Electronics

Jack// ani · Guest

Hi all,

I'm confused as how to choose a proper frequency crystal for my PIC
uC like....16f84a,f628,f877 and so on... I have few question and
confusion, here it goes.....

1.Which is better to use, A high frequency crystal(say 10Mhz) or low
frequency(like 4Mhz). Most common one I've seen is 4Mhz, I want are
the considerations as to decide which frequency should be used.

2.I have read that you can crack up your PIC up to 20Mhz. Is that
right? Will this cause overheating and large current drain, similar to
overclocked cpu?

Thanks

Neil · Guest

"Jack// ani" <nospam4u_jack@yahoo.com> wrote in message
news:1107967506.040574.78580@g14g2000cwa.googlegroups.com...

Robert Monsen · Guest

Jack// ani wrote:

Byron A Jeff · Guest

In article <1107967506.040574.78580@g14g2000cwa.googlegroups.com>,
Jack// ani <nospam4u_jack@yahoo.com> wrote:
<Hi all,
<
<I'm confused as how to choose a proper frequency crystal for my PIC
<uC like....16f84a,f628,f877 and so on... I have few question and
<confusion, here it goes.....
<
<1.Which is better to use, A high frequency crystal(say 10Mhz) or low
<frequency(like 4Mhz). Most common one I've seen is 4Mhz, I want are
<the considerations as to decide which frequency should be used.

Depends on the application. In general you want the fastest speed that will
allow your application to operate properly, but no more.

<
<2.I have read that you can crack up your PIC up to 20Mhz. Is that
<right? Will this cause overheating and large current drain, similar to
<overclocked cpu?

20 Mhz is possible. Higher current drain of course. However these chips are
rated for 2 Mhz operation. So you're not actually overclocking them.

Now the one last issue to throw in is that many of current PIC chips have
what Michrochip calls nanowatt technology. The most interesting item about
this technology is that each chip has an internal oscillator that goes up to
8 Mhz. So you don't need a crystal at all up to 8 Mhz.

In the 18 pin form factor the 16F88 is the 16F family chip that has this.

I have an overview of the 16F88 here:

http://www.finitesite.com/d3jsys/16F88.html

BAJ

Anthony Fremont · Guest

"Byron A Jeff" <byron@cc.gatech.edu> wrote

John Fields · Guest

On 9 Feb 2005 08:45:06 -0800, "Jack// ani" <nospam4u_jack@yahoo.com>
wrote:

Jack// ani · Guest

Thanks for all replies.

Byron A Jeff · Guest

In article <bQuOd.32713$sr1.125@fe2.texas.rr.com>,
Anthony Fremont <spam@anywhere.com> wrote:
<
<"Byron A Jeff" <byron@cc.gatech.edu> wrote
<
<> I have an overview of the 16F88 here:
<>
<> http://www.finitesite.com/d3jsys/16F88.html
<
<Hey Byron,
<
<You have an error on that page. You specify (incorrectly) that CMCON
<and ADCON1 must be initialized to turn off the analog. The 16F88
<actually powers up with these two registers ready for digital i/o.
<CMCON starts with a value of 0x07 and ADCON1 starts with 0x00.
<
<OTOH, ANSEL must be initialized as (for some ignorant reason) it does
<power up with all the ADC pins set as analog.

Thanks for the tip. I edited that page from my 16F628 page and I didn't
update that section. Unfortunately I can't get to it right now.

And nothing about analog powerup is ignorant. Microchip posits, and I believe
correctly so, that you can have a situation where something has an analog
input could come up with a voltage that could cause signicant current drain
if the pin is in digital mode. So they have almost always set up analog/digital
inputs as analog on powerup. It's been like that since the 16C71 days nearly
15 years ago.

BAJ

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