src/rp2_common/hardware_clocks/clocks.c - third_party/github/raspberrypi/pico-sdk - Git at Google

 /*
  * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */

 #include "pico.h"
 #include "hardware/regs/clocks.h"
 #include "hardware/platform_defs.h"
 #include "hardware/clocks.h"
 #include "hardware/watchdog.h"
 #include "hardware/pll.h"
 #include "hardware/xosc.h"
 #include "hardware/irq.h"
 #include "hardware/gpio.h"

 // The RTC clock frequency is 48MHz divided by power of 2 (to ensure an integer
 // division ratio will be used in the clocks block).  A divisor of 1024 generates
 // an RTC clock tick of 46875Hz.  This frequency is relatively close to the
 // customary 32 or 32.768kHz 'slow clock' crystals and provides good timing resolution.
 #define RTC_CLOCK_FREQ_HZ       (USB_CLK_KHZ * KHZ / 1024)

 check_hw_layout(clocks_hw_t, clk[clk_adc].selected, CLOCKS_CLK_ADC_SELECTED_OFFSET);
 check_hw_layout(clocks_hw_t, fc0.result, CLOCKS_FC0_RESULT_OFFSET);
 check_hw_layout(clocks_hw_t, ints, CLOCKS_INTS_OFFSET);

 static uint32_t configured_freq[CLK_COUNT];

 static resus_callback_t _resus_callback;

 // Clock muxing consists of two components:
 // - A glitchless mux, which can be switched freely, but whose inputs must be
 //   free-running
 // - An auxiliary (glitchy) mux, whose output glitches when switched, but has
 //   no constraints on its inputs
 // Not all clocks have both types of mux.
 static inline bool has_glitchless_mux(enum clock_index clk_index) {
     return clk_index == clk_sys || clk_index == clk_ref;
 }

 void clock_stop(enum clock_index clk_index) {
     clock_hw_t *clock = &clocks_hw->clk[clk_index];
     hw_clear_bits(&clock->ctrl, CLOCKS_CLK_USB_CTRL_ENABLE_BITS);
     configured_freq[clk_index] = 0;
 }

 /// \tag::clock_configure[]
 bool clock_configure(enum clock_index clk_index, uint32_t src, uint32_t auxsrc, uint32_t src_freq, uint32_t freq) {
     uint32_t div;

     assert(src_freq >= freq);

     if (freq > src_freq)
         return false;

     // Div register is 24.8 int.frac divider so multiply by 2^8 (left shift by 8)
     div = (uint32_t) (((uint64_t) src_freq << CLOCKS_CLK_GPOUT0_DIV_INT_LSB) / freq);

     clock_hw_t *clock = &clocks_hw->clk[clk_index];

     // If increasing divisor, set divisor before source. Otherwise set source
     // before divisor. This avoids a momentary overspeed when e.g. switching
     // to a faster source and increasing divisor to compensate.
     if (div > clock->div)
         clock->div = div;

     // If switching a glitchless slice (ref or sys) to an aux source, switch
     // away from aux *first* to avoid passing glitches when changing aux mux.
     // Assume (!!!) glitchless source 0 is no faster than the aux source.
     if (has_glitchless_mux(clk_index) && src == CLOCKS_CLK_SYS_CTRL_SRC_VALUE_CLKSRC_CLK_SYS_AUX) {
         hw_clear_bits(&clock->ctrl, CLOCKS_CLK_REF_CTRL_SRC_BITS);
         while (!(clock->selected & 1u))
             tight_loop_contents();
     }
     // If no glitchless mux, cleanly stop the clock to avoid glitches
     // propagating when changing aux mux. Note it would be a really bad idea
     // to do this on one of the glitchless clocks (clk_sys, clk_ref).
     else {
         // Disable clock. On clk_ref and clk_sys this does nothing,
         // all other clocks have the ENABLE bit in the same position.
         hw_clear_bits(&clock->ctrl, CLOCKS_CLK_GPOUT0_CTRL_ENABLE_BITS);
         if (configured_freq[clk_index] > 0) {
             // Delay for 3 cycles of the target clock, for ENABLE propagation.
             // Note XOSC_COUNT is not helpful here because XOSC is not
             // necessarily running, nor is timer...:
             uint delay_cyc = configured_freq[clk_sys] / configured_freq[clk_index] + 1;
             busy_wait_at_least_cycles(delay_cyc * 3);
         }
     }

