68300

Scott,
 
The input frequency range for the MC68332 is 25 - 50 KHz.  So, just go right ahead and us a 38.4 KHz crystal and enjoy life.
 
Now, remember, regardless of what crystal you use, you must limit the system clock frequency to what ever the part is guaranteed for.  For instance, if you buy an MC68332CFC20, you cannot run the system clock greater than 20 MHz, or 25 MHz  for a CFC25.
 
The reason that many people use the 32.768 KHz crystal is that it is a power of 2 (actually 2 to the 15th power).  Thus, if you have a 2E15 divider inside the part, you get 1 second ticks.  That's why this frequency crystal is many times referred to as a "watch" crystal.  Because this crystal is used in clocks, it enjoys very high volume, many manufacturers and good availability.
 
The big down side of 32.768 KHz crystals is that they are hard to get in automotive grade environments and temperature ranges.
 
So, if the 100 Hz tick mark is important to you and keeping "time of day" is not particularly important, use the 38.4 KHz crystal.  
 
It all depends upon your system needs.
 
Regards,
 
Charlie

-----Original Message-----
From: Scott Newell [mailto:newell@...]
Sent: Wednesday, May 22, 2002 10:47 PM
To: 68300@yahoogroups.com
Subject: [68300] Favorite clock frequency?


Just going through some old notes, and I noticed that at one time I
calculated 22.118400 MHz as a nice number: it allows you to run the UARTs
(SCI and TPU) at all the standard baud rates with no error.  Using the PLL,
it can be generated 38.400 kHz rock.  38.4 kHz also gives you a even 100 Hz
PIT rate, something 32.768 kHz can't.

So why does everyone seem to use 32.768 kHz?  Availability?

(For that matter, my next '332 design will probably use a 4.9152 MHz and a
'4060, instead of a low frequency tuning fork rock.)


newell



Yahoo! Groups Sponsor	

ADVERTISEMENT
 <http://rd.yahoo.com/M=226020.2067224.3532861.1971030/D=egroupweb/S=1706554205:HM/A=1073422/R=0/*http://www.qksrv.net/click-1084497-610300?url=http://www.tv/en/landing.shtml&HHcode=yhc3my2> 	
  <http://us.adserver.yahoo.com/l?M=226020.2067224.3532861.1971030/D=egroupmail/S=1706554205:HM/A=1073422/rand=858349274> 	

---------------------------------------------------
To unsubscribe from this group, send an email to:
68300-unsubscribe@yahoogroups.com

To learn more about Motorola Microcontrollers, please visit
http://www.motorola.com/mcu <http://www.motorola.com/mcu> 



Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service <http://docs.yahoo.com/info/terms/> .