dsPIC30F Oscillator Requirements

[Peter Restall]

Morning Chaps,

Just a quick question about the dsPIC30F (2010) oscillator/PLL if I may.
Reading the datasheet (DS70118G), the table on page 122 says that the range of
inputs for external clocks (with PLL enabled) is 0-40MHz, so long as the
maximum final frequency of 120MHz is abided by.  My thinking was to inject a
15MHz input and use the 8x PLL to obtain 120MHz - ideally I'd like to use a
7.5MHz crystal with OSC1/2 and 16x PLL, but I can't find any place that sells
that frequency quartz.

HOWEVER, on pages 156 and 157, there are two tables that list the minimum and
maximum ranges for the PLL inputs as 4-10MHz; with a 10MHz clock you would
either end up with 160MHz (x16 PLL - and exceeding specs), or 80MHz (x8 PLL -
but only 2/3 of the maximum possible).

My question is - which table should I believe ?  Am I missing something, or is
this a contradiction in the datasheet ?  My gut feeling is to take the most
conservative of the figures - ie. don't inject more than 10MHz and expect the
PLL to like it.

And a second question would be - where can I get 7.5MHz crystals from in the
UK in small quantities (ie. 10) and at a reasonable price ?  I've tried
Farnell, RS, Rapid, Digikey, eBay and Quarndon but can't see any; I can pick
up 7.3728MHz ones easily, but you lose over 0.5 MIPS with those.  Not likely
to be a problem in my case I dare say, but still...

Cheers,

Pete Restall
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[Olin Lathrop]

Peter Restall wrote:
> ideally I'd like to use a
> 7.5MHz crystal with OSC1/2 and 16x PLL, but I can't find any place
> that sells that frequency quartz.

If you need enough of a crystal you can get a custom frequency made.  If
not, the standard frequency of 7.3728MHz is widely available since it is a
multiple of common baud rates.  That means you run at 29.49 MIPs instead of
30.00 MIPs, which is only 1.7% less.  You do get the advantage that it can
hit any of the common baud rates exactly.


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[Peter Restall]

> HOWEVER, on pages 156 and 157, there are two tables that list the minimum and
> maximum ranges for the PLL inputs as 4-10MHz; with a 10MHz clock you would
> either end up with 160MHz (x16 PLL - and exceeding specs), or 80MHz (x8 PLL -
> but only 2/3 of the maximum possible).

The oscillator datasheet for the dsPIC30F family (DS70054E) actually backs
this up, so I guess I need a 7.5MHz crystal.  Shame I didn't go for one of
the dsPICs that have an (HS / 2) * PLL mode, then I could've just used one
of the abundance of 15MHz crystals I have.  C'est la vie.

Cheers,

Pete Restall
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[mcd-3]

----- Original Message -----
From: "Peter Restall" <pete@...>
To: <piclist@...>
Sent: Saturday, October 17, 2009 5:20 AM
Subject: [PIC] dsPIC30F Oscillator Requirements

As far as i can tell, the PLL input is limited to 10 MHz, so a 20 MHz
oscillator will get you a 20 MHz clock since you can't use the PLL, but a 5
MHz oscillator can get you an 80 MHz clock.


> And a second question would be - where can I get 7.5MHz crystals from in
> the
> UK in small quantities (ie. 10) and at a reasonable price ?  I've tried
> Farnell, RS, Rapid, Digikey, eBay and Quarndon but can't see any; I can
> pick
> up 7.3728MHz ones easily, but you lose over 0.5 MIPS with those.  Not
> likely
> to be a problem in my case I dare say, but still...

If you want a 7.5 MHz crystal you need to get it custom made.  These are
expensive, although perhaps not as expensive as you might think.  7.3728 MHz
crystals are a commodity item, and perhaps 1/20th to 1/50th the price of a
custom crystal.

My favorite crystal supplier is International Crystal, although there are
others.  International will take several weeks to make your crystal, and I'm
guessing will cost somewhere around 15 pounds.  You probably will not find a
7.5000 MHz crystal on the shelf, it will have to be made for you.  There are
other crystal manufacturers, some a little cheaper than International, but
IMO International is very thorough and makes sure you get exactly what you
need.

One possible strategy is to use a programmable oscillator like those from
Epson, although depending on your application, the phase noise can be a
probem.  These cost closer to 2 pounds, and can be delivered overnight.
Still expensive compared to an off the shelf crystal which can be had here
in the U.S. for as little as 27p.

--McD

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[Vicent Colomar Prats]

Hi Peter, most of dspic 30F have an 7,37MHz internal oscillator that can do
what you want without the need of an external oscillator. Of course the
maximal teorical speed would be acomplished with an 7,5MHz. 118,7 compared
to 120 is not a big difference with normal applications.

2009/10/17 Peter Restall <pete@...>

>
> > HOWEVER, on pages 156 and 157, there are two tables that list the minimum
> and
> > maximum ranges for the PLL inputs as 4-10MHz; with a 10MHz clock you
> would
> > either end up with 160MHz (x16 PLL - and exceeding specs), or 80MHz (x8
> PLL -
> > but only 2/3 of the maximum possible).
>
> The oscillator datasheet for the dsPIC30F family (DS70054E) actually backs
> this up, so I guess I need a 7.5MHz crystal.  Shame I didn't go for one of
> the dsPICs that have an (HS / 2) * PLL mode, then I could've just used one
> of the abundance of 15MHz crystals I have.  C'est la vie.
>
> Cheers,
>
> Pete Restall
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> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

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