Dipping my toe in digital electronics

[jdutky]

This is the kind of perfboard I'm talking about. Ideal for this type of project. Power lines are already laid out. It's very suitable for a digital project. Space for 12 pcs. 14...16 pin DIP ICs plus interconnects and decoupling:

https://www.conrad.de/de/p/tru-components-su527670-ic-platine-hartpapier-l-x-b-160-mm-x-100-mm-35-m-rastermass-2-54-mm-inhalt-1-st-1566656.html

Or this:

https://www.conrad.com/p/rademacher-vk-c-932-hp-laboratory-card-designed-for-twelve-16-pin-ics-with-connection-options-for-the-following-plugs-527370

I'm putting a new clock circuit together on a small prototype board I happen to have a stack of. It looks like I can fit everything on the half board, and get some nice short traces for the signals. I'm going to put the capacitors on the bottom of the board bridging the power and ground pins, in order to minimize the ground loops. I'm thinking that I will connect the ground pins in a maximally connected lattice in order to try to approximate a real ground plane (I don't have any copper clad on hand, but I''ve got some on order, which I will fall back to if this doesn't work as desired).

(the chips in the picture are just resistor arrays that I am using to get the positions and spacing right. I will replace them with the actual parts when I start soldering)

[jdutky]

Well, I set the chip up like it was a 6809 (NMI, IRQ, FIRQ, HALT, RESET, and DMA/BREQ all tied to Vcc, and a crystal across pins 38 and 39 with caps to ground) and got no clock signal out of the E and Q pins, which is a relief because I actually have a couple of (presumably genuine) 68B09s on order (in case I couldn't get the external oscillator circuit to work).

I did find something shocking, however: when I went to select the capacitors for the crystal circuit the datasheet specified 20 pF caps for a 6 MHz crystal. I have an unopened box of shiny new ceramic caps that I bought off Amazon (LORESO brand, 600 pieces! from 10 pF to 4.7uF in 24 steps! Only $19, free shipping! What a BARGAIN!) and the caps marked 20 pF measure as 35-36 pF (!!) and the ones marked 10 pF measure 38 pF (!!!). These are ceramic caps, not your sloppy electrolytic sort; I expect them to be within at least 50% of their marked values (maybe I'm naive; maybe the world no longer cares about such pedestrian details?).

So the CPU is genuine and accurately marked, but the capacitors are garbage. Fascinating.

[james_s]

A fast rise time like that on a breadboard is almost guaranteed to cause some ringing. It shouldn't really matter though, and if you want to clean it up I will second the advise to add a series resistor. That will reduce the slew rate somewhat and should help damp any ringing.

Ringing can matter if the signal integrity is sufficiently poor that the inputs incorrectly interpret the analogue voltage.

Inserting a resistor has to be near the source, and if it is sufficient to reduce ringing then it will also reduce the transition amplitude. That can be a standard problem for inputs that are not at the end of the wire.

A fast transition can also cause ground bounce, which is a classic cause of intermittent pattern-sensitive failure.

Well, it's on a breadboard, it's probably not gonna be stellar. It can be made to work though, countless people have built 8 bit computers on breadboards. If we were discussing a production design my advice would be different but in this case I'd try it and see if it works, and play around with techniques to try to reduce the ringing.

[Electro Fan]

Well, I set the chip up like it was a 6809 (NMI, IRQ, FIRQ, HALT, RESET, and DMA/BREQ all tied to Vcc, and a crystal across pins 38 and 39 with caps to ground) and got no clock signal out of the E and Q pins, which is a relief because I actually have a couple of (presumably genuine) 68B09s on order (in case I couldn't get the external oscillator circuit to work).

