It started simply...
I can't keep track of all the awesome "discrete" CPU designs on my own project. There is a list of such projects but it is "curated". Why not make my own list and invite like-minded hackers ?
If you have a similar project here, drop me a message and I'll add you to the contributors.
The group evolved towards the exploration of all the transistor-based technologies and the associated ideas. So we study how to make basic logic building blocks in standard and exotic ways.
The "Team" is a selective who's who of people who designed their own discrete computer or developed advanced logic circuits.
Come and chat with us on the open forum if you have ideas or questions !
For practical reasons (it's impossible to list everything on the 'net), the "project" is mostly about gathering people from HaD who built their CPU (or at the very least digital electronic devices). Here are some external links for those who just can't get enough:
Well, @Tim, it wasn't that hard after all ;-) How about 1.73ns at only 5V ?
OK it's ugly (bad baaaad probing) and the CDC levels are pretty much destroyed...
But it's FAST and even more POWER EFFICIENT !
I get 32MHz at 5.2V and only 77mA, or 44mW per gate, a 4.5× improvement compared to Tim's 200 mW :-)
What's my secret ? Not much, it's explained in the previous logs ;-)
ample capacitor decoupling
low Rb (150 Ohms)
finely chosen Cb (68 pF)
But this log has a newcomer : a Schottky diode. Spoiler alert : I didn't pay much attention, I found 2 reels of SMB and SMA-packaged low voltage diodes in my drawers. I don't even remember where/how/when/why I obtained them but here they are !
This version uses a ROHM RB751V-40 Schottky barrier diode in a tiny tiny package (almost 0603). It's limited to 20mA which matches well because the higher the current, the larger the junction, the more capacitance...
I also have MBR0520 diodes but the higher current rating potentially increases the capacitance, which would create more problems.
The 2N2369A is prevented from "switching hard", which has a welcome effect : less current is drawn ! At 1V the circuit sips only 3mA instead of 6mA... By 2V the difference is mostly erased, though, but at low voltages, that circuit is crazy efficient :-)
set xlabel 'V'set ylabel 'MHz'set y2label 'mA'set xr [1:5]
set yr [6:36]
set y2r [0:90]
set y2tics 3
"ringo9v2_0-68pf-RB751.dat"using1:2 axes x1y2 title "0pF current in mA" w points pt 7, \
"ringo9v2_0-68pf-RB751.dat"using1:3 title "0pF frequency in MHz" w lines, \
"ringo9v2_0-68pf-RB751.dat"using1:4 title "66pF frequency in MHz" w lines, \
"ringo9v2_0-68pf-RB751.dat"using1:5 axes x1y2 title "Schottly+66pF current in mA" w lines, \
"ringo9v2_0-68pf-RB751.dat"using1:6 title "Schottly+66pF frequency in MHz" w lines
From there we can also plot the power/frequency curves with the following script :
set key right bottom
set xlabel 'V'set xr [1:5]
set yr [0:2]
set ylabel 'mW/MHz'
plot "ringo9v2_0-68pf-RB751.dat" using 1:(($2*$1/$3))/9 title "0pF" w lines, \
"ringo9v2_0-68pf-RB751.dat" using 1:(($2*$1/$4))/9 title "66pF" w lines, \
"ringo9v2_0-68pf-RB751.dat" using 1:(($5*$1/$6))/9 title "Schottly+66pF" w lines
The result is self-explanatory :-)
These curves were measured on this simple board :
What else is there to say ?
