|Self Wake-up Timer (WKT)|
The self wake-up timer is a 32-bit, loadable down-counter. Writing any non-zero value to this timer automatically enables the counter and launches a count-down sequence. When the counter is being used as a wake up timer, this write can occur just prior to entering a reduced power mode.
When a starting count value is loaded, the self wake-up timer automatically turns on, counts from the pre-loaded value down to zero, generates an interrupt and/or a wake up request, and then turns itself off until re-launched by a subsequent software write.
The self wake-up timer can be clocked from two alternative clock sources:
The IRC-derived clock is much more accurate than the alternative, low-power clock. However, the IRC is not available in most low-power modes. This clock must not be selected when the timer is being used to wake up from a power mode where the IRC is disabled.
The alternative clock source is a (nominally) 10 kHz, low-power clock, sourced from a dedicated oscillator. This oscillator resides in the always-on voltage domain, so it can be programmed to continue operating in Deep power-down mode when power is removed from the rest of the part. This clock is also be available during other low-power modes when the IRC clock is shut-down.
The Low-Power oscillator is not accurate (approximately +/- 40 % over process and temperature). The frequency may still drift while counting is in progress due to reduced chip temperature after a low-power mode is entered.
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