantity 100 price

NXP 74LVC244AD $0.26 Bourns 4816P-T01-331LF $0.71 CTS 767-163-R330P $0.37 (not stocked)

Amazing. I guess those bleeding-edge fabs they use to make resistor networks must be really expensive! Maybe they could lower the cost by outsourcing production to TSMC :-)

Eric

16 pin SMT networks are dinosaur part, so that's no real surprise. A better choice are the 1206 4 Element ones - much lower price and less PCB area - so no one uses the Bourns ones anymore.

On a similar front, I noticed SiliconPOTs are getting better all the time, and one recent one had very good ppm matching and drift specs, and looking to be cheaper than alternate fixed-value precision resistors.

-jg