It's not the end of the adventure, of course, because it's only a ring oscillator and the diodes have destroyed the saturating...
set xlabel 'V'
set ylabel 'MHz'
set y2label 'mA'
set xr [1:5]
set yr [6:24]
set y2r [0:90]
set ytics 1
set y2tics 10
set key right bottom
plot "ringo9_2_47pF.dat" using 1:3 title "v.2 47pF Frequency in MHz" w points pt 7, \
"ringo9_2_47pF.dat" using 1:2 axes x1y2 title "v.2 47pF total current in mA" w points pt 7, \
"ringo9_2_sans.txt" using 1:3 title "v.2 sans cap Frequency in MHz" w lines, \
"ringo9_2_sans.txt" using 1:2 axes x1y2 title "V.2 sans cap current in mA" w lines
Once again the capacitor is a simple yet very effective means to go faster, yet the power curve is not affected (in a meaningful, significant way). So the efficiency is much better than v1 :-)
At 5V the circuit easily reaches 22MHz, or 2.5ns per inverter !
But is it necessary to go THAT fast ? Where is the sweet spot again ? I don't think it's a good idea to run at 5V because the speed is only marginally better for a very significant increase in power draw (42mW/gate, or 1.8mW/MHz). So maybe 5V would be reserved for special cases and places that need a serious fanout.
2V : 46mW => 5.1mW/gate, or 0.3mW/MHz/gate
2.5V : 80mW => 8.8mW/gate, or 0.48mW/MHz/gate
3V : 120mW => 13.3mW/gate, or 0.68mW/MHz/gate
3.3V : 152mW => 16.8mW/gate, or 0.83mW/MHz/gate
5V : 375mW total, 42mW/gate, or 1.8mW/MHz
It would be wise to stay under the 1mW/MHz/gate, 0.5mW/MHz/gate would be even better but the fanout would be insufficient. The standard voltage 3.3V would be a good compromise but let's wait for the results with the other cap values and the diodes !
Anyway : Going from 2.5V to 3.3V brings only 10% more speed while the power almost doubles !
But what is the right capacitor value ?
the datasheet specifies < 4pF for the gate charge. So the capacitor must be higher than that to cancel the effect. So maybe 47pF ?
OTOH I saw a speed difference that is similar between 27pF (PCB v.1) and 47pF (PCB v.2) so there would be a diminishing return, which can only be spotted by plotting the V/F curve with various capacitances.
The smallest capacitors I have are 10pF so that's a good start. I can then add 18pF in parallel to give 28pF. Adding 47pF again will give another trace...
The results for 10pF are below :
From this graph, we can only suppose that the next increase would be to 220pF...
Meanwhile, the current graph has not changed so I don't show it anymore.
Testing with 280pF gives a pretty unexpected curve, but good to know anyway :
After a promising start at very low frequency, the 47pF curve is already winning at 1.4V. I now have to check at 100, 68 and 33pF if there is another local maximum...
The 100pF curve is disappointing : why is it worse than the 280pF ?
What is so special about the 47pF I tried ? Did I fry a part ?
Trying with 33pF caps shows interesting results as well, close to the 47pF.
Apparently the 2×33pF combination has a very light advantage up to 3.5V : that's still good to take and much better than other values.
Now trying 68pF gives a result very close to 2×33pf. So close that gnuplot almost mixes the colors, unless you zoom a lot.
So 68pF wins by a tiny margin, but that's all I intended to find out :-)
The new parameters are not far from the previous one :
Rb = 150 Ohms, Rc = 470 Ohms
The change of Rb seems to have helped a bit : I now see the collector voltage saturated and not reaching Vcc (between 0.15 and 1.25V). Vb ranges from 0.18V to 0.9V => I'm now near the levels defined by CDC !
. Here is the waveform at the base : the 2N2369 is driven hard at 800mV ! Discharging it however seems to take some time...
The other change is the ample decoupling, 6×100nF + 3×10nF, I don't know if it helps but you're never too safe with that because later, I might unexpectedly scramble the local CB channels ;-)
Yet I don't see how/why I gained 30% speed with the same transistors (I replaced one by error) and almost the same resistors (ok the base resistor has lost 25% of its value... but it's worth it right ?)
Did I mistake a resistor somewhere ? Was one of the transistors "too slow" ? Is there a wrong resistor value in the first RingO ?
