Programmer’s Guide

Initialization

  1. Set the timer values WKUP_COUNT_LO, WKUP_COUNT_HI and WDOG_COUNT to zero.
  2. Program the desired wakeup pre-scaler value in WKUP_CTRL.
  3. Program the desired thresholds in WKUP_THOLD_LO, WKUP_THOLD_HI, WDOG_BARK_THOLD and WDOG_BITE_THOLD.
  4. Set the enable bit to 1 in the WKUP_CTRL / WDOG_CTRL registers.
  5. If desired, lock the watchdog configuration by writing 0 to the regwen bit in WDOG_REGWEN.

Watchdog pet

Pet the watchdog by writing zero to the WDOG_COUNT register.

Wakeup count and threshold access

The wakeup counter and threshold are both 64-bit values accessed via two 32-bit hi and lo registers. It is not possible to read or modify the 64-bit values in a single atomic access. Care must be taken to avoid issues due to race conditions caused by this. Below are some recommendations on how to access the counter and threshold to avoid problems.

Reading the counter

The counter might increment between the read of WKUP_COUNT_HI and the read of WKUP_COUNT_LO. If the WKUP_COUNT_LO value overflows between the two register reads the combined 64-bit value may be incorrect. Consider the scenario where the 64-bit counter value is 0x1_ffff_ffff. A read of the WKUP_COUNT_HI value gives 0x1. If the counter then increments to 0x2_0000_0000 then a read of WKUP_COUNT_LO gives 0x0000_00000. The final 64-bit value of 0x1_0000_0000 is incorrect. The pseudo code below provides a method to avoid this issue:

counter_hi = REG_READ(WKUP_COUNT_HI);
counter_lo = REG_READ(WKUP_COUNT_LO);
counter_hi_2 = REG_READ(WKUP_COUNT_HI);

// If WKUP_COUNT_LO overflowed between first and second read WKUP_COUNT_HI will
// have changed
if counter_hi != counter_hi_2 {
  // Read new WKUP_COUNT_LO value and use second WKUP_COUNT_HI read as top 32 bits
  counter_lo = REG_READ(WKUP_COUNT_LO);
  counter_hi = counter_hi_2;
}

counter_full = counter_hi << 32 | counter_lo;

Writing the counter

Between the two count register (WKUP_COUNT_HI and WKUP_COUNT_LO) writes the counter may increment. If the WKUP_COUNT_LO value overflows between a WKUP_COUNT_HI and WKUP_COUNT_LO write the intended counter value may be incorrect. For example an attempt to clear the counter to 0 could result in a counter value of 0x1_0000_0000. It is recommended the wakeup timer is disabled with WKUP_CTRL before writing to the WKUP_COUNT_HI and WKUP_COUNT_LO registers to avoid this problem.

Reading the threshold

The hardware does not alter the value of the WKUP_THOLD_LO and WKUP_THOLD_HI registers so there are no race conditions in reading them.

Writing the threshold

When writing to WKUP_THOLD_LO and WKUP_THOLD_HI between the two writes the 64-bit threshold is effectively an interim value that’s not intended to be the real threshold. It is possible the interim threshold is lower than the previous threshold triggering a spurious wakeup. Use the method in the pseudo code below to avoid this issue:

disable_wakeup_interrupt();

// Guaranteed 64-bit threshold greater than or equal to old threshold. This
// prevents an interrupt caused by the threshold decreasing.
REG_WRITE(WKUP_THOLD_LO, UINT32_MAX);
// Guaranteed 64-bit threshold greater than or equal to intended threshold. If
// the counter reaches this value before we've completing the final write then
// the interrupt would have happened with the intended threshold as well.
REG_WRITE(WKUP_THOLD_HI, new_thold >> 32);
// 64-bit threshold now intended value
REG_WRITE(WKUP_THOLD_LO, new_thold & 0xffff_ffff);

enable_wakeup_interrupt();

Interrupt Handling

If either timer reaches the programmed threshold, interrupts are generated from the AON_TIMER module. Disable the wakeup timer by clearing the enable bit in WKUP_CTRL. Reset the timer if desired by clearing WKUP_COUNT_HI and WKUP_COUNT_LO and renable by setting the enable bit in WKUP_CTRL. Clear the interrupt by writing 1 into the Interrupt Status Register INTR_STATE.

If the timer has caused a wakeup event (WKUP_CAUSE is set) then clear the wakeup request by writing 0 to WKUP_CAUSE.

If {WKUP_COUNT_HI, WKUP_COUNT_LO} remains above the threshold after clearing the interrupt or wakeup event and the timer remains enabled, the interrupt and wakeup event will trigger again at the next clock tick.

Device Interface Functions (DIFs)