     // Set aux mux first, and then glitchless mux if this clock has one
     hw_write_masked(&clock->ctrl,
         (auxsrc << CLOCKS_CLK_SYS_CTRL_AUXSRC_LSB),
         CLOCKS_CLK_SYS_CTRL_AUXSRC_BITS
     );

     if (has_glitchless_mux(clk_index)) {
         hw_write_masked(&clock->ctrl,
             src << CLOCKS_CLK_REF_CTRL_SRC_LSB,
             CLOCKS_CLK_REF_CTRL_SRC_BITS
         );
         while (!(clock->selected & (1u << src)))
             tight_loop_contents();
     }

     // Enable clock. On clk_ref and clk_sys this does nothing,
     // all other clocks have the ENABLE bit in the same position.
     hw_set_bits(&clock->ctrl, CLOCKS_CLK_GPOUT0_CTRL_ENABLE_BITS);

     // Now that the source is configured, we can trust that the user-supplied
     // divisor is a safe value.
     clock->div = div;

     // Store the configured frequency
     configured_freq[clk_index] = (uint32_t)(((uint64_t) src_freq << 8) / div);

     return true;
 }
 /// \end::clock_configure[]

 void clocks_init(void) {
     // Start tick in watchdog, the argument is in 'cycles per microsecond' i.e. MHz
     watchdog_start_tick(XOSC_KHZ / KHZ);

     // Everything is 48MHz on FPGA apart from RTC. Otherwise set to 0 and will be set in clock configure
     if (running_on_fpga()) {
         for (uint i = 0; i < CLK_COUNT; i++) {
             configured_freq[i] = 48 * MHZ;
         }
         configured_freq[clk_rtc] = RTC_CLOCK_FREQ_HZ;
         return;
     }

     // Disable resus that may be enabled from previous software
     clocks_hw->resus.ctrl = 0;

     // Enable the xosc
     xosc_init();

     // Before we touch PLLs, switch sys and ref cleanly away from their aux sources.
     hw_clear_bits(&clocks_hw->clk[clk_sys].ctrl, CLOCKS_CLK_SYS_CTRL_SRC_BITS);
     while (clocks_hw->clk[clk_sys].selected != 0x1)
         tight_loop_contents();
     hw_clear_bits(&clocks_hw->clk[clk_ref].ctrl, CLOCKS_CLK_REF_CTRL_SRC_BITS);
     while (clocks_hw->clk[clk_ref].selected != 0x1)
         tight_loop_contents();

     /// \tag::pll_init[]
     pll_init(pll_sys, PLL_COMMON_REFDIV, PLL_SYS_VCO_FREQ_KHZ * KHZ, PLL_SYS_POSTDIV1, PLL_SYS_POSTDIV2);
     pll_init(pll_usb, PLL_COMMON_REFDIV, PLL_USB_VCO_FREQ_KHZ * KHZ, PLL_USB_POSTDIV1, PLL_USB_POSTDIV2);
     /// \end::pll_init[]

     // Configure clocks
     // CLK_REF = XOSC (usually) 12MHz / 1 = 12MHz
     clock_configure(clk_ref,
                     CLOCKS_CLK_REF_CTRL_SRC_VALUE_XOSC_CLKSRC,
                     0, // No aux mux
                     XOSC_KHZ * KHZ,
                     XOSC_KHZ * KHZ);

     /// \tag::configure_clk_sys[]
     // CLK SYS = PLL SYS (usually) 125MHz / 1 = 125MHz
     clock_configure(clk_sys,
                     CLOCKS_CLK_SYS_CTRL_SRC_VALUE_CLKSRC_CLK_SYS_AUX,
                     CLOCKS_CLK_SYS_CTRL_AUXSRC_VALUE_CLKSRC_PLL_SYS,
                     SYS_CLK_KHZ * KHZ,
                     SYS_CLK_KHZ * KHZ);
     /// \end::configure_clk_sys[]

     // CLK USB = PLL USB 48MHz / 1 = 48MHz
     clock_configure(clk_usb,
                     0, // No GLMUX
                     CLOCKS_CLK_USB_CTRL_AUXSRC_VALUE_CLKSRC_PLL_USB,
                     USB_CLK_KHZ * KHZ,
                     USB_CLK_KHZ * KHZ);

