https://github.com/raspberrypi/pico-sdk/blob/1.3.0/src/rp2_common/hardware_clocks/clocks.c#L121


    rosc @ 1-12 MHz

    xosc @ 12 MHz
        |
        \-- clk_ref @ 12 MHz
                |
                \-- watchdog tick 1:12, @ 1 Mhz
                |       |
                |       \-- timer/alarm: get_absolute_time() in micro seconds
                |
                \-- pll_sys @ 125 MHz
                |       |
                |       \-- clk_sys @ 125 MHz
                |               |
                |               \-- clk_peri @ 125 MHz
                |               |        |
                |               |        \-- UART (but not I2C)
                |               |
                |               \-- DMA pacing timers
                |
                \-- pll_usb @ 48 MHz
                        |
                        \-- clk_usb
                        |
                        \-- clk_adc
                        |
                        \-- clk_rtc 1:1024 @ 46,875 Hz
                                |
                                \-- RTC 1:46875 @ 1Hz