Something else is interesting : I'm now at 30mA but the last "record" was at held at 50mA so something serious is going on here ! Efficiency has jumped too !
The signal falls in about 5ns on the 200MHz scope, which is close to the limits. There is some overshoot, very likely caused by the ground clip and the limited BW of the whole system.
Another good sign is that the falling edge (at the collector) is now the fast one, in 5ns :-) (we were puzzled that the rising edge was the fast one on the other board, might have been mistaken for the base ? nah...)
The rising edge takes about 12 ns to completely reach 1.1V and this will get only longer with more loading. But in 8ns, 1V is reached.
At 2.5V and 470 ohms shorted in DC to 0V, the collector current is drawing 5mA (approx.)
Add to this the other current source (the base capacitance and the transistor might have 10mA transients... So once again it's in line with the CDC specs :-)
The base current is defined by (Vc - Vb) / Rb = (1.2 - 0.85) / 150 => Ib = 2.3mA (at 2.5V, during DC ON) => in line with the expected values :-)
The circuit alternates between 5mA and 2.5mA, this averages to 3.7mA/9.3mW per inverter (FO1).
The delay :
This plot is from the base and collector of the same transistor, so we see the latency of the signal : about 5ns between the middle point of the rising edge on the base and the middle point of the falling edge of the collector. It takes about 8ns from the start of the base's rising edge to the end of the collector's falling edge...
The reverse however takes more time, due to RC loading.
set xlabel 'V'
set ylabel 'MHz'
set y2label 'mA'
set xr [1:4]
set yr [6:18]
set y2r [0:120]
set ytics 1
set y2tics 10
set key right bottom
plot "27pf.dat" using 1:3 title "v.1 27pF Frequency in MHz" w points pt 7, \
"27pf.dat" using 1:2 axes x1y2 title "v.1 27pF total current in mA" w points pt 7, \
"ringo9_2_sans.txt" using 1:3 title "v.2 sans cap Frequency in MHz" w lines, \
"ringo9_2_sans.txt" using 1:2 axes x1y2 title "V.2...
Ring oscillator with 9 levels of low-grade 2369 (according to their hFE).
Rb = 220, Rc = 470, like before.
1nF to decouple a pair of transistors.
But this time I add more capacitors : 100nF on the power input and 27pF to short each base resistor ! As usual, it's a step by step modification to help with understanding the effect of every change.
From the beginning, starting at about 10MHz, I saw the incremental increase of frequency : about 500KHz for each capacitor I added. I tested very often because I didn't want to spend any time spotting soldering error.
After a while I had the 9 capacitors wired and *bim* 16MHz without effort !
Some tuning later, a lot of blowing, and the best frequency I got was 16.8MHz !
That's at least 50% better than without the capacitors.
So I was Falstad'ing some ECL/differential amplifier topologies and playing with the resistor values ratios...
I found some strange behaviours with this single-ended circuit when the collector and emitter resistors are equal.
At 5V the turning point is at 3.1V, so I created a sine wave centered around 3V with +/- 1V peaks. The output looks like a rectified version...
The effect disappears when the ratio of the resistors is modified. This might be a desired effect or an unwanted behaviour, and since I'm playing with ECL topologies, I want to avoid this so I need to understand what is going on.
This is important because I would like to save a transistor at the common emitter node so the resistor value must be well chosen. It's good to know that a 1/1 ratio is BAD, and changing it affects the kink point...
But OTOH it opens up potential for fun, such as sound effects :-P
With a sporadic and limited access to the workshop (at last !) I can finally try new ideas ! I have meanwhile received 9K PMBT2369 in SMD but I decided to use the old stock of 50pc 2N2369A in metal can, that was waiting in a small bag that I received from various sources... What can be closer to the CDC era ? (A motor-generator ? :-P)
This is a "mixed bag" with at least 2 sources or makers, some with golden legs, and I decided to test them. Just because I now have a better tester and it's good to see if/how the different types differ...
