crystal frequency, machine cycle and speed of my PC

[PG1995]

Hi

I have always wondered what that GHz thing means about the processors. They say it's speed of a processor. For example, my computer used 2 GHz processor. After reading some of the stuff about 8051 I understand a little what crystal frequency, machine cycle, etc. are. So, I presume 2 GHz is crystal frequency.

As it says in the attachment that the 8051 could use crystal frequency from 4 MHz to 30 MHz. Suppose a 8051 MCU is first connected to a crystal with frequency 5 MHz and then the same MCU is connected to 30 MHz. Would there be a difference in terms of performance. For 5 MHz machine cycle would 2.4 μs and for the 30 MHz it would be 0.4 μs.

If using a higher frequency crystal really raises the performance bar, then, theoretically speaking, can I use higher frequency crystal for my PC so that it perform well?

Please keep it simple so that I can understand it. Thank you.

Regards
PG

 

[3v0]

On a PC you generally do not have to change crystals to boost the speed. Google overclocking. It is easier to do with some motherboards then others.

On a PC there are a lot of parts that play together and boost just the processor speed without increasing memory speed etc etc may not get you as much boost as you think. Not to mention the possible reliability issues.

 

[crutschow]

PC manufacturers run the CPUs as fast as possible while still insuring that the those with the worst manufacturing tolerance limits will still operate. Individual PCs can be overclocked depending upon how fast the particular CPU is, which depends upon the PC. Overclocking can achieve some minor increase in speed (which the gamer's are always looking for) but it does increase CPU heat generation and may reduce reliability. So generally it's not worth the effort unless you are a gamer.

 

[MrAl]

Hi,

Increasing the frequency of the CPU in the PC makes the CPU go faster. That means that once it gets a set of instructions to perform, it can perform them faster. That's overclocking the CPU. However, it has to interface with the RAM memory so if the RAM isnt faster too then the CPU will have to waste more clock cycles waiting for the use of the memory. You can also try to overclock your RAM to keep up with the CPU.
One of the main limiting factors today is the heat generation. The faster the CPU is clocked, the more heat it generates. This is partly because of the capacitance and partly because we have to increase the core voltage to get a serious increase in speed. If you are not serious about overclocking then it wont be worth bothering with because you wont want to invest in better heat sinking for the CPU, which could potentially allow you to overclock much more.

Increasing the frequency of the microcontroller however can have a much more profound effect because it has it's own internal memory that goes as fast as the controller chip. That's if the chip allows it. The chip will use more power however unless it is in standby or sleep.

One thing that is often overlooked in today's discussions is *under clocking*. Under clocking allows you to run the PC CPU at a lower frequency then originally designed for. This not only means MUCH less heat in the CPU, it also means a lower electric bill. There's also the possibility of shutting down some of the cores and only allowing those cores that are needed to run either at full speed or reduced speed. For example my old 4 core CPU used something like 95 watts when it was running all 4 cores at top speed, but only 25 watts or less when only one core was being used. That's 1/4 the power. My newer 8 core CPU uses only 35 watts at reduced speed when im typing or doing other stuff that doesnt required super fast CPU access. If i had to use it full throttle all the time it would use almost 200 watts, and this almost never happens except for a few seconds. The drawback to high speed and multiple cores is the required heat pipe heat sink, and would probably do better with a water cooled.

So the speed can be increased and the more you cool the CPU the faster you can run it, to a point. Of course more cores also uses more power and thus more heat, so shutting down some of the cores is also a good idea when the full power of the CPU is not needed.

 

[PG1995]

Thank you very much for the informative replies, 3v0, Carl, MrAl.

The 8051 has two pins, 18 and 19, for interfacing with external crystals. What's the purpose of two crystals? Could you please briefly tell me? Thanks.

Kind regards
PG

 

[Sceadwian]

The 8051 has two pins, 18 and 19, for interfacing with external crystals. What's the purpose of two crystals? Could you please briefly tell me? Thanks.

For interfacing with an external crystal, not plural, just one, you can't use two crystals at the same time, at least not generally and never in common practice.

Another note is that the processors of modern CPU's don't actually have tuned crystals in the GHZ range, crystals typically are not tuned for frequencies higher than about 20mhz, anything that uses a 100mhz crystal generally isn't a crystal shaped for 100mhz, it could be a 20mhz crystal with the filter/amplifier network set up to catch the 5th order harmonic. You can design the oscilator circuit to filter out any frequency you want, a 20mhz crystal can be run at any harmonic of the native frequency of the crystal, and can often be 'bent' a little bit with the right additional circuitry for fine tuning.

This base frequency is sent through a PLL and frequency multiplied to derive the processor clock. Modern clock generators are incredibly flexible which is why overclocking is possible.