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While looking into some custom built CPUs I've noticed the frequencies at which they operate are relatively low compared to modern CPUs (in the order of several MHz). Is there an electronics engineering reason for that limitation, e.g. for breadboards? If yes, how to determine the max frequency that is possible to achieve with your design?

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  • \$\begingroup\$ Like...home made VLSI chips? I'd imagine it's a cost limitation. The precision that multi-million dollar equipment affords just can't be duplicated at home and as a result the chips must run slower. \$\endgroup\$ – Nate Nov 1 '12 at 1:24
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    \$\begingroup\$ @Nate, I imagine that he's referring to homebrew multi-chip CPUs built from TTL. For example: homebrewcpu.com \$\endgroup\$ – Alfred Centauri Nov 1 '12 at 1:30
  • \$\begingroup\$ @Nate i've updated my question to be more specific. \$\endgroup\$ – Eugen Nov 1 '12 at 1:32
  • \$\begingroup\$ @AlfredCentauri - yes, you are right ;) \$\endgroup\$ – Eugen Nov 1 '12 at 1:33
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It mostly has to do with the lengths of interconnections and propagation delays through the gates. If we reduce a CPU to its essence, it is a feedback machine. A bunch of combinatorial logic circuits compute some boolean functions over the current state of the machine, and those functions determine the new state, which is latched in by sequential circuitry when a new clock edge arrives. The combinatorial circuits all have delays. The clock period cannot be shorter than the time it takes for the slowest path through these gates to produce a stable result because a single incorrect bit stops the show.

Furthermore, the sequential logic has timing requirements. Before the clock edge arrives, there is some minimum setup time that the inputs have to be stable and then afterward they have to be stable for some hold time. If these are violated, the state becomes garbage.

The propagation delays are caused by things like how fast parasitic capacitances can charge, how fast current can build in the face of an inductance and how fast silicon devices can switch. For example, a bipolar transistor with a smaller base can switch faster than one with a larger base, so a tiny transistor on a chip will be faster than a discrete one.

In an earlier answer that I deleted, I wrote about transmission line effects. But I didn't consider that these effects don't even come into the picture at the speeds we are talking about because, say, at 10 Mhz, the wavelength is still about 30 meters. So on the scale of an ordinary sized circuit board, pulses on the time scale of a few megahertz still reach all parts of a copper network simultaneously.

So, if you make a CPU out of discrete components, you're simply not achieving the small components with fast switching times, and the same proximity which minimizes stray capacitances and inductances.

Nevertheless, ancient discrete-component machines in the 1960's did run quite a bit faster than these homebrew machines. It took some time and cunning to get there. For instance the IBM 360 Model 44 (1964) ran at 4 Mhz. That may still be "homebrew speed", but the CDC 7600 released just a few years later in 1969 surpassed 36 Mhz. The Wikipedia article http://en.wikipedia.org/wiki/CDC_7600 gives a hint at some of the tricks that were pulled, for instance:

"As always, Cray's design also focussed on packaging to reduce size, shorten signal paths, and thereby increase operating frequency. ... [E]ach circuit module actually consisted of up to six PC boards, each one stuffed with subminiature resistors, diodes, and transistors. The six boards were stacked up and then interconnected along their edges, making for a very compact, but basically unrepairable module."

So the homebrew CPU's are not necessarily built to their true potential due to some confounding effects having to do with the build quality and layout. Still, anyone who builds a CPU out of individual integrated circuits and discrete components which runs at several megahertz should be applauded.

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  • \$\begingroup\$ Depending upon the complexity of the data path, I would think that a homebrew device should be able to clock in at 20Mhz or more without difficulty using modern technology and conventional techniques. Not multi-GHz, but not a total slouch. I suspect, though, that it most cases where homebrew CPUs are used, ease of troubleshooting is more important than speed. Incidentally, the master clock crystal of the original Pong(R) brand arcade machine was 14.3818Mhz, though it was divided down pretty early on; I think the only thing gated by something that speed is the center-line of the playfield. \$\endgroup\$ – supercat Nov 2 '12 at 15:16
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    \$\begingroup\$ A lot of homebrew CPUs use EPROMs to store microcode, but also to implement complex logic and/or truth tables (many of them even have an ALU made out of one or more ROMs). The access speed of the ROMs can limit the top speed of the machine significantly, but they're popular because they make it easy to debug, repurpose and tweak the processor without significant rewiring. \$\endgroup\$ – Alexios Oct 19 '13 at 12:29
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As a former high schooler who built a special-purpose computer with 7400 series TTL, which won some kinda award at the science fair, I observed these things that kept it from running as fast as possible:

  • Stray capacitance in the breadboard. A few pF between every adjacent pair of connectors. That limited pulse edge rise/fall times and in places added crosstalk. This was probably the biggest factor.

  • Variations of grab-bag chips. (Anyone remember Poly-Paks?) 74LSxx, 74Hxx, 74xx with different propagation delays and other characteristics, made it impossible to have signals stay in sync at higher clock speeds than a few MHz.

  • Cheapo static memory chips, again from a grab-bag or other no-quality source. They just couldn't read or write reliably beyond a certain rate.

  • My test instrumentation was limited to homebrew signal generators, a 5MHz bandwidth oscilloscope, and temporary jury-rigged digital circuitry. Hard to check signal integrity, timing, amplitudes of digital signals that have been low-pass filtered into wobbly mush.

