Do 4000-series CMOS inputs have any problems coping with the much faster edge rates of 74HC outputs? Everything would be running on 5V, of course. It's all "just" digital logic, but sometimes the transistors inside forget that... And yes, I know I could use a processor and write code instead of using a few packages of random logic. That's not my desire.
Mixing 4000-series CMOS and 74HC in a 5V system - any issues?
Started by ●April 13, 2017
Reply by ●April 13, 20172017-04-13
On Thu, 13 Apr 2017 20:26:38 -0400, Steve Goldstein <sgoldHAM@alum.mit.edu> wrote:>Do 4000-series CMOS inputs have any problems coping with the much >faster edge rates of 74HC outputs? Everything would be running on 5V, >of course. It's all "just" digital logic, but sometimes the >transistors inside forget that... > >And yes, I know I could use a processor and write code instead of >using a few packages of random logic. That's not my desire.No problem in the fast-to-slow direction. In the 4000-to-HC direction, you might check that the HC input slew rate isn't too slow; schmitts might be prudent. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●April 13, 20172017-04-13
On 4/13/2017 8:26 PM, Steve Goldstein wrote:> Do 4000-series CMOS inputs have any problems coping with the much > faster edge rates of 74HC outputs? Everything would be running on 5V, > of course. It's all "just" digital logic, but sometimes the > transistors inside forget that... > > And yes, I know I could use a processor and write code instead of > using a few packages of random logic. That's not my desire.Using hard logic is your desire? Your passion is to wire up a circuit and to strap down the chips? Very kinky! One day you may find a need for the softer side of things. When that happens, don't be afraid to step out of your cable closet and explore a new path. -- Rick C
Reply by ●April 13, 20172017-04-13
On Thu, 13 Apr 2017 17:32:37 -0700, John Larkin <jjlarkin@highland_snip_technology.com> wrote:>On Thu, 13 Apr 2017 20:26:38 -0400, Steve Goldstein ><sgoldHAM@alum.mit.edu> wrote: > >>Do 4000-series CMOS inputs have any problems coping with the much >>faster edge rates of 74HC outputs? Everything would be running on 5V, >>of course. It's all "just" digital logic, but sometimes the >>transistors inside forget that... >> >>And yes, I know I could use a processor and write code instead of >>using a few packages of random logic. That's not my desire. > >No problem in the fast-to-slow direction. In the 4000-to-HC direction, >you might check that the HC input slew rate isn't too slow; schmitts >might be prudent.Good point. Luckily this little project looks like it's all 74HC driving 4000 or 4000 driving 4000, at least at this stage of the design. But I'll keep that in mind in case I have to drive 74HC from 4000. Schmitts are cheap enough and I normally use them anywhere I need an inverter, just because.
Reply by ●April 14, 20172017-04-14
On Friday, 14 April 2017 01:53:09 UTC+1, Steve Goldstein wrote:> On Thu, 13 Apr 2017 17:32:37 -0700, John Larkin > <jjlarkin@highland_snip_technology.com> wrote: > >On Thu, 13 Apr 2017 20:26:38 -0400, Steve Goldstein > ><sgoldHAM@alum.mit.edu> wrote: > > > >>Do 4000-series CMOS inputs have any problems coping with the much > >>faster edge rates of 74HC outputs? Everything would be running on 5V, > >>of course. It's all "just" digital logic, but sometimes the > >>transistors inside forget that... > >> > >>And yes, I know I could use a processor and write code instead of > >>using a few packages of random logic. That's not my desire. > > > >No problem in the fast-to-slow direction. In the 4000-to-HC direction, > >you might check that the HC input slew rate isn't too slow; schmitts > >might be prudent. > > Good point. Luckily this little project looks like it's all 74HC > driving 4000 or 4000 driving 4000, at least at this stage of the > design. But I'll keep that in mind in case I have to drive 74HC from > 4000. Schmitts are cheap enough and I normally use them anywhere I > need an inverter, just because.Whether you need to schmitt things depends on the circuit. Each logic gate has a fair bit of gain so speeds up rise & fall times. Whether that's good enough just depends. A bank vault system that calls the flying squad if an edge goes high for 0.5uS would not want the results of a sluggish input to a gate causing a brief output error. But if your circuit controls bulb brightness, who cares if it gets 0.5uS of full brightness. If a 4000 output goes through 2 spare HC inverters before it hits the nand gate it'll be fast by then. NT
Reply by ●April 14, 20172017-04-14
On Thu, 13 Apr 2017 20:26:38 -0400, Steve Goldstein wrote:> Do 4000-series CMOS inputs have any problems coping with the much faster > edge rates of 74HC outputs? Everything would be running on 5V, > of course. It's all "just" digital logic, but sometimes the transistors > inside forget that... > > And yes, I know I could use a processor and write code instead of using > a few packages of random logic. That's not my desire.AFAIK pretty much anything that's currently available in 4000-series is available in 74HC4000-series. What's not there? -- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com
