Early microprocessors often used NMOS or PMOS transistor technology (see this question for their use in early Intel chips). Techniques such as implementing registers with dynamic memory cells (instead of static logic) and precharging the buses were often used to save on the transistor counts. However, such techniques meant that the processor could not tolerate a stopped clock signal, as charge would eventually drain away.

In contrast, CMOS processors are more likely tolerate a stopped clock. The RCA 1802 is a good example; the clock may be stopped indefinitely. The PowerPC 750 was CMOS and dynamic, but its radiation-hardened version RAD750 is CMOS and static.

Were there any NMOS/PMOS processors that would still function properly after the clock was stopped and restarted?



As mentioned, being able to operate static isn't tied to production process, but the logic design. Thus there were of course static CPUs. A good example might be the Valvo-Signetics 2650 one of the more successful of the lesser known ones. Another example might be Texas' 9900, but I'm not entirely sure without checking its data sheet.

  • Is the Valvo version different than the original? Jun 21 at 21:12
  • @MauryMarkowitz Erm, I somehow don't understand your question. Using Valvo, Signetics or Philips was a marketing thing defined due region and time.
    – Raffzahn
    Jun 21 at 21:32
  • Indeed, which is why I asked - was there a reason to say "Valvo-Signetics" as opposed to simply "Signetics"? Jun 21 at 22:47
  • Nice find. Wikipedia link added for the 2650; the WP article confirms NMOS, and an advertisement confirms that it is static. The 9900 is also NMOS, but the data sheet is unclear on clock rates. Table 4.4 lists timing requirements, but only "typical"; the minimum and maximum columns are entirely blank.
    – DrSheldon
    Jun 22 at 3:42
  • @MauryMarkowitz Simply for understanding. As the names have been used different in different regions, not everyone will relate to either alone - at some point I simply started to use it that way - not to mention that it has been used by them hyphenated as well :))
    – Raffzahn
    Jun 22 at 11:28

Early NMOS Z80 CPUs were "half"-static -- they tolerated indefinite holding of the clock in '1' state, but not in '0'.

Z80 tech manual, pg.75

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