I did find something shocking, however: when I went to select the capacitors for the crystal circuit the datasheet specified 20 pF caps for a 6 MHz crystal. I have an unopened box of shiny new ceramic caps that I bought off Amazon (LORESO brand, 600 pieces! from 10 pF to 4.7uF in 24 steps! Only $19, free shipping! What a BARGAIN!) and the caps marked 20 pF measure as 35-36 pF (!!) and the ones marked 10 pF measure 38 pF (!!!). These are ceramic caps, not your sloppy electrolytic sort; I expect them to be within at least 50% of their marked values (maybe I'm naive; maybe the world no longer cares about such pedestrian details?).

So the CPU is genuine and accurately marked, but the capacitors are garbage. Fascinating.

Watch at about 16 min (admittedly electrolytic rather than ceramic as you point out but maybe tolerances aren’t what they used to be), also maybe scan the comments on capacitor tolerances

Looks like the moral of the story is when in doubt, measure - so it’s a good thing we like to measure

[tggzzz]

A fast rise time like that on a breadboard is almost guaranteed to cause some ringing. It shouldn't really matter though, and if you want to clean it up I will second the advise to add a series resistor. That will reduce the slew rate somewhat and should help damp any ringing.

Ringing can matter if the signal integrity is sufficiently poor that the inputs incorrectly interpret the analogue voltage.

Inserting a resistor has to be near the source, and if it is sufficient to reduce ringing then it will also reduce the transition amplitude. That can be a standard problem for inputs that are not at the end of the wire.

A fast transition can also cause ground bounce, which is a classic cause of intermittent pattern-sensitive failure.

Well, it's on a breadboard, it's probably not gonna be stellar. It can be made to work though, countless people have built 8 bit computers on breadboards. If we were discussing a production design my advice would be different but in this case I'd try it and see if it works, and play around with techniques to try to reduce the ringing.

"not be stellar" is a euphemism for "crap".

"can be made to work" implies it is error-prone and requires experience. The OP is a beginner.

"see if it works" is a euphemism for "haven't spotted the failure yet".

[rstofer]

I think it has been covered but the rule is:  One 0.1ufd ceramic capacitor per IC  mounted as close as possible to Vcc and Gnd at the IC.  It used to be possible to buy wire-wrap sockets with the capacitor already fitted to the socket.  No chance of violating the rule.

It isn't possible to get up close and personal with the capacitor when  using a breadboard.  Get it as close as you can.  Inductance in the leads won't help!  Keep the path short.

I always add 100 ufd at the far end of a board, away from the power entrance and possibly another at the entrance.

[james_s]

A fast rise time like that on a breadboard is almost guaranteed to cause some ringing. It shouldn't really matter though, and if you want to clean it up I will second the advise to add a series resistor. That will reduce the slew rate somewhat and should help damp any ringing.

Ringing can matter if the signal integrity is sufficiently poor that the inputs incorrectly interpret the analogue voltage.

Inserting a resistor has to be near the source, and if it is sufficient to reduce ringing then it will also reduce the transition amplitude. That can be a standard problem for inputs that are not at the end of the wire.

A fast transition can also cause ground bounce, which is a classic cause of intermittent pattern-sensitive failure.

Well, it's on a breadboard, it's probably not gonna be stellar. It can be made to work though, countless people have built 8 bit computers on breadboards. If we were discussing a production design my advice would be different but in this case I'd try it and see if it works, and play around with techniques to try to reduce the ringing.

"not be stellar" is a euphemism for "crap".

"can be made to work" implies it is error-prone and requires experience. The OP is a beginner.

"see if it works" is a euphemism for "haven't spotted the failure yet".

It's a solderless breadboard, it's not going to be great, so what? I think I've had this same pedantic argument with you in the past, you think solderless breadboards are crap, I get the point, so what? That doesn't stop many thousands of people from building stuff that works. An excellent way of getting experience is to experiment, make errors and learn from them. This is not some kind of safety critical system where a crash will kill somebody or cause serious hardship. It's an experimental 8 bit computer put together as a learning exercise, no sane person is going to expect absolute perfection.

