S9KEAZN8 minimum PIT or FTM period

ralfvandenbooga — Thu, 20 Sep 2018 11:22:26 GMT

Hello,

Currently I'm working with an S9KEAZN8AMTG micro and I'm trying to achieve a very high timer interrupt frequency.

My goal is to have a FTM timer which I can use to generate sub-microsecond interrupts because I need an accurate way to execute actions on t=4 microseconds and t=10 microseconds. Until so far I have not been able to set the interrupt rate faster than ~1 microsecond.

I have set the 32kHz IRC to supply the FLL with a clock, and this clock is then passed to the FTM module without any clock prescaling active. As far as I can see this should give the FTM module a clock of ~48MHz. I assume that when I set the modulo value to 24, I should have an interrupt every 0.5 microseconds.

void clock_init (void) { /* FLL Enabled with Internal clock */
OSC_CR = 0x00; /* (default value) */
                           /* OSCEN=0: OSC module disabled */
                           /* OSCSTEN=0: OSC clock disabled in Stop mode */
                           /* OSCOS=0: Ext clk source (don't care here) */
                           /* RANGE=0: Low Freq range of 32 KHz */
                           /* HGO=0: low power High Gan Osc mode (don't care here) */
ICS_C2 = 0x20; /* Use defaults until dividers configured (default) */
                           /* BDIV=1: divided by 2 */
                           /* LP = 0: FLL Not disabled in bypass mode */
ICS_C1 = 0x04; /* Internal ref clock is FLL source (default)*/
                           /* CLKS=0: Output of FLL is selected to control bus freq */
                           /* RDIV=0: Ref divider = 1 since RANGE = 0 */
                           /* IREFS=0: Int Ref clock is selected */
                           /* IRCLKEN=0: ICSIRCLK is inactive */
                           /* IREFSTEN=0: Int ref clk is disabled in Stop mode */
while ((ICS_S & ICS_S_LOCK_MASK) == 0); /* Wait for FLL to lock*/
SIM_CLKDIV = 0x01000000; /* OUTDIV1 = 0; Core/sysclk is ICSOUTCLK div by 1 */
                                                /* OUTDIV2 = 1 bus/flash is OUTDIV1/2 */
                                                /* OUTDIV3 = 1; FTMs, PWT is ICSOUTCLK div by 2 */
ICS_C2 = 0x00;    /* BDIV div 1:1 - increases bus/flash freq */
}

void init_FTM(void) {

ftm0_nvic_irq_init();
SIM_SCGC |= SIM_SCGC_FTM0_MASK; /* Sys Clk Gate Ctrl: enable bus clock to FTM0 */

FTM0_SC = 0x00000048; /* FTM0 module settings for desired channel modes: */
                                          /* CWMS (Center aligned PWM Select) = 0 (default, up count) */
                                          /* TOIE (Timer Overflow Interrupt Ena) = 0 (default) */
                                          /* CLKS (Clock source) = 0 (default, no clock; FTM disabled) */
                                          /* PS (Prescaler factor) = 0 (default). Prescaler = 2**0 = 1 */
FTM0_MOD = 24; /* FTM0 counter final value */
                              /* FTM0 Period = MOD-CNTIN+0x0001 ~= 24 ctr clks */
                              /* 24 clks x 1 sec/48M clks = 0.5us (2MHz) */
}

Is it possible to get the FTM timer faster than an interrupt every ~1 microsecond, something like 0.2 microseconds, or is this not possible?

Thank you,

Ralf

Re: S9KEAZN8 minimum PIT or FTM period

ralfvandenbooga — Fri, 21 Sep 2018 11:33:42 GMT

It turned out the settings I used to configure the timer were correct. The pitfall was in the fact that I didn't account for the overhead/cpu-cycles required to jump to the ISR of the timer. This caused my interrupt frequency not to get higher than ~1MHz.

I only needed to do 1 thing in my ISR and after that the ISR was not required anymore. So I set the timer to give an interrupt at the time I required it and that solved my issue. Now I have a resolution of ~20ns / timer tick which is fine for me.

topic Re: S9KEAZN8 minimum PIT or FTM period in Kinetis Microcontrollers

S9KEAZN8 minimum PIT or FTM period

Re: S9KEAZN8 minimum PIT or FTM period