faheyr@silver.ucs.indiana.edu (Bob Fahey <NECR0SIS>) (10/10/90)
Hi there. I tried doing the 14Mhz a500 hack a while back, and I was wondering if one of you hardware gurus could tell me something: I couldn't get the 74f74 d-flip flop, but got a 74als74 instead. I was told that it would function just as the 74f74 would (i.e. pos. edge), but I couldnt get the thing to work properly. I don't think it was my 68000, since I tried 3 different ones. Is it the flip flop that causes the 14Mhz side to not come up? I mean, is there something about the 74als74 that is different from the 74f74? Bob Fahey | ///| a.k.a. NECR0SIS oo faheyr@silver.ucs.indiana.edu -or- | | \\\ ///_| \__/ faheyr@rose.ucs.indiana.edu | | \XXX/ | M I G A | | "'Ere long done do does did..." -The Smiths |
jeremym@brahms.udel.edu (Jeremy A Moskowitz) (10/10/90)
In article <63606@iuvax.cs.indiana.edu> faheyr@silver.ucs.indiana.edu (Bob Fahey <NECR0SIS>) writes: >I couldn't get the 74f74 d-flip flop, but got a 74als74 instead. I was told >that it would function just as the 74f74 would (i.e. pos. edge), but I >couldnt get the thing to work properly. I don't think it was my 68000, since >I tried 3 different ones. > >Is it the flip flop that causes the 14Mhz side to not come up? I mean, is >there something about the 74als74 that is different from the 74f74? The 74F74 chip is a Flip Flop D gate type. And it is damn fast. The als (which I've never heard of) has an LS in the title of the chip! It's slower! No wonder it wont work. Call 1-800-Fone-ACP and get the 16MHZ 68000 and 74F74. This is the place to go. -jeremym@brahms.udel.edu
trantow@csd4.csd.uwm.edu (Jerry J Trantow) (10/10/90)
In article <63606@iuvax.cs.indiana.edu> faheyr@silver.ucs.indiana.edu (Bob Fahey <NECR0SIS>) writes: >I couldn't get the 74f74 d-flip flop, but got a 74als74 instead. I was told >that it would function just as the 74f74 would (i.e. pos. edge), but I >couldnt get the thing to work properly. I don't think it was my 68000, since > >Is it the flip flop that causes the 14Mhz side to not come up? I mean, is >there something about the 74als74 that is different from the 74f74? > >Bob Fahey The logical FUNCTION of the the 74als74 and 74f74 d-flip flop is the same, but the transition speed and drive characteristics are different. The propagation delay from low to high for the f seris is somewhere between 3 and 10 nanoseconds. The f series parts are built for speed. (f- FAST) The Wait a second, I found some better numbers. The average propogation delay of 74ALS parts is cited as ~4.5 nseconds and the average propogation delay of 74F parts is cited as <3 nseconds in the SAMSUNG High Performance CMOS Logic Data Book. The speed is probably close enough to work. But wait, the simple charts in books like this don't consider the load that the gate has to drive. I suspect that the ALS part you put into the circuit is interfaced to F parts and is having a hard time driving the other gates which changes the propogation delay times and causes your problems. DON'T mix logic families unless you know whhat you are doing! _____________________________________________________________________________ Jerry J. Trantow | When you call a thing mysterious, 1560 A. East Irving Place | all it means is that you don't understand it. Milwaukee, Wi 53202-1460 | Lord Kelvin (414) 289-0503 | _____________________________________________________________________________