Minimum clock speed for pic 16f877a

[binukannur08]

Hai all
May I know the minimum speed for pic16f877a
According to datasheet it is 0-20MHz , but at 0MHz how it possible.

 

[FvM]

0-20 MHz means fully static logic. There's no minimum clock speed. You can use arbitrary low clock frequencies and you can start and stop the clock whenever you want. Some peripherals e.g. the charge balancing ADC might not work sufficiently at very clock frequencies.

 

[Analog Ground]

Great answer with the catch of the possible A/D problem. The A/D has an option to use an RC oscillator local to the A/D. This may be a work around if a low frequency main clock is a problem.

 

[binukannur08]

Is it work as stand by mode in 0Mhz speed?
what happen when I choose 0 clock
Iam using a crystal oscillator then how can I chane it during each operation mentined above A/D especially

 

[wp100]

Hi,

Wonder what your thinking behind using such low clock frequencies is ?
Why cannot you use the software to delay the program code instead ?

You say you are using a 'crystal oscillator ' do you mean a crystal attached to osc1/2 pin or a totally separate crystal based oscillator ? which if containing its own divider network would allow you to change frequencies to be fed to the 877A.

You probably want to look at the more modern 16F and 18F chips which have the features you are probably after like 'sleep' mode and selectable oscillator speed etc etc.
Look at the 18F4520 and 18F45K22 datasheets to see what I mean.

 

[binukannur08]

Hai,
Actually I didnot understand what it
mean clock speed 0-20MHz

 

[wp100]

Hai,
Actually I didnot understand what it
mean clock speed 0-20MHz

Hi,

When you program a micro like a Pic, it creates a set of 'Instructions' that the micro executes to complete your task.

Most instruction take 1 'machine cycle' to complete.

A machine cycle is the oscillators frequency typically divided by 4.

So if you are using a 4 mhz crystal divided by 4 to give a machine cycle /speed of 1 mhz.

Typical crystal speeds for diy work are anything from 1mhz to 20 mhz though sometimes a 32,678hz ( watch crystal) is used because that readily divides down to exactly 1 second.

Does that answer your question ok ... ?

 

[PA3040]

Frequency domain starts by 0 or DC F = 1/T

In the PIC 16f877a supports fully static logic as pointed by FvM that range in this case max frequency is 20MHz

 

[binukannur08]

Hai.
Thank u for all your helps
In most of the tutorial and example I found discussing about 4MHz and 20MHz clock.
Can I use a 10MHz or 7MHz etc.
Or it is needed to exactly divisible by 4, ? ie for giving exact machine code

 

[wp100]

Hai.
Thank u for all your helps
In most of the tutorial and example I found discussing about 4MHz and 20MHz clock.
Can I use a 10MHz or 7MHz etc.
Or it is needed to exactly divisible by 4, ? ie for giving exact machine code

Hi,

You can use whatever frequency you like; virtually all crystals will divide exactly by 4

You will find the majority of projects use 4,8,16 or 20 mhz crystals and these are common cheap to buy values.

Using something like 7mhz is fine if you are creating a project from scratch, but if you used it on a project that was designed around say, 4 or 8 or 16 mhz you might find it the program does not run properly as all the timings will be affected to some degree.

Don't forget that a lot of Pic chips have inbuilt oscillators so no need for a crystal.

 

[binukannur08]

can you give information about how to use internal clock in our main programe.
I think yet it is for A/D and other such things

 

[wp100]

can you give information about how to use internal clock in our main programe.
I think yet it is for A/D and other such things

Hi,

First you have to check the datasheet or microchips site to see what chips have the optional internal oscillator function.

Whatever chip you use, you have to specify the oscillator type in the Config lines of your program code, it could be for a crystal, R/C or internal etc.

Some chips also have several different internal frequencies you can select, again from within your program code.

I think yet it is for A/D and other such things

No, its is for everything, every single instruction is controlled by the speed of whatever oscillator you use; like what I said earlier, FOsc/4 = 1 machine cycle = 1 instruction (most of them)

If you start doing some programming, like a simple flashing led program, then I think it will all become a lot clearer for you.