> H.C. Cron wrote > > HCC> As I see the problem, you can not solve it in your way. What you are > HCC> doing can be compared with an inverter the output of which is fed > HCC> back to the input. Look at D. D is a constituent of the Reset signal > HCC> and just this Reset signal has to reset D by voltage levels (no edge > HCC> triggering). This will result in the voltage of D to hang somewhere > HCC> between the logic levels. > > are you really sure about that? > You have a natural delay in Martin's circuit, since the D output will > not instantaneously flip back to the high-state once the reset signal > disappears, but only after 8 external clock signals.
How can you reset D with the D signal itself? These are level signals, no edge trigering. > Getting the reset signal from a counter is not very uncommon in the > way Martin describes it, I have myself used such circuits and if I'm > not completely mistaken it even is part of either the datasheet or the > application notes. When you do it via the clock input it will work. In the case of level switching it is not inpossible that some circuit will work, depending on the delays of the different flip=flops, but there is no garantee and in factyou have critical races in that case. You have to treat this problem as asynchronous logic. > > However, unlike Martin's idea with a switch, I also would prefer a > solid logic solution, which would select either the B or C output to > contribute to the reset signal. Just doing a quick estimation with > normal logic gates you would need at least four gates with different > (NOR, NAND) functions. Therefore I would suggest a 2-1 data > multiplexer such as > 74xx157 (153) > 74xx257 (253) > These chips offer 4(2) multiplexers which select 1 out of 2(4) input > lines to be repeated at the output pin. These are not analog switches > and they only work in one direction. > > Uwe. > > > HCC> The 7493 does not give you much room for playing the game. An option > HCC> would be to use an extra one-shot which is triggered by the condition > HCC> you need. The output of the oneshot will deliver a pulse which can be > HCC> designed long enough to reset the counter. > HCC> A tric, which I don't like, is to charge a capacitor on your > HCC> condition via a diode and use the capacitors voltage as the input of > HCC> a CMOS gate the output of which delivers the reset signal. This way > HCC> of designing however frustrates the design principles of the logic > HCC> family with the consequence that you can not rely on the build in > HCC> noise levels. > > HCC> Regards, Harry > > >> What I have is a 7493 configured to reset when the count > >> reaches 12 or when outputs D and C go high. At other times, I want > >> the 7493 to reset when the count reaches 10 or when outputs D and B go > >> high. For that, I can directly connect D to one reset and the other > >> reset could go to a double-pole, single-throw switch with the other > >> two terminals of the switch going to B and C. I wanted the switch, > >> however, to be made of digital logic so I only need ground a line to > >> change from B--Reset to C--reset. > >> -- > >> Author: Martin McCormick > >> INET: [EMAIL PROTECTED] > >> > >> Fat City Network Services -- 858-538-5051 http://www.fatcity.com > >> San Diego, California -- Mailing list and web hosting services > >> --------------------------------------------------------------------- > >> To REMOVE yourself from this mailing list, send an E-Mail message > >> to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > >> the message BODY, include a line containing: UNSUB CHIPDIR-L > >> (or the name of mailing list you want to be removed from). You may > >> also send the HELP command for other information (like subscribing). > >> > > -- > Author: Uwe Zimmermann > INET: [EMAIL PROTECTED] > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > San Diego, California -- Mailing list and web hosting services > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > the message BODY, include a line containing: UNSUB CHIPDIR-L > (or the name of mailing list you want to be removed from). You may > also send the HELP command for other information (like subscribing). > -- Author: H.C. Croon INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