     // CLK ADC = PLL USB 48MHZ / 1 = 48MHz
     clock_configure(clk_adc,
                     0, // No GLMUX
                     CLOCKS_CLK_ADC_CTRL_AUXSRC_VALUE_CLKSRC_PLL_USB,
                     USB_CLK_KHZ * KHZ,
                     USB_CLK_KHZ * KHZ);

     // CLK RTC = PLL USB 48MHz / 1024 = 46875Hz
     clock_configure(clk_rtc,
                     0, // No GLMUX
                     CLOCKS_CLK_RTC_CTRL_AUXSRC_VALUE_CLKSRC_PLL_USB,
                     USB_CLK_KHZ * KHZ,
                     RTC_CLOCK_FREQ_HZ);

     // CLK PERI = clk_sys. Used as reference clock for Peripherals. No dividers so just select and enable
     // Normally choose clk_sys or clk_usb
     clock_configure(clk_peri,
                     0,
                     CLOCKS_CLK_PERI_CTRL_AUXSRC_VALUE_CLK_SYS,
                     SYS_CLK_KHZ * KHZ,
                     SYS_CLK_KHZ * KHZ);
 }

 /// \tag::clock_get_hz[]
 uint32_t clock_get_hz(enum clock_index clk_index) {
     return configured_freq[clk_index];
 }
 /// \end::clock_get_hz[]

 void clock_set_reported_hz(enum clock_index clk_index, uint hz) {
     configured_freq[clk_index] = hz;
 }

 /// \tag::frequency_count_khz[]
 uint32_t frequency_count_khz(uint src) {
     fc_hw_t *fc = &clocks_hw->fc0;

     // If frequency counter is running need to wait for it. It runs even if the source is NULL
     while(fc->status & CLOCKS_FC0_STATUS_RUNNING_BITS) {
         tight_loop_contents();
     }

     // Set reference freq
     fc->ref_khz = clock_get_hz(clk_ref) / 1000;

     // FIXME: Don't pick random interval. Use best interval
     fc->interval = 10;

     // No min or max
     fc->min_khz = 0;
     fc->max_khz = 0xffffffff;

     // Set SRC which automatically starts the measurement
     fc->src = src;

     while(!(fc->status & CLOCKS_FC0_STATUS_DONE_BITS)) {
         tight_loop_contents();
     }

     // Return the result
     return fc->result >> CLOCKS_FC0_RESULT_KHZ_LSB;
 }
 /// \end::frequency_count_khz[]

 static void clocks_handle_resus(void) {
     // Set clk_sys back to the ref clock rather than it being forced to clk_ref
     // by resus. Call the user's resus callback if they have set one

     // CLK SYS = CLK_REF. Must be running for this code to be running
     uint clk_ref_freq = clock_get_hz(clk_ref);
     clock_configure(clk_sys,
                     CLOCKS_CLK_SYS_CTRL_SRC_VALUE_CLK_REF,
                     0,
                     clk_ref_freq,
                     clk_ref_freq);

     // Assert we have been resussed
     assert(clocks_hw->resus.status & CLOCKS_CLK_SYS_RESUS_STATUS_RESUSSED_BITS);

     // Now we have fixed clk_sys we can safely remove the resus
     hw_set_bits(&clocks_hw->resus.ctrl, CLOCKS_CLK_SYS_RESUS_CTRL_CLEAR_BITS);
     hw_clear_bits(&clocks_hw->resus.ctrl, CLOCKS_CLK_SYS_RESUS_CTRL_CLEAR_BITS);

     // Now we should no longer be resussed
     assert(!(clocks_hw->resus.status & CLOCKS_CLK_SYS_RESUS_STATUS_RESUSSED_BITS));

     // Call the user's callback to notify them of the resus event
     if (_resus_callback) {
         _resus_callback();
     }
 }

 static void clocks_irq_handler(void) {
     // Clocks interrupt handler. Only resus but handle irq
     // defensively just in case.
     uint32_t ints = clocks_hw->ints;

     if (ints & CLOCKS_INTE_CLK_SYS_RESUS_BITS) {
         ints &= ~CLOCKS_INTE_CLK_SYS_RESUS_BITS;
         clocks_handle_resus();
     }

 #ifndef NDEBUG
     if (ints) {
         panic("Unexpected clocks irq\n");
     }
 #endif
 }

 void clocks_enable_resus(resus_callback_t resus_callback) {
     // Restart clk_sys if it is stopped by forcing it
     // to the default source of clk_ref. If clk_ref stops running this will
     // not work.

