Datasheet


NOTE
the value is rounded up to the nearest integer so the wait time will be just over 1ms
2.16.4. XOSC Counter
The COUNT register provides a method of managing short software delays. Writing a value to the COUNT register
automatically triggers it to start counting down to zero at the XOSC frequency. The programmer then simply polls the
register until it reaches zero. This is preferable to using NOPs in software loops because it is independent of the core
clock frequency, the compiler and the execution time of the compiled code.
2.16.5. DORMANT mode
In DORMANT mode (see Section 2.11.3) all of the on-chip clocks can be paused to save power. This is particularly
useful in battery-powered applications. The RP2040 is woken from DORMANT mode by an interrupt either from an
external event such as an edge on a GPIO pin or from the on-chip RTC. This must be configured before entering
DORMANT mode. If the RTC is being used to trigger wake-up then it must be clocked from an external source. To enter
DORMANT mode the programmer must then switch all internal clocks to be driven from XOSC or ROSC and stop the
PLLs. Then a specific 32-bit value must be written to the DORMANT register in the chosen oscillator (XOSC or ROSC) to
stop it oscillating. When exiting DORMANT mode the chosen oscillator will restart. If XOSC is chosen then the frequency
will be more precise but the restart time is longer due to the startup delay (>1ms on the RP2040 reference design (see
Hardware design with RP2040, Minimal Design Example)). If ROSC is chosen then the frequency is less precise but the
start-up time is very short (approximately 1μs).

NOTE
the PLLs must be stopped before entering DORMANT mode
SDK: https://github.com/raspberrypi/pico-sdk/tree/master/src/rp2_common/hardware_xosc/xosc.c Lines 50 - 55
50 void xosc_dormant(void) {
51 // WARNING: This stops the xosc until woken up by an irq
52 xosc_hw->dormant = XOSC_DORMANT_VALUE_DORMANT;
53 // Wait for it to become stable once woken up
54 while(!(xosc_hw->status & XOSC_STATUS_STABLE_BITS));
55 }

WARNING
If no IRQ is configured before going into DORMANT mode the XOSC or ROSC will never restart.
See Section 2.11.5.2 for a complete example of DORMANT mode using the XOSC.
2.16.6. Programmer’s Model
SDK: https://github.com/raspberrypi/pico-sdk/tree/master/src/rp2040/hardware_structs/include/hardware/structs/xosc.h Lines 15 - 24
15 typedef struct {
16 io_rw_32 ctrl;
17 io_rw_32 status;
18 io_rw_32 dormant;
RP2040 Datasheet
2.16. Crystal Oscillator (XOSC) 241