Ruth Hendrix

HCS Bus Frequency

Discussion created by Ruth Hendrix Employee on Jan 26, 2006
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Posted: Sun Jun 05, 2005 8:58 pm    
Hi Guys
I am relatively new to the HCS12 and one thing I notice strait way is that the info on the datasheets seems to be a bit illusive in some cases.
The thing that really surprises me is the way that Freescale deals with the Frequency that the Micro can work at. It seems that they are giving you a value but are almost afraid of saying it… On the Datasheet for the micro I am using ( MC9S12C32), they say that the max bus frequency is 25MHz so Can I understand by that, that the crystal to be used can be 50MHz. As any of you used it up to that frequency? Because to me, after reading the datasheet and how much it talks about a 16MHz Crystal, it seems that there are issues about using a higher value crystal. When I see this kind of sort of playing down the capabilities of such a great product, it makes me a bit nervous.
What are your experiences?
I would appreciate some feedback on this as it may help others.
Posted: Sun Jun 05, 2005 9:58 pm
The S12s all have a PLL which can multiply the crystal frequency up to what you require. Most people use either 4, 8 or 16MHz because they are cheap.
Posted: Mon Jun 06, 2005 9:21 am    

The datasheet (link) p°111 of the User Guide V1 indicates the XTAL max depending on the configuration of the oscillator.
It is either 16MHz (Colpitts) or 40MHz (Pierce).
But you could feed the MCU with a frequency of 50MHz if you want, with an oscillator or output of another chip.

I think datasheet is written knowing how most people would use the MCU. As Freegeeks said, because of the cost, I've never seen a S12 clocked from a 40MHz XTAL... Especially when you can have the same job done from a 16MHz.

You mention "Issues" with higher crystals. Well, each crystal has its own characteristics and the USER should design a proper oscillator to clock the micro. Safety factor of the oscillator is what you have to evaluate, independantly of what the MCU manufacturer is telling you (FSL or others). FSL, by its datasheet, guarantee you can put a 16/40MHz XTAL and it will work if you designed properly. If you put more it is not guaranteed.

How do you think you get limited by this?
(knowing that the PLL let you get almost all frequencies)

Posted: Mon Jun 06, 2005 12:33 pm    
I generally use an 8MHz crystal. It is generally safer to run a slower crystal due to noise depending on your enviornment I belive. Here is how I clock it up to a 24MHz bus:

movb    #2,SYNR                       ;pll clock is defined as (2 * (SYNR + 1)) * Crystal                                          ;core clock =   pll                                          ;bus clock  =   pll/2                                          ;!!my C32 dev board uses an 8MHz crystal!!                                  brclr   CRGFLG,%00001000,*            ;wait for PLL to lock on to the new frequency                                          ;and branch if bit not set back onto self    movb    #%10000000,CLKSEL             ;select PLL clock as our new frequency

Posted: Tue Jun 07, 2005 3:13 pm    
Thank you all for your help.
It seems that I have to look into the use of the PLL.
The friend that introduced me to the Motorola Micros as always said that they are far superior to any other competitor but it still comes very hard that they are so packed with features. I normally stayed away from the PLL as I had this impression that it will mean more dificulties, but it's usage seems more the norm than the exception.
thanks for the heads-up.
best regards