     // Store user's resus callback
     _resus_callback = resus_callback;

     irq_set_exclusive_handler(CLOCKS_IRQ, clocks_irq_handler);

     // Enable the resus interrupt in clocks
     clocks_hw->inte = CLOCKS_INTE_CLK_SYS_RESUS_BITS;

     // Enable the clocks irq
     irq_set_enabled(CLOCKS_IRQ, true);

     // 2 * clk_ref freq / clk_sys_min_freq;
     // assume clk_ref is 3MHz and we want clk_sys to be no lower than 1MHz
     uint timeout = 2 * 3 * 1;

     // Enable resus with the maximum timeout
     clocks_hw->resus.ctrl = CLOCKS_CLK_SYS_RESUS_CTRL_ENABLE_BITS | timeout;
 }

 void clock_gpio_init_int_frac(uint gpio, uint src, uint32_t div_int, uint8_t div_frac) {
     // Bit messy but it's as much code to loop through a lookup
     // table. The sources for each gpout generators are the same
     // so just call with the sources from GP0
     uint gpclk = 0;
     if      (gpio == 21) gpclk = clk_gpout0;
     else if (gpio == 23) gpclk = clk_gpout1;
     else if (gpio == 24) gpclk = clk_gpout2;
     else if (gpio == 25) gpclk = clk_gpout3;
     else {
         invalid_params_if(CLOCKS, true);
     }

     // Set up the gpclk generator
     clocks_hw->clk[gpclk].ctrl = (src << CLOCKS_CLK_GPOUT0_CTRL_AUXSRC_LSB) |
                                  CLOCKS_CLK_GPOUT0_CTRL_ENABLE_BITS;
     clocks_hw->clk[gpclk].div = (div_int << CLOCKS_CLK_GPOUT0_DIV_INT_LSB) | div_frac;

     // Set gpio pin to gpclock function
     gpio_set_function(gpio, GPIO_FUNC_GPCK);
 }

 static const uint8_t gpin0_src[CLK_COUNT] = {
     CLOCKS_CLK_GPOUT0_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_GPOUT0
     CLOCKS_CLK_GPOUT1_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_GPOUT1
     CLOCKS_CLK_GPOUT2_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_GPOUT2
     CLOCKS_CLK_GPOUT3_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_GPOUT3
     CLOCKS_CLK_REF_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0,    // CLK_REF
     CLOCKS_CLK_SYS_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0,    // CLK_SYS
     CLOCKS_CLK_PERI_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0,   // CLK_PERI
     CLOCKS_CLK_USB_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0,    // CLK_USB
     CLOCKS_CLK_ADC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0,    // CLK_ADC
     CLOCKS_CLK_RTC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0,    // CLK_RTC
 };

 // Assert GPIN1 is GPIN0 + 1
 static_assert(CLOCKS_CLK_GPOUT0_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_GPOUT0_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
 static_assert(CLOCKS_CLK_GPOUT1_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_GPOUT1_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
 static_assert(CLOCKS_CLK_GPOUT2_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_GPOUT2_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
 static_assert(CLOCKS_CLK_GPOUT3_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_GPOUT3_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
 static_assert(CLOCKS_CLK_REF_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1    == (CLOCKS_CLK_REF_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0    + 1), "hw mismatch");
 static_assert(CLOCKS_CLK_SYS_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1    == (CLOCKS_CLK_SYS_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0    + 1), "hw mismatch");
 static_assert(CLOCKS_CLK_PERI_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1   == (CLOCKS_CLK_PERI_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0   + 1), "hw mismatch");
 static_assert(CLOCKS_CLK_USB_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1    == (CLOCKS_CLK_USB_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0    + 1), "hw mismatch");
 static_assert(CLOCKS_CLK_ADC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1    == (CLOCKS_CLK_ADC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0    + 1), "hw mismatch");
 static_assert(CLOCKS_CLK_RTC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1    == (CLOCKS_CLK_RTC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0    + 1), "hw mismatch");

 bool clock_configure_gpin(enum clock_index clk_index, uint gpio, uint32_t src_freq, uint32_t freq) {
     // Configure a clock to run from a GPIO input
     uint gpin = 0;
     if      (gpio == 20) gpin = 0;
     else if (gpio == 22) gpin = 1;
     else {
         invalid_params_if(CLOCKS, true);
     }

