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I've read that Z3, first electromechanical general purpose computer operated at a frequency of about 5–10 Hz, and the ENIAC had a 100 kHz clock, though each instruction took 20 cycles.

What microprocessor has the lowest clock speed? I'm curious about fixed speed processors and the lowest minimum and maximums on devices with variable clock speeds.

By microprocessor I mean the definition given by wikipedia: A computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits.

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    You might need to be more specific on what you mean by "microprocessor." Do you mean something on a single integrated circuit?
    – JRN
    Commented Jul 3, 2021 at 2:04
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    Most (fully static) CMOS CPUs can go down to zero with their minimum clock speed. So, asking for the minimum is not really a useful question for such chips. I guess you want the slowest maximum speed.
    – tofro
    Commented Jul 3, 2021 at 8:40
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    Then your question is not very useful (or rather, very ambigous). Even a Z80A, which is nominally clocked at 4MHz, runs on a clock as low as 250kHz according to the datasheet.
    – tofro
    Commented Jul 3, 2021 at 14:25
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    You seriously believe that a site like Wikipaedia is a reliable source for such a definition?
    – Chenmunka ♦
    Commented Jul 3, 2021 at 17:47
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    Let me rephrase that. Wikipedia is not a reliable source for ANY information. Please see the questions on Meta discussing this.
    – Chenmunka ♦
    Commented Jul 3, 2021 at 17:58

4 Answers 4

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Any CPU that is fully-static can be clocked down to 0 Hz and still function. Such CPUs are in wide use today. Here are some examples:

  • RCA 1802
    Used in a few early micros, as an embedded processor, and in space applications such as Galileo (Jupiter) and Magellan (Venus). It also was the initial platform for CHIP-8.
  • Intel 80386EX
    A variant of the 386 targeting embedded applications.
  • WDC W65C02S
    Fully-static version of the ubiquitous 65C02.
  • WDC W65C816S
    Fully-static version of the 65C816 (enhanced 8-/16-bit version of the 65C02).
  • Freescale 683XX
    Microcontroller using a fully-static 68000-based core.
  • The new tinyAVR chips (0-, 1-, and 2-series, e.g. ATtiny204) seem to be fully static. They're rated down to 0 Hz and I've personally run them as low as ~683 Hz.
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    Obviously, the intent of the question was different. What was the lowest maximum speed of a microprocessor?
    – Leo B.
    Commented Jul 3, 2021 at 3:11
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    @LeoB. I think a more interesting question is what non-static processor has/had the lowest minimum clock speed.
    – Alex Hajnal
    Commented Jul 3, 2021 at 3:13
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    afaik any AVR mcu, incl. the oldest ones, was static, as well as all 8bit PICs by microchip, and all AT89 mcs-51-compatible mcus by atmel.
    – lvd
    Commented Jul 3, 2021 at 8:30
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    @LeoB. I would argue that there is no intent of the question. It's a random "gee I wonder" question that has nothing to do on any stack exchange site, except curiously retrocomputing where such questions are accepted.
    – pipe
    Commented Jul 3, 2021 at 11:53
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    Yes, this is just a trivia question. But I find you can still learn a lot from asking these kind of questions
    – alessandro
    Commented Jul 3, 2021 at 15:08
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Going with the spirit of the question.

A quick glance at Wikipedia's Microprocessor Chronology we find:

  • PPS-4 from Rockwell at 200 kHz
  • PPS-8 from Rockwell at 256 kHz
  • TMS 1000 from TI at 400 kHz
  • PPS-25 from Fairchild at 400 kHz
  • IMP-4 from National at 500 kHz
  • 8008 from Intel at 500 kHz
  • IMP-8 & -16 from National at 715 kHz
  • the 4004 and 4040 from Intel at 740 kHz

So, I would say the PPS-4 "wins", while the Intel 8008 is probably the slowest chip that may be commonly known.

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    The PPS-4 runs at 4× the base frequency (i.e. ~800 kHz on a 199 kHz "A" clock). See here and here. However, the datasheet (p. 14) for the TMS1000 says that it'll run down to 100 kHz (and possibly lower; setting the internal oscillator to 40 kHz is described on p. 15).
    – Alex Hajnal
    Commented Jul 3, 2021 at 5:27
  • The TMS1000 also wasn't static (it used DRAM for its registers). See here for details on how it was implemented.
    – Alex Hajnal
    Commented Jul 3, 2021 at 12:52
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Not the slowest in clock speeds (a whopping 1MHz), but probably the simplest and slowest in raw performance of its time may be the Motorola MC14500 CPU - It only has a 1-bit data bus and only 16 instructions.

Given that not all (especially industry control) applications need more than "on" and "off" as inputs and outputs, it was still a very versatile thing to have.

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Other answers are correct about other microprocessors that can run at very low speeds and still be functional. For example, I have a RocketChip for my Apple II which is a 6502 that can be controlled via software to run at the default 1MHz or accelerated 5MHz, but can also be tuned down to 50kHz.

However, I think the nature of the question is more around CPUs and their native speeds. If that is the case, if I exclude minis and mainframes and focus on desktop, I think the Intel 4004 might be the lowest since its native clock was 750kHz and I can't recall many others in the kHz range that go lower than that.

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