Most "golden" parts fall in the lower bins and the tinned ones have overall the best gain. I made 3 bins :
< 60 (lowest is 46)
higher (a few up to 114 and one at 119)
and then I use the lower gain ones to build the RingO, with 9 parts to give a low-enough frequency that makes 'scoping reasonable.
I could have made a > 100 bin but
I just wanted to have a look at the spec spread
I wanted to weed out the lemons (and use them first to establish a baseline)
Rb = 220 (that's what I have in stock right now, close enough)
Afterthought : I should have tried 100 Ohms for Rb. Or even 47/50 ohms maybe....
After-afterthought : or 330 ohms (see near the end)
And the soldering iron was turned on !
For the sake of simplicity I omitted the caps. They used too much room. Next time I'll look at the SMD stock.
I added 1nF to decouple every pair of transistor (that's 5nF but spread to ease HF transients)
I found some partial reels of SMD Schottky diodes but once again, decided to not use them yet.
So I wanted to establish a baseline for speed and more importantly : explore the power vs speed envelope because... Tim found that a LOT of power was wasted. I would like to get a gate that is still "pretty fast" and yet consumes at least 10 times less power.
For the measurements I used a 200MHz digital scope with 10x probe. The output waveform is pretty nice and quite square-y :-) No funky feature is noticed, it's plain old RTL and I didn't bother to measure the rise/fall time because the measurement circuit is not optimised.
Still it's very telling.
Rise time is about 5ns. Which is odd since I expected that RC would dominate it.
In fact something else is happening : it seems that the transistor has a harder work to totally saturate and keep Vce sat to a sufficiently low value. This in turn reduces the frequency because the transistor turns off later.
At 5V the circuit doesn't seem to get hot, maybe thanks to the help of the metal cans and the resistors directly soldered to thick metal that can spread the heat.
You can find a curve that is similar to what Tim found already :
Vcc Total Freq.
5V 123mA 6.98MHz <= never mind.
4.5V 109 7.37
4V 97 7.77
3.5V 81 8.61
3V 69 9.42 <= why waste so much power ?
2.5V 55 10.16
2.25 47 10.48
2V 40 10.55 <= sweet spot !
1.75 33 10.48
1.5V 26 10.06
1.25 18 9.13
1V 11 7.34 <= wow, that's still good :-D
The frequency is measured by the scope but not finely calibrated. My HP freqmeter wouldn't accept the raw signal, something to do with ringing and probe impedance, I'll check that later. Still you can find the important features.
With my choice of parts, I find that the sweet spot is in the 1.75V-2.25V range, as roughly expected, so I'm pretty happy ! But there is more to that curve.
Discussions on the chat with @Tim and his reseach into weird latches made me look back at my MUX-based latch. The early idea was not working but later revisions did, once I created a fully functional MUX2.
This one works down to 2V. It uses the same values for all resistors, it has a /Q output as well, a single-phase clock input, and only 5 transistors.
So far there are 5 transistors but for a whole register, the clock transistor can be shared among several bits.
At one point, I had to add a 1G resistor to prevent totally bonkers behaviour of the simulator between the two PN junctions of the transistors in series, ignore it for real circuits. In rare cases however, Falstad indicated 25GV at that node...
This time, I understood that there is one case that looks wrong but has no real effect on the circuit : when C is low and Din is high and feeds current into the /Q node, but it does not matter. In previous versions I added diodes to prevent current from flowing back but when absent, the working supply voltage can go lower and the noise immunity is better (or it can dissipate less power).
The /Q output might not be required. In this case, the 1K resistor that feeds the base of the output transistor could be omitted. This would also speed the circuit up a bit.