     // Work out sources. GPIN is always an auxsrc
     uint src = 0;

     // GPIN1 == GPIN0 + 1
     uint auxsrc = gpin0_src[clk_index] + gpin;

     if (has_glitchless_mux(clk_index)) {
         // AUX src is always 1
         src = 1;
     }

     // Set the GPIO function
     gpio_set_function(gpio, GPIO_FUNC_GPCK);

     // Now we have the src, auxsrc, and configured the gpio input
     // call clock configure to run the clock from a gpio
     return clock_configure(clk_index, src, auxsrc, src_freq, freq);
 }
	/*
	* Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
	*
	* SPDX-License-Identifier: BSD-3-Clause
	*/

	#include "pico.h"
	#include "hardware/regs/clocks.h"
	#include "hardware/platform_defs.h"
	#include "hardware/clocks.h"
	#include "hardware/watchdog.h"
	#include "hardware/pll.h"
	#include "hardware/xosc.h"
	#include "hardware/irq.h"
	#include "hardware/gpio.h"

	// The RTC clock frequency is 48MHz divided by power of 2 (to ensure an integer
	// division ratio will be used in the clocks block). A divisor of 1024 generates
	// an RTC clock tick of 46875Hz. This frequency is relatively close to the
	// customary 32 or 32.768kHz 'slow clock' crystals and provides good timing resolution.
	#define RTC_CLOCK_FREQ_HZ (USB_CLK_KHZ * KHZ / 1024)

	check_hw_layout(clocks_hw_t, clk[clk_adc].selected, CLOCKS_CLK_ADC_SELECTED_OFFSET);
	check_hw_layout(clocks_hw_t, fc0.result, CLOCKS_FC0_RESULT_OFFSET);
	check_hw_layout(clocks_hw_t, ints, CLOCKS_INTS_OFFSET);

	static uint32_t configured_freq[CLK_COUNT];

	static resus_callback_t _resus_callback;

	// Clock muxing consists of two components:
	// - A glitchless mux, which can be switched freely, but whose inputs must be
	// free-running
	// - An auxiliary (glitchy) mux, whose output glitches when switched, but has
	// no constraints on its inputs
	// Not all clocks have both types of mux.
	static inline bool has_glitchless_mux(enum clock_index clk_index) {
	return clk_index == clk_sys \|\| clk_index == clk_ref;
	}

	void clock_stop(enum clock_index clk_index) {
	clock_hw_t *clock = &clocks_hw->clk[clk_index];
	hw_clear_bits(&clock->ctrl, CLOCKS_CLK_USB_CTRL_ENABLE_BITS);
	configured_freq[clk_index] = 0;
	}

	/// \tag::clock_configure[]
	bool clock_configure(enum clock_index clk_index, uint32_t src, uint32_t auxsrc, uint32_t src_freq, uint32_t freq) {
	uint32_t div;

	assert(src_freq >= freq);

	if (freq > src_freq)
	return false;

	// Div register is 24.8 int.frac divider so multiply by 2^8 (left shift by 8)
	div = (uint32_t) (((uint64_t) src_freq << CLOCKS_CLK_GPOUT0_DIV_INT_LSB) / freq);

	clock_hw_t *clock = &clocks_hw->clk[clk_index];

	// If increasing divisor, set divisor before source. Otherwise set source
	// before divisor. This avoids a momentary overspeed when e.g. switching
	// to a faster source and increasing divisor to compensate.
	if (div > clock->div)
	clock->div = div;

