MK20 LPTMR clocked from ERCLK32K

[simonhaines]

I am trying to clock the low power timer on a MK20FX512 from a 32.768kHz crystal attached to XTAL32/EXTAL32, but the timer does not work from that clock source.

Here is what I have tried:

• Clocking the LPTMR from the 1kHz LPO works as expected.
• Clocking the LPTMR from the OSC0ERCLK works as expected (this is a 16MHz clock).
• The RTC derived from XTAL32/EXTAL32 works as expected and is accurate.

The documentation states that setting LPTMR0_PSR[PCS] to 0b10 uses 'ERCLK32K — secondary external reference clock' as the clock source, however the timer does not operate from that clock source.

Here is some code that demonstrates how I am setting up the peripherals.

// Enable real-time clock
SIM->SCGC6 |= SIM_SCGC6_RTC_MASK;
// Disable time counter for setup
RTC->CR = 0;
RTC->SR = 0;
// RTC oscillator (EXTAL32/XTAL32) is 12.5pF +/-10ppm, enable oscillator and enable output
RTC->CR = RTC_CR_SC8P_MASK | RTC_CR_SC4P_MASK | RTC_CR_CLKO_MASK | RTC_CR_OSCE_MASK;
RTC->TSR = 0x01;  // Reset the seconds count, clearing the 'time invalid' flag
RTC->SR = RTC_SR_TCE_MASK;  // Enable time counter
// Select RTC oscillator as source for ERCLK32K
SIM->SOPT1 |= SIM_SOPT1_OSC32KSEL_MASK;

// Enable LPTIMER trigger interrupt at 10Hz
SIM->SCGC5 |= SIM_SCGC5_LPTIMER_MASK;
LPTMR0->CSR = 0;  // Disable timer for setup

LPTMR0->PSR = LPTMR_PSR_PCS(0b10) | LPTMR_PSR_PRESCALE(0b0110);  // Select RTC 32.768kHz clock /32768 = 1kHz
LPTMR0->CMR = 100;  // Set compare value: 1kHz / 100 = 10Hz

// Enable interrupts
NVIC_SetPriority(LPTimer_IRQn, 0x02);
NVIC_EnableIRQ(LPTimer_IRQn);

// Enable timer and interrupt
LPTMR0->CSR = LPTMR_CSR_TIE_MASK | LPTMR_CSR_TEN_MASK;

volatile int rtc, status;
while (true) {
 rtc = RTC->TSR;    // This increments as expected
 status = RTC->SR;  // This is 0b10000 = 'time counter enabled'
}‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

The timer interrupt simply toggles a pin that can monitor timer operation.

void LPTimer_IRQHandler() {
 // Clear timer compare flag
 LPTMR0->CSR |= LPTMR_CSR_TCF_MASK;

 // Toggle a status pin SPARE1 (PTC12)
 PTC->PTOR |= (1 << 12);
}‍‍‍‍‍‍‍

When setting LPTMR0_PSR[PCS] to either 0b01 (LPO) or 0b11 (OSC0ERCLK), the interrupt triggers as expected. When the clock source is set to 0b10 (ERCLK32K), the interrupt is never triggered.

Am I missing something in my setup?

[mjbcswitzerland · Accepted solution]

Hello Simon

// RTC oscillator (EXTAL32/XTAL32) is 12.5pF +/-10ppm, enable oscillator and enable output
RTC->CR = RTC_CR_SC8P_MASK | RTC_CR_SC4P_MASK | RTC_CR_CLKO_MASK | RTC_CR_OSCE_MASK;

This configuration is not correct.

To enable the RTC output to other peripherals the CLKO mask needs to be set to '0' and not '1'. At '1' it is 'masking' the output and so it is not allowing the LPTMR to operate from it.

PTC->PTOR |= (1 << 12);

has redundancy, correct and more efficient use

PTC->PTOR = (1 << 12);

Regards

Mark

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[simonhaines]

Hello Mark, thanks for spotting my mistake. I had confused that bit to mean 'enable the clock output to other peripherals'. It seems that no matter how many times I read the data sheets, I still get tripped up over simple things like this.

Thanks again for all your valuable work on this forum. Every time I need to implement a new feature, I'll look for your comments here for guidance. Cheers,

Simon.