Reply by ●April 14, 20172017-04-14
On 4/13/2017 11:20 PM, tabbypurr@gmail.com wrote:> On Friday, 14 April 2017 01:53:09 UTC+1, Steve Goldstein wrote: >> On Thu, 13 Apr 2017 17:32:37 -0700, John Larkin >> <jjlarkin@highland_snip_technology.com> wrote: >>> On Thu, 13 Apr 2017 20:26:38 -0400, Steve Goldstein >>> <sgoldHAM@alum.mit.edu> wrote: >>> >>>> Do 4000-series CMOS inputs have any problems coping with the much >>>> faster edge rates of 74HC outputs? Everything would be running on 5V, >>>> of course. It's all "just" digital logic, but sometimes the >>>> transistors inside forget that... >>>> >>>> And yes, I know I could use a processor and write code instead of >>>> using a few packages of random logic. That's not my desire. >>> >>> No problem in the fast-to-slow direction. In the 4000-to-HC direction, >>> you might check that the HC input slew rate isn't too slow; schmitts >>> might be prudent. >> >> Good point. Luckily this little project looks like it's all 74HC >> driving 4000 or 4000 driving 4000, at least at this stage of the >> design. But I'll keep that in mind in case I have to drive 74HC from >> 4000. Schmitts are cheap enough and I normally use them anywhere I >> need an inverter, just because. > > Whether you need to schmitt things depends on the circuit. Each logic gate has a fair bit of gain so speeds up rise & fall times. Whether that's good enough just depends. A bank vault system that calls the flying squad if an edge goes high for 0.5uS would not want the results of a sluggish input to a gate causing a brief output error. But if your circuit controls bulb brightness, who cares if it gets 0.5uS of full brightness. If a 4000 output goes through 2 spare HC inverters before it hits the nand gate it'll be fast by then.I'm not sure why people are making such a big deal of this issue. If logic is designed properly the speed of an edge has no impact on the logical result other than on clock lines. For purely combinatorial logic changing edges can always generate race conditions through multiple paths, so you should expect anomalous results until the input change has settled down. Designing logic to be race free is a special task which takes a lot more design effort than standard logic design, so it is seldom used. To the OP, are you counting on combinatorial logic outputs to be race free, that is for one input edge to result in a single output edge? If not, don't sweat the slower inputs driving faster logic. -- Rick C
Reply by ●April 14, 20172017-04-14
Tim Wescott wrote:> On Thu, 13 Apr 2017 20:26:38 -0400, Steve Goldstein wrote: > >> Do 4000-series CMOS inputs have any problems coping with the much faster >> edge rates of 74HC outputs? Everything would be running on 5V, >> of course. It's all "just" digital logic, but sometimes the transistors >> inside forget that... >> >> And yes, I know I could use a processor and write code instead of using >> a few packages of random logic. That's not my desire. > > AFAIK pretty much anything that's currently available in 4000-series is > available in 74HC4000-series. What's not there? >You cannot SEE the MOSS...
Reply by ●April 14, 20172017-04-14
On Thu, 13 Apr 2017 22:45:06 -0500, Tim Wescott <tim@seemywebsite.com> wrote:>On Thu, 13 Apr 2017 20:26:38 -0400, Steve Goldstein wrote: > >> Do 4000-series CMOS inputs have any problems coping with the much faster >> edge rates of 74HC outputs? Everything would be running on 5V, >> of course. It's all "just" digital logic, but sometimes the transistors >> inside forget that... >> >> And yes, I know I could use a processor and write code instead of using >> a few packages of random logic. That's not my desire. > >AFAIK pretty much anything that's currently available in 4000-series is >available in 74HC4000-series. What's not there?CD4026 and CD4033 don't seem to have direct 74HC equivalents, and CD4033 is perfect for what I want to do. I haven't searched for a similar 74HC function since CD4033 is still readily available. Another poster asked about race conditions. That's not an issue, it's a synchronous design, and slow.
Reply by ●April 14, 20172017-04-14
On Thu, 13 Apr 2017 22:45:06 -0500, the renowned Tim Wescott <tim@seemywebsite.com> wrote:>On Thu, 13 Apr 2017 20:26:38 -0400, Steve Goldstein wrote: > >> Do 4000-series CMOS inputs have any problems coping with the much faster >> edge rates of 74HC outputs? Everything would be running on 5V, >> of course. It's all "just" digital logic, but sometimes the transistors >> inside forget that... >> >> And yes, I know I could use a processor and write code instead of using >> a few packages of random logic. That's not my desire. > >AFAIK pretty much anything that's currently available in 4000-series is >available in 74HC4000-series. What's not there?There are a whole lot that are not, but they're not not usually what we would strongly consider for a production situation because they're probably on the way out. Eg. 4054,55, 56, 4062, 4068, 4089, etc. etc. --sp -- Best regards, Spehro Pefhany
