# What is meant by instruction cycle and clock cycle ?

Discussion in 'Embedded Systems and Microcontrollers' started by abhimanyu143, Jan 28, 2015.

1. ### abhimanyu143 Thread Starter Member

Aug 25, 2014
211
1
hello ,
I need some help , I am confused on some terms.
1. The instruction time ?
2. instruction cycle ?
3. clock cycle ?

instruction cycle
fetch-and-execute cycle, fetch-decode-execute cycle
8051 runs on 12MHz

What is meant by instruction cycle and clock cycle ?
How is the 'clock cycle' of 8051 MCU related to the 'crystal' oscillator ?

2. ### sevenfold4 Member

Jan 12, 2015
80
7
The clock cycle is the period of the clock that you're feeding the to the micro (e.g. , a clock cycle takes 1us if the oscillator's frequency is 1MHz). The instruction cycle depends on the internal structure of the microcontroller or processor you use. E.g. , for a PIC an instruction cycle takes 4 clock cycles, for an 8051 it takes 12 cycles. This is because an instruction has multiple stages, like fetch, decode, execute and writeback.

http://www.8051projects.net/plugins/forum/forum_viewtopic.php?41545.0#post_41554

This page shows how Timer values are calculated, maybe it will serve you as an example.

Last edited: Jan 28, 2015
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3. ### abhimanyu143 Thread Starter Member

Aug 25, 2014
211
1
I am explaining with your example.
Example: a clock cycle takes 1us if the oscillator's frequency is 1MHz
fetch-and-execute cycle,
explanations: 1 us for fetch cycle and 1 us for execute cycle. that means it take 2 us
fetch-decode-execute cycle
explanations: it take 3 us.

4. ### sevenfold4 Member

Jan 12, 2015
80
7
Anyways
1 Clock period = 1/frequency of the oscillator
1 Machine cycle = 12 oscillator periods (In case of 8051, because of the inner structure)
1 Instruction cycle = 1-4 machine cycles (Depending on the instruction given for 8051)
So if 1 machine cycle would calculate to 1us, then 1 instruction would take 1us (Depending on the instruction)
I think... In the past i just did the calculations and never really had to go too deep into it.
Correct me if i am wrong

5. ### abhimanyu143 Thread Starter Member

Aug 25, 2014
211
1
actually I don't understand what do you mean, still I am confused on following terms.
1. The instruction time ?
2. instruction cycle ?
3. clock cycle ?

6. ### sevenfold4 Member

Jan 12, 2015
80
7
1. I guess that would be the time it takes to run 1 cycle?
2. Is the time to run the instructions.
3. Is the speed of the clock depending of the crystal and structure of your mcu

7. ### vikasbly44 New Member

Jan 19, 2015
13
1
The instruction cycle is the time period during which one instruction is fetched from memory and executed when a computer is given an instruction to machine language.

The clock cycle is the time between two adjacent pulses of the oscillator that sets the tempo of the computer processor.