Question on PIC's instruction cycle time

valemike · Guest

I'm using an 8MHz crystal.

Is my instruction time 4/800000 = 500 ns?
Or is it 1/8000000 = 125 ns?

What does the 4X PLL do for me? Gives me 1 / (4 x 8000000) = 31ns per instruction?

I've tried deciphering the timing diagrams on the datasheet, but am not quite sure.

Reason why i ask is that i'm dealing with a motor whose encoder feeds into RB0 at rates of up to 546 uS. I'll be stuck in ISRs all the time then! (Yeah i know, i should have fed the encoders to externally clock TOCKI and TOCKO, but too late - hardware is done).

Thanks,
Mike

Chas · Guest

The instruction cycle is 4 clocks long. Therefore, at 8MHz, each instruction will take 500ns. At 546us per interrupt, you still can execute more than 1000 instructions between interrupts. If you enable the 4X PLL you would quadruple that number.

If you don't have any other interrupts and have a bit of courage, you might look at using the global interrupt. I have used it for a clock tick interrupt via timer 1 and it requires very few instructions to get into the interrupt and back out. Search this forum for other threads.

valemike · Guest

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

valemike · Guest

Thanks for verifying the math Mark! I'm relieved to find out that my hardware is still feasible.

rwyoung · Joined: 12 Nov 2003 Posts: 563 Location: Lawrence, KS USA

I didn't see where you said specifically WHICH PIC you are using.

Some of the PIC18Fs (I'm assuming you are using those since you mention the x4 PLL) have a maximum clock speed of 25MHz while others run to 40MHz. It depends on the part and in some cases in what mode you run the part.

If your part has a maximum of 25MHz clock, then you cannot use an 8MHz plus x4 PLL option.

Check the data sheet first.
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

Without looking at every pic18 datasheet, the max clock frequency is 25MHz. If you want 40MHz then you have to use the 4X with a 10MHz clock. I don't know of any 18's that will only run at 25MHz unless there is a problem and an errata indicates the fix is to run at a slower speed.

rwyoung · Joined: 12 Nov 2003 Posts: 563 Location: Lawrence, KS USA

A good example is the PIC18F6620/6720/8620/8720 which has a top speed of 25MHz in microcontroller mode and 16MHz in microprocessor mode. So as I understand the chip, you can put a 6MHz and use the x4 PLL to get to 24MHz for example, but you can't put a 10MHz out there and expect the chip to run as a 40MHz part. Find a 6.25MHz crystal and you can boost yourself upto the full 25MHz.

Check the "External Clock Timing Requirements" table in the Electrical Characteristics sections of the datasheets, YMMV.

In technical terms, the '8720 has a zillion I/O and oodles of memory and can be run in either the single chip or multi-chip (microprocessor) mode.

On the other hand, the new and improved '8722 (should just be coming on-line at the usual distributor sources) is supposed to run out to 40MHz.

And a few of the PIC18s are listed as 48MHz parts.
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!

valemike · Guest

It's the 18F448 i'm using. It says in the manual that it can go to 40Mhz.

rwyoung · Joined: 12 Nov 2003 Posts: 563 Location: Lawrence, KS USA

newguy · Joined: 24 Jun 2004 Posts: 1909

Amen to that, Rob.

I'm presently teaching a pic programming course to two groups of technologists. Roughly 95% of them are now at the point where I can give them a task that they'll have to get the pic to perform, and I never have to help them. They simply call me over when it's done. If there's anything they don't know, they look it up themselves. That's the group that I'd like to hire from, if I was an employer.

The rest......ugh. You ever heard the phrase "you can lead a horse to water, but drowning one is a real b***h." It doesn't matter how many times I try to point them in the right direction, and gently suggest that they read the data sheet for the part they have to interface to, but.....I guess I have to take solace in the fact that most of them get it.