0
\$\begingroup\$

Has someone of you seen a XOR gate with 16 or 32 inputs? (Integrated in a single chip, of course)

I would need it to compare two 16bits variables to each other.

Or are there better options for comparing such a high input count?

Clock would be 40MHz so it should be really fast (5-10ns delay).

Logic level: preffered 5V or 3.3V

\$\endgroup\$
3
\$\begingroup\$

You can cascade 2 or 4 74HC85 chips which will give you an '=' output as well as magnitude comparison.

Edit: Total delay will be much too long for your 5-10ns requirement, if you can't work with the propagation delay, I think some kind of programmable logic will be about your only choice.

You could use a small CPLD such as the XC2C32.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ Using 5 74F85s, set up as 2 levels, will (typically)(just about) work. Typical delay is 22 nsec. \$\endgroup\$ – WhatRoughBeast Sep 5 '15 at 23:47
2
\$\begingroup\$

For which kind of logic levels : TTL, LVTTL ...? What speed ?

There are chips like this one : CD74HC688 : 8-Bit Identity/Magnitude Comparators

http://www.nxp.com/documents/data_sheet/74HC_HCT688_CNV.pdf

Which are almost obsolete.

You could instead use a CPLD from Lattice, Altera.

\$\endgroup\$
1
  • \$\begingroup\$ TTL or CMOS, speed should be 40MHz. \$\endgroup\$ – marco-a Sep 5 '15 at 16:24
1
\$\begingroup\$

The best device I know of is the 74AS885 which runs on 5 volts and will compare two 8-bit numbers in ~6 nsec. You'll need two, of course, and they come in a 24-pin DIP. The big problem is that two ICs will require typically 1.3 watts, and perhaps as much as 2 watts (400 mA). If this is a problem, I'd recommend TEMLIBs suggestion of a CPLD - actually several, but you should be able to use, for instance, something like the Altera EPM240, although I'm not certain of the final speed you'd get for this sort of multilevel logic which the logic structure requires.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.