Experiments with the 74LS138 - Making an 8 Bit pipelined CPU - Part 5

[@petesapwell]

Gotta love the humble LS138, and hats of to the people that thought long and hard about design for this wonderful series of IC’s

[@weirdboyjim]

Indeed! I end up using them all over the place.

[@KingHuffy1]

Thank you for this series. I’m already addicted and dreaming of starting my own breadboard CPU. I looked ahead at some of your newest videos and WOW you have done amazing things. Keep up the awesome work.

[@weirdboyjim]

You are welcome! It's always good to hear people are finding it interesting.

[@edgeeffect]

I love the 74138... it makes a really good iO expander for microcontrollers too.

[@weirdboyjim]

Yes! It seemed weird they way they have different enable inputs until you start trying to use them in circuits.

[@edgeeffect]

@@weirdboyjim I've never thought of your cascading using the enables.... that's really economical.... I think you've just saved me loads of complexity in my synthesizer control panel I'm working on at the moment..... really good idea.... I'm hoping I can use it on my analogue demultiplexer too.

[@UpcycleElectronics]

I'm 6 videos into your playlist and only had to order some 74x193's so far. It's very nice of you to stay within my current inventory like this :-) Any suggestions are obviously too late for relevance here, but I picked up some 74x4514's to play around with. They are not as common as the x138's but it's a 4 to 16 line mux decoder with latched inputs. It might prove useful. I really like how you're delving into the details of propagation delay at higher frequency. I look forward to your 40 MHz 555 circuit thingy as well ;-) My inexperience was leading me to believe a pipeline design would involve multiple clock signals divided down but it looks like most people use the inverted clock and rising/falling edges. Very interesting. Thanks for sharing. -Jake

[@weirdboyjim]

That's weird, we had a few comments on this that have disappeared?

[@jerril42]

I'm absolutely loving this series. I've got a lot to catch up. Thank you James.

[@weirdboyjim]

I'd be building the CPU regardless but comments like yours make the extra effort to record and edit the videos worth it. Thank you! I hope you find the rest interesting.

[@electron7373]

Great series really enjoying the process of building it up from logic chips.

[@weirdboyjim]

Glad yopu are finding it interesting!

[@tonyfremont]

Amazing. I've been tinkering with micro stuff since the mid 70s when I built a Netronics COSMAC Elf II. I consider myself to have a pretty good handle on things going on inside computers, but have never delved this derp into the internal workings of a CPU. Really looking forward to the pipeline and CPU microcode. I'm watching them in order after accidentally stumbling onto your stuff and being stunned by a non CPU processor appearing to be moving at a pretty good clip. You and Ben Eater are doing incredible things. Have you considered recondensing your work into an FPGA?

[@weirdboyjim]

Glad you are finding it interesting! There are some people on discord trying to put together an fpga version but it's not one of my goals for this version. When I'm done I want to start a new build to experiment with some more advanced features and fpga would be on the cards for that!

[@Otakutaru]

FPGA FPGA FPGA!!

[@migry]

When I was first watching this I was shouting out (well in my mind anyway!) 74LS154. Many years ago my local electronics club used 2 (3,4?) of these devices to make a LED roulette wheel. It proved quite a popular money maker at the school fete!

[@weirdboyjim]

This is one of my flaws, I found the 138 first and since I can solve the 16 line problem easily with 2 of those I didn't look further.

[@jarisipilainen3875]

26:34 when bit 5 is active first led counting when it off last leds counting. kinda reverse order with 8 leds

[@weirdboyjim]

There isn’t any counting near the time index you linked. Can you explain more about the problem you think you have seen?

[@weirdboyjim]

Your reply to my comment is "held for review", you accidentally made a link to a spam website. Add a space after the full stop.

[@jarisipilainen3875]

@@weirdboyjim its fine i trust you it works xD cool project. many man hour :)

[@DavidLindes]

24:02 - In late 2018, had you been watching This Old Tony videos? ;) (Not that that's the only possible source of inspiration here, just... somehow feels like that vibe to me?)

[@weirdboyjim]

I never once chopped a breadboard into with my hand! Couldn't get the knack of it.

[@DavidLindes]

@@weirdboyjim haha, ah well. You did a nice job on the insta-population of one, though. :) Also: is it just me, or would breadboards with 3 or even 4 horizontal sections be nice, for stuff with LED indicators in particular? I guess there'd need to be a longer jumper for power to ICs, but that's one line... I'd love to have LED's on the same columns as the pins sometimes... Ah well.

[@weirdboyjim]

@@DavidLindes I keep meaning to make some mini pcb's with a 8 led's and the resistors on. Essentially a SIL led array.

[@jlawrence71]

Probably more expeensive, but the 74HC154EN is a 4 to 16 line in one chip. Thoughts?

[@weirdboyjim]

It's a nice chip, to be honest I was struggling to get hold of any at the start of the build but the combination of enable lines on the 138 is useful.

[@jlawrence71]

@@weirdboyjim yes, i saw that hat trick this afternoon. i do like you showing these tricks. i have watched ben eater then james woods. now i am watching yours. a nice progression on concepts. i am breadboarding now too. however my final will be based on the rc2014 bus. i have extended it to 63 lines as i am going to use breadboards as my backplane. i had already figured out the need to multiplex by control bus, so i immediately followed your same approach. appreciate the reply. you are providing valuable insight and it good to see these things taken one step further. cheers

[@twobob]

some nice tricks. good thoughts

[@weirdboyjim]

I've been thinking about doing a series of feature explorations for logic chips as a separate series.

[@twobob]

@@weirdboyjim interesting. a "Logical Lego" approach with a Binary 101 primer should cover the 74 series (with the odd nod to other bases, *cough* 74x24x). I would certainly welcome anything of that nature, for any series. Ideally with large friendly letters saying DON'T PANIC

[@theleviathan3902]

Did you try to do a snap transition, but then mess it up and just cut in footage of the datasheet?

[@weirdboyjim]

You would need to provide a time index for me to comment on a specific case but this is a very old video, my production standards have improved since then.

[@theleviathan3902]

​@@weirdboyjimit's at timestamp 12:29

[@weirdboyjim]

Yep, I was being clever and messed up.

[@weirdboyjim]

But that wouldn't have been me "cutting in" data sheet. My pc captures the audio, so I probably left a gap on the editor timeline so whatever I'm capturing on the pc then shows through. In this case the data sheet.

[@robertgrayson2177]

You should have disconnected the bottom of the top solderless breadboard first ?.

[@weirdboyjim]

It's a bad habit to work on circuits with the power on, but I still do it.

[@szymoniak75]

Did you come up with this chaining at the end of the video yourself?

[@weirdboyjim]

I didn't get it from anywhere but I would say "I came up with it myself", I regard that as an obvious use case. I do exactly this in several places in the cpu build later on.

[@szymoniak75]

@@weirdboyjim that's impressive, thanks for answering

[@Every_thing717]

Anyone know the serial number of the registers?

[@weirdboyjim]

Not sure what you are asking there?

[@Every_thing717]

@@weirdboyjim about the number which is found on any IC for example the IC timer has a number (555) on it , therefore I want the number that found on the register so I can buy them

[@weirdboyjim]

@@Every_thing717 ok, the term to use there is “part number”.

[@weirdboyjim]

@@Every_thing717 all the latch chips I use for the registers in this build (that are not counters) are 74xx574, I’m careful to use common parts as much as possible.