MC9s12DG128 RTI

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MC9s12DG128 RTI

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desponia_
Contributor I
dear warriors
 
could you say me how i can calculate real time interrupt period for the following configuration on MC9s12DG128 microcontroller
 
RTICTL = 0x1F;   fbus=12 Mhz
 
could you say me what is the my rti period according to  configuration written over.
 
best regards

Message Edited by Alban on 02-28-2006 11:48 AM

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peg
Senior Contributor IV

Hi Serkan,

The RTI period is based on the Xtal frequency directly and you have selected Fxtal/16384. But you have only mentioned the Fbus which is determined by the PLL setup in conjunction with the Fxtal.

Regards

Peg

P.S. This should probably have been a new subject in the 16 bit forum!

[Alban changed subject after moved thread to proper board]

Message Edited by Alban on 02-28-2006 11:49 AM

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desponia_
Contributor I

Dear warrior

i m using in real time interrupt system clock which is driven by PLL. that is  PLLCLK=SYSCLK=RTI main clock (see attachment please)

my PLL  sysclk=12Mhz

then for RTICTL= 0x1F my ral time int period will be T= 1/[f/16384]

T= 1/ [12Mhz/16384] =1 / 732.421Hz = 1.365milisecond.

but i m seeing it on asciloscope screen as 4.1 milisecond. what is the problem.

best regards

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peg
Senior Contributor IV

Hi,

The only time the RTI clock is derived from PLLCLK is when SCM is set. This is Self Clocked Mode which is meant to keep everything alive during clock loss. In SCM the PLLCLK runs at fSCM which is quoted in the electrical specs as 1-5.5MHz.

So if your measuring 4.1 ms period of RTI then this particular device has a fSCM of 3.996MHz (16384/0.0041). Well within spec!

From CRG manual:

SCM — Self Clock Mode Status Bit

SCM reflects the current clocking mode. Writes have no effect.

1 = MCU is operating in Self Clock Mode with OSCCLK in an unknown state. All clocks are derived from PLLCLK running at its minimum frequency fSCM.

0 = MCU is operating normally with OSCCLK available.

From DG manual:

Rating                   Symbol  Min Typ Max Unit

Self Clock Mode frequency  fSCM          1              5.5   MHz

Regards

Peg

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