Two circuits with diferent clock speeds

Thread Starter

The Kadet

Joined Apr 27, 2016
7
Two circuits with diferent clock speeds How to get them to work togueder?

For example in a computer the memory clock speed is diferent from the processor's clock speed, the bus clock speed is diferent from the piriferals bus and so on...

How are these things coordinated?
 

BR-549

Joined Sep 22, 2013
4,938
A computer, the memory for a computer and a processor are an array of switches.

And everything that the computer or processor does, is just a matter of turning the right switch, on or off, at the right time.

A program is a list of switching instructions.

Everything is controlled by switching switches.
 

Thread Starter

The Kadet

Joined Apr 27, 2016
7
So the processor clock gives impulses in the from of square wave to the entire sisthem?

How is that possible when the power suply cables are conected to the motherboard and the circuits runing the the processor arent that thick?
And then is not there power dissipation on board?

The memory flip flops work at the same 2+ ghz as the processor!?
 

jpanhalt

Joined Jan 18, 2008
10,505
Clocks of different speeds can be synchronized, usually as a simple 2^n multiple. You can also have master-slave relationships for communication wherein the slave follows the master's clock only during the communication. Clock signals do not require heavy cables, since the current supplies is quite small.

John
 

Thread Starter

The Kadet

Joined Apr 27, 2016
7
How can it be sincronized just for comunication in the bus? with multiple's is a good idea.



Clocks of different speeds can be synchronized, usually as a simple 2^n multiple. You can also have master-slave relationships for communication wherein the slave follows the master's clock only during the communication. Clock signals do not require heavy cables, since the current supplies is quite small.

John
 

AnalogKid

Joined Aug 1, 2013
8,687
For example in a computer the memory clock speed is diferent from the processor's clock speed, the bus clock speed is diferent from the piriferals bus and so on... How are these things coordinated?
Internally, the main processor clock is divided down to run all of the internal sections (instruction decode, cache management, ALU, etc.) in synchronism. It is the two external buses, memory and I/O, that need to be managed. Both are clocked by a frequency derived from the system clock, either by straight division, phase-locked loop, direct digital synthesis.

The external buses do not have to be directly related to the CPU clock. For example, the PCI bus runs at 66 MHz or 133 MHz no matter what the CPU clock is. The processor requests something, then goes away a does something else until the subsystem signals that the result is ready. Or if the processor is writing a string of data, it sends out some bytes or words, then does something else until the subsystem signals that it has digested that data and is ready for more.

Of course, the details vary a lot from one system to another.

ak
 
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hp1729

Joined Nov 23, 2015
2,304
Why is there a need for being synchronized? A system has many different clock sources. No need for synchronism. No concern for power systems.
 

hp1729

Joined Nov 23, 2015
2,304
Huh???

In a computer/microprocessor, all of the clocks need to be synchronized.
Some do. I can see processor and memory being synchronized..Video runs on its own clock speed. Serial I/O runs on its own clock speed. How many crystal oscillators are in your computer? Interrupts and polling take care of the differences.
PLL? Crystals and ceramic resonators.
 

Picbuster

Joined Dec 2, 2013
1,022
You are all right and wrong its depends on the system designed.
sync- or async sync all hardware functions use the same master async all different clocks running freely and handover information on handshake, polling or interrupt.
Handshake: you may send now or I have data for you. polling: read prog loops and collect, when flag set by sender, the data in senders xmit register clear after successful read. (eq loop circuit runs at 2GB and sender(eq RTC) processor runs at 32Khz from a different source .)
Interrupt: set flag when data is to be collected. ( do not mix up with handshake looks similar but is not)
Each mechanism, or in combination, has it's own unique pro and contra to be selected by the designer to create an optimal usage of the resources.
 

hp1729

Joined Nov 23, 2015
2,304
Some do. I can see processor and memory being synchronized..Video runs on its own clock speed. Serial I/O runs on its own clock speed. How many crystal oscillators are in your computer? Interrupts and polling take care of the differences.
PLL? Crystals and ceramic resonators.
Okay, not even memory needs to be synchronous. I agree the design is popular with simple microprocessor but larger systems may not be so. There are / were asynchronous bus systems where the processor starts a memory or I/O cycle but the length of the cycle is up to the memory or I/O device. 68K (Motorola) microprocessor based systems had this carry over from DEC PDP-11 type designs.
 

WBahn

Joined Mar 31, 2012
26,299
Why is there a need for being synchronized? A system has many different clock sources. No need for synchronism. No concern for power systems.
Huh? No concern for power systems? Try connecting two megawatt generators together without synchronizing them.

If the clocks aren't properly synchronized, then what happens when you try to sample data with a part of the system running from one clock at the same time that it is being changed by another part of the system running from a different clock. In general ... pandemonium. Some of the bits are from the old data, some are from the new data, and some may be metastable. Clock synchronization prevents this. If you can't synchronize clocks then you have to register them across the clock domain boundaries using a FIFO or FIFO-like approach. This adds complexity, circuitry, delay, and power consumption that can all be avoided if the clock domains can be synchronized.
 
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