Dear All,
I am looking to determine the precise periodic frequency of the LPIT peripheral when operating in Compare Mode. Unfortunately, Figure 46-5 on p1392 (Rev 8. 06/2018) does not show the sequence in sufficient detail.
In short, my question is which formula is correct?
f_periodic = f_peripheral / (TVAL);
or
f_periodic = f_peripheral / (TVAL + 1);
The cook-book example ("hello_interrupts") suggests the first version since is specifies a divider of 40000000:
void LPIT0_init (void) {
PCC->PCCn[PCC_LPIT_INDEX] = PCC_PCCn_PCS(6); /* Clock src=6 (SPLL2_DIV2_CLK)*/
PCC->PCCn[PCC_LPIT_INDEX] |= PCC_PCCn_CGC_MASK; /* Enable clk to LPIT0 regs */
LPIT0->MCR = 0x00000001; /* DBG_EN-0: Timer chans stop in Debug mode */
/* DOZE_EN=0: Timer chans are stopped in DOZE mode */
/* SW_RST=0: SW reset does not reset timer chans, regs */
/* M_CEN=1: enable module clk (allows writing other LPIT0 regs)*/
LPIT0->MIER = 0x00000001; /* TIE0=1: Timer Interrupt Enabled fot Chan 0 */
LPIT0->TMR[0].TVAL = 40000000; /* Chan 0 Timeout period: 40M clocks */
LPIT0->TMR[0].TCTRL = 0x00000001; /* T_EN=1: Timer channel is enabled */
/* CHAIN=0: channel chaining is disabled */
/* MODE=0: 32 periodic counter mode */
/* TSOT=0: Timer decrements immediately based on restart */
/* TSOI=0: Timer does not stop after timeout */
/* TROT=0 Timer will not reload on trigger */
/* TRG_src=0: External trigger soruce */
/* TRG_SEL=0: Timer chan 0 trigger source is selected*/
}However, the reference manual says the TVAL value is loaded when the timer reaches zero. So, if the peripheral spends a full clock at zero value, then the periodic value in the example is slightly wrong since there are (TVAL+1) clocks per interrupt.
This will matter more when the timer values are short (e.g. TVAL = 100), where it would result in a timing error of roughly 1%.