[tggzzz]

A fast rise time like that on a breadboard is almost guaranteed to cause some ringing. It shouldn't really matter though, and if you want to clean it up I will second the advise to add a series resistor. That will reduce the slew rate somewhat and should help damp any ringing.

Ringing can matter if the signal integrity is sufficiently poor that the inputs incorrectly interpret the analogue voltage.

Inserting a resistor has to be near the source, and if it is sufficient to reduce ringing then it will also reduce the transition amplitude. That can be a standard problem for inputs that are not at the end of the wire.

A fast transition can also cause ground bounce, which is a classic cause of intermittent pattern-sensitive failure.

Well, it's on a breadboard, it's probably not gonna be stellar. It can be made to work though, countless people have built 8 bit computers on breadboards. If we were discussing a production design my advice would be different but in this case I'd try it and see if it works, and play around with techniques to try to reduce the ringing.

"not be stellar" is a euphemism for "crap".

"can be made to work" implies it is error-prone and requires experience. The OP is a beginner.

"see if it works" is a euphemism for "haven't spotted the failure yet".

It's a solderless breadboard, it's not going to be great, so what? I think I've had this same pedantic argument with you in the past, you think solderless breadboards are crap, I get the point, so what? That doesn't stop many thousands of people from building stuff that works. An excellent way of getting experience is to experiment, make errors and learn from them. This is not some kind of safety critical system where a crash will kill somebody or cause serious hardship. It's an experimental 8 bit computer put together as a learning exercise, no sane person is going to expect absolute perfection.

You would do well to note solderless breadboards enable:

• many thousands of people building stuff that fails
• beginners being put off electronics due to frustration with flaky circuits that don't work due to inevitable stray LCR

Hence the points aren't pedantic, for the simple reason that I have seen those consequences too often.

[rstofer]

There are solder type prototype boards with a pattern that matches the solderless variety.  As each module is completed and tested, it can be ported to a solder type board.

http://www.chipquik.com/datasheets/SBB830.pdf

I think I prefer wire-wrap for this kind of thing.  I have a cut-strip-wrap gun that makes fast work of the process.  There's no reason not to use wire-wrap sockets with these boards.  Solder in the discrete components and wire-wrap all of the signals.

[jdutky]

So I've made some progress on the clock generator circuit, first we have the circuit that I'm actually trying to build (from the MC68B09E datasheet), and the breadboard I've put together to implement it (the chips are, clockwise from upper left: 74LS74 dual D-type flip-flops, 74LS04 hex inverters, 74LS76 dual JK flip-flops, and a 74S51 standing in for the 74LS10 3-input NAND while I'm waiting on a purchase to arrive. On the right hand side you can see my improved probe discipline to sample the output Q and E signals. So long as I don't make any sudden moves, or breath, the probes will sit nicely in the breadboard sockets and let me take pictures. The tactile switch in the upper right allows me to trigger the MRDY input). Next we have several pictures of the oscilloscope display showing 1) the clock sampled at the oscillator's output, 2) the clock sampled where at the 74LS76 clock inputs, and 3) the Q and E output signals.

Clearly something is terribly wrong, I suspect that it is a problem with how I have wired the circuit together, but I'm not entirely sure of that. The factors that suggest that I have mis-wired the circuit are that the Q high time is much longer than the low time, and the fact that I repeatedly mis-wired the connections around JK flip-flops several times.

The scope settings are 200ns/div horizontal and 5V/div vertical. The oscillator is 3.088 MHz, though I'm planning to use an 8 MHz oscillator in the final version (I'm waiting on a purchase to arrive).

UPDATE: looking at it now, and at the two samples of the clock signal, I'll bet I could clean up the clock signals just by taking them all directly from the output of the oscillator, rather than doing a running line like that, but I don't think that the clock signal is the problem here because while it gets a little worse, it doesn't get bad enough to be confusing the logic gates. I think I've clearly got something wired wrong and am inducing an unstable state in the JK flip-flops. I dimly recall (from college courses) that this was a problem even with ideal logic circuits run in simple simulations.