Today, it would be hard to find a 5MHz scope unless one is an antique buyer. Better chips of all kinds are just as easy to get, even in 0.1" spaced DIP packages, except I haven't seen much in the way of grab-bags in a long time. Socket breadboards, however, haven't changed much. Stray capacitance is still a speed killer for any edgy creative digital projects.

Avoiding breadboards by using a homebrew PCB is the best way to avoid stray capacitance, but of course requires more effort and time.

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I'd think the primary reason is that as you increase frequency, the impedance of your breadboard's connections will increase and limit the final speed of your circuit.

Every connection in your breadboard has a low, but non-zero inductance. As your frequency gets higher and higher, you need to take these effects into account. The impedance of the wires can be found by:

This formula

where L is the inductance of the wire. Eventually, Z will become high enough that not current will flow and your circuit will stop working. Finding the exact numerical value for this number will be very complicated, especially because breadboards have traces next to each other and that will change each wire's impedance a bit from this formula. If you really want an (imprecise) number, you can try here to calculate the inductance (and therefore impedance) of your wires. If you know the lowest current that a part can operate from, you can determine the maximum frequency before you hit that limit.

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  • \$\begingroup\$ Could you explain the relationship between the impendance and speed of the circuit? \$\endgroup\$ – Eugen Nov 1 '12 at 1:42
  • \$\begingroup\$ So how does this explain that CPUs like Intel Core I7 work at clock speeds of 2.5GHz and CPUs build on breadboards cannot achieve this speed? I thought initially that there is a connection with the length of the wire between the CPU and RAM chips. \$\endgroup\$ – Eugen Nov 1 '12 at 1:59
  • \$\begingroup\$ Not to deny the laws of physics, but I don't think that inductance is really the main limiter of speed. After, similar wires of similar ranges of lengths are used in homebrew radios and other projects, at much higher frequencies. One just has to be careful about matching impedances, lengths, layouts, avoid accidental couplings, etc. \$\endgroup\$ – DarenW Nov 1 '12 at 2:20
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    \$\begingroup\$ @DarenW: Stray inductance and capacitance are indeed the problems. In a radio system, you typically have only one wire of non-trivial length. In a processor implementation, you have hundreds, with mutual inductance depending on the spacing. The frequency-dependent interactions are unmanageable due to the complexity. Characteristic impedance depends strongly on things like distance from ground traces... which aren't well controlled on a breadboard. \$\endgroup\$ – Ben Voigt Nov 1 '12 at 2:26
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    \$\begingroup\$ I think you're right, I guess it should probably just be reactances in general. Capacitance will result in similar issues, but for things like hard rising/falling edges and state changes. The equations for finding the impedance is similar though, and if he wants a numerical answer those could probably be applied in a similar fashion. \$\endgroup\$ – Nate Nov 1 '12 at 2:29
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Others have answered the "why". Here's how to determine the maximum speed.

  1. For each flip-flop, look up it's clock to Q.
  2. Total the wire length of all the wires from the flip-flop to the next flip flop. Turn this length to time. Wire is ~2/3 speed of light
  3. Total any gate delays, including through asynchronous RAM.
  4. Take the setup time at the next flip-flop.
  5. Add 1-4. This is you minimum clock period. Invert to get frequency.
  6. Consider clock skew. If the clock gets to the second ff before the first, add skew in with 1-4.
  7. If the clock gets to the second ff before the first, calculate the minimum of 1-3. Make sure that they are less than the hold time required by the second ff plus the clock skew.
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  • \$\begingroup\$ Which wire length you are talking about: the length from the power source to CPU out pins, CPU out pins to RAM chips ... ? Also I am not really clear what you mean in the first step. \$\endgroup\$ – Eugen Nov 1 '12 at 20:00
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    \$\begingroup\$ @Eugen - I think (but not my area of expertise) he's referring to its internal propagation delay - time from being clocked to having a stable output. \$\endgroup\$ – mickeyf Nov 2 '12 at 1:45
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Apart from all the electrical reasons that limit the speed, there is also one on the logical level:

You cannot throw as many resources at making things run faster, such as pipelined operation with branch prediction, faster arithmetic and whatnot. Caches don't make much sense either if they aren't faster than your main memory.

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For homebrew machines it comes down to two factors. The propagation delay for the chips you are using and the number of chips you need to use on the longest path through your CPU design.

For example, a 74HC574 (8 bit register) has a maximum propagation delay of about 41ns (taken from its datasheet). Now lets say the longest path through your CPU design requires it to pass through 8 different chips. Add up the propagation delays for each of the 8 and lets imagine it comes to 333ns. With 1000ns being the same as 1Mhz that would give you a maximum speed of 3Mhz.

In practice you might want to restrict yourself to something slower like 2Mhz, in order to ensure a stable design. Even if you think you will only miss the timing once per billion cycles, then your still in trouble. 10 billion divided by 3 million means you miss-execute once every 3,333 seconds, which is about once per hour. Crashing your machine every hour is not good!

To make it go faster you can use faster chips and/or change the design to reduce the number of chips in the slowest path. About the fastest homebrew speed you see is around 4Mhz which gives you 250ns to complete each cycle.

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