	// If switching a glitchless slice (ref or sys) to an aux source, switch
	// away from aux first to avoid passing glitches when changing aux mux.
	// Assume (!!!) glitchless source 0 is no faster than the aux source.
	if (has_glitchless_mux(clk_index) && src == CLOCKS_CLK_SYS_CTRL_SRC_VALUE_CLKSRC_CLK_SYS_AUX) {
	hw_clear_bits(&clock->ctrl, CLOCKS_CLK_REF_CTRL_SRC_BITS);
	while (!(clock->selected & 1u))
	tight_loop_contents();
	}
	// If no glitchless mux, cleanly stop the clock to avoid glitches
	// propagating when changing aux mux. Note it would be a really bad idea
	// to do this on one of the glitchless clocks (clk_sys, clk_ref).
	else {
	// Disable clock. On clk_ref and clk_sys this does nothing,
	// all other clocks have the ENABLE bit in the same position.
	hw_clear_bits(&clock->ctrl, CLOCKS_CLK_GPOUT0_CTRL_ENABLE_BITS);
	if (configured_freq[clk_index] > 0) {
	// Delay for 3 cycles of the target clock, for ENABLE propagation.
	// Note XOSC_COUNT is not helpful here because XOSC is not
	// necessarily running, nor is timer...:
	uint delay_cyc = configured_freq[clk_sys] / configured_freq[clk_index] + 1;
	busy_wait_at_least_cycles(delay_cyc * 3);
	}
	}

	// Set aux mux first, and then glitchless mux if this clock has one
	hw_write_masked(&clock->ctrl,
	(auxsrc << CLOCKS_CLK_SYS_CTRL_AUXSRC_LSB),
	CLOCKS_CLK_SYS_CTRL_AUXSRC_BITS
	);

	if (has_glitchless_mux(clk_index)) {
	hw_write_masked(&clock->ctrl,
	src << CLOCKS_CLK_REF_CTRL_SRC_LSB,
	CLOCKS_CLK_REF_CTRL_SRC_BITS
	);
	while (!(clock->selected & (1u << src)))
	tight_loop_contents();
	}

	// Enable clock. On clk_ref and clk_sys this does nothing,
	// all other clocks have the ENABLE bit in the same position.
	hw_set_bits(&clock->ctrl, CLOCKS_CLK_GPOUT0_CTRL_ENABLE_BITS);

	// Now that the source is configured, we can trust that the user-supplied
	// divisor is a safe value.
	clock->div = div;

	// Store the configured frequency
	configured_freq[clk_index] = (uint32_t)(((uint64_t) src_freq << 8) / div);

	return true;
	}
	/// \end::clock_configure[]

	void clocks_init(void) {
	// Start tick in watchdog, the argument is in 'cycles per microsecond' i.e. MHz
	watchdog_start_tick(XOSC_KHZ / KHZ);

	// Everything is 48MHz on FPGA apart from RTC. Otherwise set to 0 and will be set in clock configure
	if (running_on_fpga()) {
	for (uint i = 0; i < CLK_COUNT; i++) {
	configured_freq[i] = 48 * MHZ;
	}
	configured_freq[clk_rtc] = RTC_CLOCK_FREQ_HZ;
	return;
	}

	// Disable resus that may be enabled from previous software
	clocks_hw->resus.ctrl = 0;

	// Enable the xosc
	xosc_init();

	// Before we touch PLLs, switch sys and ref cleanly away from their aux sources.
	hw_clear_bits(&clocks_hw->clk[clk_sys].ctrl, CLOCKS_CLK_SYS_CTRL_SRC_BITS);
	while (clocks_hw->clk[clk_sys].selected != 0x1)
	tight_loop_contents();
	hw_clear_bits(&clocks_hw->clk[clk_ref].ctrl, CLOCKS_CLK_REF_CTRL_SRC_BITS);
	while (clocks_hw->clk[clk_ref].selected != 0x1)
	tight_loop_contents();

	/// \tag::pll_init[]
	pll_init(pll_sys, PLL_COMMON_REFDIV, PLL_SYS_VCO_FREQ_KHZ * KHZ, PLL_SYS_POSTDIV1, PLL_SYS_POSTDIV2);
	pll_init(pll_usb, PLL_COMMON_REFDIV, PLL_USB_VCO_FREQ_KHZ * KHZ, PLL_USB_POSTDIV1, PLL_USB_POSTDIV2);
	/// \end::pll_init[]