UPDATE the 2nd: upon further reflection, it occurs to me that I have made this more complicated than it needs to be: I only need the "optional" portion of the circuit if I am going to support other bus masters in the system. I can omit the optional portion, lose two ICs (the two on the left) and get a circuit that is probably simple enough to debug at my current skill leve 

[tggzzz]

There are solder type prototype boards with a pattern that matches the solderless variety.  As each module is completed and tested, it can be ported to a solder type board.

http://www.chipquik.com/datasheets/SBB830.pdf

Solves the intermittent R problem, does little for the parasitic L&C problem.

Easier and cheaper to simply use rats nest (if you like the "solderless breadboard look"!) or manhattan techiques, e.g.



or proprietary "BusBoard" boards with a full backplane but no through-hole plating, so you can select which pads are/aren't ground, e.g.

I think I prefer wire-wrap for this kind of thing.  I have a cut-strip-wrap gun that makes fast work of the process.  There's no reason not to use wire-wrap sockets with these boards.  Solder in the discrete components and wire-wrap all of the signals.

Been there, done that, still got lots of wire and a manual wrapper, and a few (expensive!) sockets

The decoupling caps are a pain (unless the flat Rogers caps or built into the board) and you have to be disciplined enough to create a decent ground grid.

[Doctorandus_P]

I prefer to use "enameled" wire for prototyping, very much like the board shown in:
https://en.wikipedia.org/wiki/Wiring_pencil

And I combine that with (high quality) sockets for all DIP IC's, because the chance to damage a chip is relatively big with prototyping and it's a real *&^%$#@! to loosen all the wires and put them back if a chip has to be replaced.

If you insist on using a breadboard, then at least devise some way of taking the crystal off the board.
I've had boards lock up because a digital pin next to the crystal was switching, and the capacitive coupling through the breadboard was enough to upset the oscillator to lock up the CPU. Using such a square canned crystal oscillator is a much better way.

[joeqsmith]

...

 clockwise from upper left: 74LS74 dual D-type flip-flops, 74LS04 hex inverters, 74LS76 dual JK flip-flops, and a 74S51 standing in for the 74LS10 3-input NAND
...

Clearly something is terribly wrong, I suspect that it is a problem with how I have wired the circuit together, but I'm not entirely sure of that. The factors that suggest that I have mis-wired the circuit are that the Q high time is much longer than the low time, and the fact that I repeatedly mis-wired the connections around JK flip-flops several times.

Look at the datasheet for the 7451 and then look at how you have it wired.  I did not check anything else.

Everything will need to be correct for it to work.  The white protoboard should be fine for what you are doing. 

[Electro Fan]

So I've made some progress on the clock generator circuit, first we have the circuit that I'm actually trying to build (from the MC68B09E datasheet), and the breadboard I've put together to implement it (the chips are, clockwise from upper left: 74LS74 dual D-type flip-flops, 74LS04 hex inverters, 74LS76 dual JK flip-flops, and a 74S51 standing in for the 74LS10 3-input NAND while I'm waiting on a purchase to arrive. On the right hand side you can see my improved probe discipline to sample the output Q and E signals. So long as I don't make any sudden moves, or breath, the probes will sit nicely in the breadboard sockets and let me take pictures. The tactile switch in the upper right allows me to trigger the MRDY input). Next we have several pictures of the oscilloscope display showing 1) the clock sampled at the oscillator's output, 2) the clock sampled where at the 74LS76 clock inputs, and 3) the Q and E output signals.

Clearly something is terribly wrong, I suspect that it is a problem with how I have wired the circuit together, but I'm not entirely sure of that. The factors that suggest that I have mis-wired the circuit are that the Q high time is much longer than the low time, and the fact that I repeatedly mis-wired the connections around JK flip-flops several times.

