Basic question that I'm surprised I can't find the answer. I'm more marketing than micro-engineering :)

If the processor says is rated to 120 Mhz and the board uses a 12Mhz crystal, is the hardware limited to 12Mhz? Or it can still operate at 120 Mhz? Somewhere in between? Depends on the firmware?

Reason: The product is exactly as above and I'm writing up the spec sheet for customers. So am I being dishonest if I say it is powered by a 120Mhz micro in this situation? Or that is perfectly true if the firmware guys make it so?

edit: The processor is STMicro F205 Cortex M3. They market it as "120Mhz" but then the datasheet says to use a "4 to 26 MHz crystal oscillator", which our people chose a 12Mhz.

  • \$\begingroup\$ Likely the hardware people use a clock multiplier (usually on board PLL) to scale the clock internally. It is not dishonest to say it is a 120MHZ device if it uses a 12 MHz clock X10 since the logic is still clocked at 120MHz. If the Cortex recomends a 4-26MHz crystal and says it operates up to 120MHz, then there is surely an internal clock multiplier available. This is typically configured during device programming. \$\endgroup\$
    – crasic
    Jul 15, 2015 at 4:14

2 Answers 2


The STMicro F205 Cortex M3 contains an internal PLL that can clock the processor all the way to 120 MHz. It can use an internal 16 MHz RC oscillator to drive the PLL or an external 4-26 MHz clock source.

So technically the processor can run up to 120 Mhz, however, that top speed depends on how your firmware programmers set up the PLL.

  • \$\begingroup\$ OK, thanks. So your opinion: if I don't know what the firmware guys plan on doing (but realize they won't really be pushing it to 120Mhz) am I being honest by saying "up to 120Mhz" given the 12 Mhz crystal? Or is it limted to, for example, 55Mhz (120 / 26 * 12)? \$\endgroup\$
    – bcsteeve
    Jul 14, 2015 at 23:23
  • 2
    \$\begingroup\$ For your equipment, if you can't get the information from your development team, "up to 120 MHz" is not accurate. Since you don't know what frequency the processor is running at, you cannot honestly put a number on your data sheet. Go pound on someone's desk. \$\endgroup\$ Jul 15, 2015 at 1:15
  • \$\begingroup\$ Perhaps it isn't a typical application. There is no firmware. At all. So it will run at whatever it is eventually told to run at. What I was asking was whether the crystal is setting the limitation, or is it the firmware. I know it sounds weird that I don't have that information. Unusual circumstances. Anyway, I believe i have my answer: it can run up to 120Mhz if the firmware eventually tells it to. I was worried we were stuck at 12Mhz due to the crystal. \$\endgroup\$
    – bcsteeve
    Jul 15, 2015 at 7:00
  • \$\begingroup\$ If the end customer is uploading the firmware and programming the device then up to 120 Mhz is an acceptable way to market the product. If you are selling a product that is not customer programmable then there has to be some software loaded in the device or it would just be just a brick :-) \$\endgroup\$ Jul 16, 2015 at 19:22
  • \$\begingroup\$ I didn't really answer your other question about speed. A PLL basically works by multiplying the reference frequency by some number. I'm not a 100% sure of the ratios that the F205 supports but 10x is fairly common. \$\endgroup\$ Jul 16, 2015 at 19:28

Depends on the device. There are plenty of systems which have internal PLLs so that the clock frequency can be higher than the crystal frequency. But why not ask the hardware people?

  • \$\begingroup\$ I figured it would be easier to ask here. The "hardware people" were outsourced. I've been asked by customers what the specs are - not that it really matters - and I'm trying to supply that information from an honest, but marketed, stance. I've updated the question to include the micro model if that helps. I'm scouring the datasheet for info regarding the PLL but it is mostly Greek to me. \$\endgroup\$
    – bcsteeve
    Jul 14, 2015 at 23:15

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