	// Configure clocks
	// CLK_REF = XOSC (usually) 12MHz / 1 = 12MHz
	clock_configure(clk_ref,
	CLOCKS_CLK_REF_CTRL_SRC_VALUE_XOSC_CLKSRC,
	0, // No aux mux
	XOSC_KHZ * KHZ,
	XOSC_KHZ * KHZ);

	/// \tag::configure_clk_sys[]
	// CLK SYS = PLL SYS (usually) 125MHz / 1 = 125MHz
	clock_configure(clk_sys,
	CLOCKS_CLK_SYS_CTRL_SRC_VALUE_CLKSRC_CLK_SYS_AUX,
	CLOCKS_CLK_SYS_CTRL_AUXSRC_VALUE_CLKSRC_PLL_SYS,
	SYS_CLK_KHZ * KHZ,
	SYS_CLK_KHZ * KHZ);
	/// \end::configure_clk_sys[]

	// CLK USB = PLL USB 48MHz / 1 = 48MHz
	clock_configure(clk_usb,
	0, // No GLMUX
	CLOCKS_CLK_USB_CTRL_AUXSRC_VALUE_CLKSRC_PLL_USB,
	USB_CLK_KHZ * KHZ,
	USB_CLK_KHZ * KHZ);

	// CLK ADC = PLL USB 48MHZ / 1 = 48MHz
	clock_configure(clk_adc,
	0, // No GLMUX
	CLOCKS_CLK_ADC_CTRL_AUXSRC_VALUE_CLKSRC_PLL_USB,
	USB_CLK_KHZ * KHZ,
	USB_CLK_KHZ * KHZ);

	// CLK RTC = PLL USB 48MHz / 1024 = 46875Hz
	clock_configure(clk_rtc,
	0, // No GLMUX
	CLOCKS_CLK_RTC_CTRL_AUXSRC_VALUE_CLKSRC_PLL_USB,
	USB_CLK_KHZ * KHZ,
	RTC_CLOCK_FREQ_HZ);

	// CLK PERI = clk_sys. Used as reference clock for Peripherals. No dividers so just select and enable
	// Normally choose clk_sys or clk_usb
	clock_configure(clk_peri,
	0,
	CLOCKS_CLK_PERI_CTRL_AUXSRC_VALUE_CLK_SYS,
	SYS_CLK_KHZ * KHZ,
	SYS_CLK_KHZ * KHZ);
	}

	/// \tag::clock_get_hz[]
	uint32_t clock_get_hz(enum clock_index clk_index) {
	return configured_freq[clk_index];
	}
	/// \end::clock_get_hz[]

	void clock_set_reported_hz(enum clock_index clk_index, uint hz) {
	configured_freq[clk_index] = hz;
	}

	/// \tag::frequency_count_khz[]
	uint32_t frequency_count_khz(uint src) {
	fc_hw_t *fc = &clocks_hw->fc0;

	// If frequency counter is running need to wait for it. It runs even if the source is NULL
	while(fc->status & CLOCKS_FC0_STATUS_RUNNING_BITS) {
	tight_loop_contents();
	}

	// Set reference freq
	fc->ref_khz = clock_get_hz(clk_ref) / 1000;

	// FIXME: Don't pick random interval. Use best interval
	fc->interval = 10;

	// No min or max
	fc->min_khz = 0;
	fc->max_khz = 0xffffffff;

	// Set SRC which automatically starts the measurement
	fc->src = src;

	while(!(fc->status & CLOCKS_FC0_STATUS_DONE_BITS)) {
	tight_loop_contents();
	}

	// Return the result
	return fc->result >> CLOCKS_FC0_RESULT_KHZ_LSB;
	}
	/// \end::frequency_count_khz[]

	static void clocks_handle_resus(void) {
	// Set clk_sys back to the ref clock rather than it being forced to clk_ref
	// by resus. Call the user's resus callback if they have set one

	// CLK SYS = CLK_REF. Must be running for this code to be running
	uint clk_ref_freq = clock_get_hz(clk_ref);
	clock_configure(clk_sys,
	CLOCKS_CLK_SYS_CTRL_SRC_VALUE_CLK_REF,
	0,
	clk_ref_freq,
	clk_ref_freq);

	// Assert we have been resussed
	assert(clocks_hw->resus.status & CLOCKS_CLK_SYS_RESUS_STATUS_RESUSSED_BITS);

