| ??? 10/05/10 06:44 Read: times |
#178913 - That does work if you have one functional part for "seed" Responding to: ???'s previous message |
Per Westermark said:
Don't bother with trying to control pins at high speed to look at propagation delays.
Do what I suggested. Send square wave at high speed through chip and let delayed and non-delayed signal mix in XOR gate of best available technology. The variation in delay will vary the low-pass-filtered voltage from the XOR. This is a cheap method to guestimate propagation delays between different families or detect that a chip of known family doesn't drive the output with full current capability. The square wave thing works with known parts consisting of gates, but with shift registers and counters it can be less than ideal. Further, with ripple counters, depending on how you register failure, it may not work at all. XOR's won't verify tristate parts without tying the enables active. That's why my XELTEK programmer can't distinguish between 7408 and 74126, for example. Further, the XOR, being two logic levels, is the slowest of all gates, and will often be too slow for speed-testing. Most importantly, for an XOR-based gate checker, a known-good sample of the same part is needed. However, if you don't know what it is, i.e. if it's stock-numbered rather than bearing the sandard part number, you won't know what it is. Building up a repertoire of vectors derived from databook truth tables works very well. You start with the table for the one you have, and test it, then go on to the next number. Eventually, you have all the numbers you use worked out. If you don't know what it is, you try all the tables you have assembled. If you don't get a match, at least you know it's not one of them, though it could be defective. RE |
