Discreet Logic oppinions - Page 1
Forum section: electronics / general

author: Supercharged
author_rank: Regular Contributor
timestamp: January 28, 2018, 09:10:17 pm
content: What are your opinions on using discreet logic in personal projects, where a microcontroller would do the trick.While i have no problem with using a micro, i feel its just not as much fun as using discreet logic ICs.

author: rstofer
author_rank: Super Contributor
timestamp: January 28, 2018, 09:47:12 pm
content: I really liked using discrete logic - back in the early '80s. It's 2018 and I doubt that I'm going to do that stuff any more.I have a cut-strip-wrap gun that makes wiring almost tolerable but my eyesight isn't what it was so there's a downside.Most of my projects involve building CPUs and peripherals and that takes an enormous number of ICs. I'm not even sure where to buy large prototype boards.So, since I really like hardware design, I'm messing around with CPLDs and FPGAs - mostly FPGAs, Writing a line of code to create a 32 bit register is a lot easier than actually wiring one and consuming 4 D-flop chips.Nevertheless, it is certainly possible to use discrete logic and there is something satisfying about getting a discrete system to work. I have done a full CPU design with discrete logic using blown-link PROMs for holding the microcode. I did that back in '74 and it was really interesting.Go for it! Post about your results!

author: Benta
author_rank: Super Contributor
timestamp: January 28, 2018, 10:18:50 pm
content: I love using discrete logic, mainly stone-age technology like metal-gate CMOS (4000 series).Why? Because most of my designs are mixed-signal using opamps mostly running at +/-12 V. Integrating analog and logic without level-shifting is just so much easier, power supply design also. And speed is not an issue, I'm below 5 MHz mostly.That being said, the 4000-series contains a lot of crap (asynchronous/ripple logic), but there are devices that are my absolute favourites:The gates, of course.4027 JK FFs (I also use 4013 D FFs, but only in exceptions)4526 synchronous counter.4046 PLL, after I've designed a loop controller (loop filter) thatreallyworks.And of course special functions when needed.It's fun!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2018, 10:47:45 pm
content: FYI, "discreet" = quiet, low-key; "discrete" = in individual units.I suppose CD4000 series logic is pretty discreet though (it's so slow, it's hard to get much EMI off it).As for use, if a logic function takes more than a few chips, it's better to use an integrated solution. If it's a more fixed-logic, real time application, an FPGA may be better, otherwise an MCU is probably the way to go.This does not include interface ICs, which are usually required anyway. Regardless which technology is used in the core, the circuit will still need, say, RS-422 transceivers to connect to such an interface properly. (If improper connection is acceptable, then there are all kinds of hacks possible. Like, I don't know, maybe there's an MCU or FPGA with LED driver pins -- just regular CMOS or open-drain pins, but with higher current capacity, which would make it also suitable for RS-422 or I2C or something like that. But this gets very device-specific, and you need to read a lot of datasheets to select a suitable part for this kind of treatment. It's not a first-try sort of thing.)Tim

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2018, 03:01:33 am
content: Discrete logic has a place in mixed-signal designs, any anyplace where interfacing to the real world is necessary, and where the ultimate in performance is required. Sometimes even logic built from discrete transistors is the best option like with fast level shifting.

author: Supercharged
author_rank: Regular Contributor
timestamp: January 29, 2018, 06:45:49 pm
content: Quote from: T3sl4co1l on January 28, 2018, 10:47:45 pmThank you. I thought the spelling looked wrong, but i couldn't figure out why

author: Supercharged
author_rank: Regular Contributor
timestamp: January 29, 2018, 06:48:25 pm
content: Quote from: Benta on January 28, 2018, 10:18:50 pmI hadn't even thought about that, but it definitely makes sense.

author: james_s
author_rank: Super Contributor
timestamp: January 29, 2018, 08:11:47 pm
content: Sometimes it's fun to build something with old techniques, even if a microcontroller can do it. One thing about discrete logic is you're not going to run into random bugs in your code because there's not any code. If you're building something simple, logic has fewer opportunities for bugs to creep in.

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2018, 08:22:05 pm
content: Quote from: james_s on January 29, 2018, 08:11:47 pmSometimes there is no choice. Low jitter requires differential signalling and controlling return currents to prevent threshold voltage shifts which is not something you can do with microcontrollers and programmable logic.In mixed signal designs, problems with this show up as poor power supply rejection and excessive noise.

author: Hypernova
author_rank: Unknown
timestamp: February 02, 2018, 05:44:28 am
content: One risk with replacing discrete logic with MCU/CPLD is that a few years down the line you might not be able to find the firmware source code due to what ever reasons. Whereas with logic chips you can be confident that the same design can be replicated from a sheet of circuit diagram.

author: james_s
author_rank: Super Contributor
timestamp: February 02, 2018, 05:46:45 am
content: That is certainly a valid concern, on a few occasions I've lost source code for various projects I've created and later wished I'd been more careful. I've also worked on equipment that had a failed microcontroller, I knew exactly what was wrong with the hardware but was unable to repair it due to the fact that the firmware was not available.