The scope settings are 200ns/div horizontal and 5V/div vertical. The oscillator is 3.088 MHz, though I'm planning to use an 8 MHz oscillator in the final version (I'm waiting on a purchase to arrive).

UPDATE: looking at it now, and at the two samples of the clock signal, I'll bet I could clean up the clock signals just by taking them all directly from the output of the oscillator, rather than doing a running line like that, but I don't think that the clock signal is the problem here because while it gets a little worse, it doesn't get bad enough to be confusing the logic gates. I think I've clearly got something wired wrong and am inducing an unstable state in the JK flip-flops. I dimly recall (from college courses) that this was a problem even with ideal logic circuits run in simple simulations.

This clock generator circuit is very cool and inspirational - has me motivated to try something similar.  Between your good progress and the split differences comments on the tradeoffs in construction techniques in this thread I’m learning some good stuff.  Based on how far and fast you have moved out on this project I’m betting dollars to donuts you get it running. Thanks for sharing your cool project and for spawning the very educational discussion in this thread.

fwiw, I’m excited about this project because it has just right balance between challenging complexity and for me, barely but possibly understandable insight to what I’m pretty sure is foundational logic and related circuitry for 8 bit computer building, along with other basic analog and digital circuit theory and practice learning opportunities in general, and/also if I can get though all that I’m eager to try moving/porting in steps from breadboarding to manhattan and to do some wire wrapping.  I find manhattan to be fun and when done well kind of artistically/aesthetically beautiful - it provides a very pretty view of form and function, and for some reason I find wire wrapping almost therapeutic.  So this project looks like a an excellent mountain to try climbing and exploring.  It has a bunch of stuff I’d like to learn and this thread is clearing just enough paths that I’m inclined to give it a try.  And if this gets anywhere the whole Ben Eater-type 8 bit computer building project might be feasible.  This thread is a gateway to a lot of possibilities.   

[jdutky]

Oh! I was looking at the arrangement for the 74LS51, not the 74S51! That is certainly a rookie mistake.

But, I went ahead and removed the 74LS74 and 74S51 from the circuit (they are marked as "optional" in the datasheet; you only need them if you are going to stretch cycles for slow memory, or maybe for other bus masters?) and the smaller circuit is much more within my current grasp. This looks much better, though still not quite perfect (I'm probing the base oscillator and one of the Q or E outputs).

[Electro Fan]

...

 clockwise from upper left: 74LS74 dual D-type flip-flops, 74LS04 hex inverters, 74LS76 dual JK flip-flops, and a 74S51 standing in for the 74LS10 3-input NAND
...

Clearly something is terribly wrong, I suspect that it is a problem with how I have wired the circuit together, but I'm not entirely sure of that. The factors that suggest that I have mis-wired the circuit are that the Q high time is much longer than the low time, and the fact that I repeatedly mis-wired the connections around JK flip-flops several times.

Look at the datasheet for the 7451 and then look at how you have it wired.  I did not check anything else.

Everything will need to be correct for it to work.  The white protoboard should be fine for what you are doing.

Wow, Joe - looks like you inserted the diagram on top of jdutky’s breadboard? 
Very clever, nice, and helpful.

[joeqsmith]

If you don't care too much about the ringing and just want to see the states, you could go back to the hook and long ground lead.  If you want to use the short ground leads like you show, you may want to consider finding some sort of support for the probes.  The tips are not typically very robust and are easy to snap off. 

[tggzzz]

I prefer to use "enameled" wire for prototyping, very much like the board shown in:
https://en.wikipedia.org/wiki/Wiring_pencil

And I combine that with (high quality) sockets for all DIP IC's, because the chance to damage a chip is relatively big with prototyping and it's a real *&^%$#@! to loosen all the wires and put them back if a chip has to be replaced.

If you insist on using a breadboard, then at least devise some way of taking the crystal off the board.
I've had boards lock up because a digital pin next to the crystal was switching, and the capacitive coupling through the breadboard was enough to upset the oscillator to lock up the CPU. Using such a square canned crystal oscillator is a much better way.