At some points I have problems with Falstad's sim (how surprising !) because the circuit wouldn't want to hold the High state on Q. I added a 470p capacitor to hold a bit of charge during the clock transition but later retries (and a full screen refresh) work without it... The value is approximate and the transistors' inherent capacitances would normally do the trick and add the tiny delay. Of course, a full simulation with xSPICE will work better :-)
Of course Falstad is a bit buggy but in practice, it's a lot like in real circuits, which are not perfect either. If you can make your circuit work in many conditions in Falstad's sim, the real circuit has good chances to work in real life.
The next step is obviously to create a DFF with a pair of these cells but in practice, timing is everything and with discrete computers, the clock strobe can be made very short, or successive latches can be strobed by clock signals with a little delay...
Not all weird discrete logic has to be based on ancient components.
Most projects in discrete logic families focus on recreating ancient circuit styles (like RTL,DTL, DCTL or, as a bastard abberation, LTL) with the components that are still available today. It turns out that many of the specialized transistors are long gone. How about doing it the other way: Pick a minimal building block that is easily available today and base logic on that?
Browsing distributor listings I found an interesting category of small devices: Analog switches and multiplexers.
Basic building Block: The 2:1 Multiplexer
An example of an 2:1 analog multiplexer is shown above. These are not digital devices, but actual analog switches. The connection between B and A will be low ohmic when it is active and assume a very high resistance when deselected. This means that it can be used in both directions.
There is an abundance of 2:1 switches available in very small SOT-363 packages (above) from different sources for prices that rival that of discrete transistors. A short listing of some of the devices I found on LCSC is in the table below:
Jiangsu Qin Hang
Youtai Semiconductor Co
Unfortunately I was not able to find any spice model of these devices that is suitable for LTspice. So I made my own behavioral model as shown below, next to the entity symbol. Many parasitics are not considered here. Neither is the delay that the control logic is causing, so it can only be seen as a crude approximation.
Note the biasing resistor on the output, which is very important to prevent LTspice from getting stuck in a metastable state. Spice does not really like switches...
Building basic logic gates
Most basic two-input gates can be realized with one or two analog multiplexers. It's interesting to note that the MUX is more accomodating to positive logic. Inversions typically require adding an additional multiplexer.
The OR gate can be realized in a very similar manner. NOR and NAND require an additional inverter.
XOR can be realized by an multiplexer that selectes between an inverted and non-inverted version of the secondary input. XNOR is realized by swapping multiplexer inputs.
Latches are the achilles heel of any logic family. Building a latch with a digital multiplexer is actually fairly easy and can be done with a single multiplexer by routing the output back to one of the inputs. However, this is not so easy with analog multiplexers, as they only act as a switch without any buffering or amplification.
Instead, we will revert to a dynamic latch as shown below.
The first multiplexer acts as a path gate. If the clk is high, the input data will be routed to the output where the storage capacitor is charged. If the clk is low, the output will be connected to a floating input, so that the charge on the capacitor is held. The second multiplexer acts as an output buffer.
Obviously this is a bit tricky in operation as some of the charge will dissipate through leakage into the buffer control input and internal leakage in the multiplexer. A sufficiently high clock is required to allow cyclicated refreshing of the latch content.
The figure above shows simulation results of the latch in operation. Since loading of the capacitor causes a current surge on the input, spikes are seen on the input signal. Proper buffers and dimensioning of the storage capacitor is necessary.
To verify the functionality in a more complex circuit, I designed an 8 bit counter in spice. You can see some of the output traces above. I hope that some of the transient spikes disappear once real-world parasitics are added.
This looks like a potential approach to build discrete...
These days I'm contemplating "tasting" BFP740 (44GHz GBW but not in stock so far) and 2N2369 gates (I have a fistful but not enough to make anything interesting)...
I propose to create a new project/page where we gather all the ring oscillators experiments, sort them by technologies, discuss on measurement details (and gotchas) and agree on a standard "size" to help tally and compare speeds, efficiencies etc.
I was thinking that with my BFS480 (rated at 7GHz) I would need 9 inverters in series to have a reasonably observable waveform and a frequency that my HP5335A could accurately follow.