Valid points.

I've never really liked the wiring pencils, but that is a matter of personal preference.

[jdutky]

If you don't care too much about the ringing and just want to see the states, you could go back to the hook and long ground lead.  If you want to use the short ground leads like you show, you may want to consider finding some sort of support for the probes.  The tips are not typically very robust and are easy to snap off. 

Ive been draping the probe cables over nearby things on the bench (e.g. a multimeter that was sitting next to the breadboard) in order to take their weight off the probe tip, not to protect the probe tip (thanks for the warning) but because the probe will simply pop out of the breadboard if there is even the slightest tension from the cable. I read the article by Paulo Renato about making your own low-Z probe, and that looks like it might be useful here.

[joeqsmith]

Working at low frequencies and using a breadboard, you may find resistive probes not making much of a difference.   

IMO, with your seemingly interest in the signal shape and with the NanoVNAs being so inexpensive, I would highly recommend you get one and start learning some of the basics.   This simple device could help you understand that wires are not just wire and resistors are not just resistors.   These concepts may become important depending how far you decide to take this new hobby of yours.   You may also want to download the free Micro-Cap simulator.   This simulator would allow you to play with high speed designs without having to buy the equipment or build anything.  You could also share your circuits with others who could then maybe help you out.   

A few probe links.   This is my Harbor Freight probe holder with vintage Tektronix high Z probe:
https://www.eevblog.com/forum/testgear/hi-z-probe-for-50-ohm-spectrum-analyzer/msg3241778/#msg3241778   

Some links to homemade low impedance probes:
https://www.eevblog.com/forum/testgear/fifty-ohm-probes/
https://www.eevblog.com/forum/projects/lo-z-probe/

I'm sure there are many others.  Try the advanced search option.

[tggzzz]

Working at low frequencies and using a breadboard, you may find resistive probes not making much of a difference.   

Indeed, but the OP is working at hundreds of MHz,  due to the signal's rise/fall time.

[joeqsmith]

Working at low frequencies and using a breadboard, you may find resistive probes not making much of a difference.   

Indeed, but the OP is working at hundreds of MHz,  due to the signal's rise/fall time.

I doubt they care about emissions and the fundamental is so low timing shouldn't pose any problems.   

Back in the days of the 6800,  my school required each student to design and construct a similar computer on whiteboard.  We were doing this with an analog scopes, 10X probes and burning EPROMs.   I have no doubt that the OP can pull this off if they stick with it.  But still to your point, it's good to start learning some of the basics as well which is why I mentioned the NanoVNA. 

[tggzzz]

Working at low frequencies and using a breadboard, you may find resistive probes not making much of a difference.   

Indeed, but the OP is working at hundreds of MHz,  due to the signal's rise/fall time.

I doubt they care about emissions and the fundamental is so low timing shouldn't pose any problems.   

Sigh.

Emissions are probably irrelevant to the OP, and are certainly irrelevant to waveforms on a breadboard.

If you miss hold times due to unacceptable signal integrity then it doesn't matter whether the clock is 1GHz or 1Hz.

If a clock waveform is unacceptable due to transmission line effects then it doesn't matter whether the clock is 1GHz or 1Hz.

If ground bounce causes problems (in a clock or signal) then it doesn't matter whether the clock is 1GHz or 1Hz.

All that matters is the rise/fall time - because that is the core parameter for all those problems.

[joeqsmith]

Not that we would run at a Hz but even at 3MHz, the time we have to settle is very long.  Normally, it won't pose a problem. 

[tggzzz]

Not that we would run at a Hz but even at 3MHz, the time we have to settle is very long.  Normally, it won't pose a problem.

Settling time is irrelevant.

I suggest you do some basic research on these topics

• ground bounce, cause and effect
• hold time
• signal propagation in a transition line
• clock signals that don't have a clean and fast risetime
• effect of signal and clock voltages that are >Vcc and <0