	// Now we have fixed clk_sys we can safely remove the resus
	hw_set_bits(&clocks_hw->resus.ctrl, CLOCKS_CLK_SYS_RESUS_CTRL_CLEAR_BITS);
	hw_clear_bits(&clocks_hw->resus.ctrl, CLOCKS_CLK_SYS_RESUS_CTRL_CLEAR_BITS);

	// Now we should no longer be resussed
	assert(!(clocks_hw->resus.status & CLOCKS_CLK_SYS_RESUS_STATUS_RESUSSED_BITS));

	// Call the user's callback to notify them of the resus event
	if (_resus_callback) {
	_resus_callback();
	}
	}

	static void clocks_irq_handler(void) {
	// Clocks interrupt handler. Only resus but handle irq
	// defensively just in case.
	uint32_t ints = clocks_hw->ints;

	if (ints & CLOCKS_INTE_CLK_SYS_RESUS_BITS) {
	ints &= ~CLOCKS_INTE_CLK_SYS_RESUS_BITS;
	clocks_handle_resus();
	}

	#ifndef NDEBUG
	if (ints) {
	panic("Unexpected clocks irq\n");
	}
	#endif
	}

	void clocks_enable_resus(resus_callback_t resus_callback) {
	// Restart clk_sys if it is stopped by forcing it
	// to the default source of clk_ref. If clk_ref stops running this will
	// not work.

	// Store user's resus callback
	_resus_callback = resus_callback;

	irq_set_exclusive_handler(CLOCKS_IRQ, clocks_irq_handler);

	// Enable the resus interrupt in clocks
	clocks_hw->inte = CLOCKS_INTE_CLK_SYS_RESUS_BITS;

	// Enable the clocks irq
	irq_set_enabled(CLOCKS_IRQ, true);

	// 2 * clk_ref freq / clk_sys_min_freq;
	// assume clk_ref is 3MHz and we want clk_sys to be no lower than 1MHz
	uint timeout = 2 * 3 * 1;

	// Enable resus with the maximum timeout
	clocks_hw->resus.ctrl = CLOCKS_CLK_SYS_RESUS_CTRL_ENABLE_BITS \| timeout;
	}

	void clock_gpio_init_int_frac(uint gpio, uint src, uint32_t div_int, uint8_t div_frac) {
	// Bit messy but it's as much code to loop through a lookup
	// table. The sources for each gpout generators are the same
	// so just call with the sources from GP0
	uint gpclk = 0;
	if (gpio == 21) gpclk = clk_gpout0;
	else if (gpio == 23) gpclk = clk_gpout1;
	else if (gpio == 24) gpclk = clk_gpout2;
	else if (gpio == 25) gpclk = clk_gpout3;
	else {
	invalid_params_if(CLOCKS, true);
	}

	// Set up the gpclk generator
	clocks_hw->clk[gpclk].ctrl = (src << CLOCKS_CLK_GPOUT0_CTRL_AUXSRC_LSB) \|
	CLOCKS_CLK_GPOUT0_CTRL_ENABLE_BITS;
	clocks_hw->clk[gpclk].div = (div_int << CLOCKS_CLK_GPOUT0_DIV_INT_LSB) \| div_frac;

	// Set gpio pin to gpclock function
	gpio_set_function(gpio, GPIO_FUNC_GPCK);
	}

	static const uint8_t gpin0_src[CLK_COUNT] = {
	CLOCKS_CLK_GPOUT0_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_GPOUT0
	CLOCKS_CLK_GPOUT1_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_GPOUT1
	CLOCKS_CLK_GPOUT2_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_GPOUT2
	CLOCKS_CLK_GPOUT3_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_GPOUT3
	CLOCKS_CLK_REF_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_REF
	CLOCKS_CLK_SYS_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_SYS
	CLOCKS_CLK_PERI_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_PERI
	CLOCKS_CLK_USB_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_USB
	CLOCKS_CLK_ADC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_ADC
	CLOCKS_CLK_RTC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0, // CLK_RTC
	};

	// Assert GPIN1 is GPIN0 + 1
	static_assert(CLOCKS_CLK_GPOUT0_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_GPOUT0_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
	static_assert(CLOCKS_CLK_GPOUT1_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_GPOUT1_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
	static_assert(CLOCKS_CLK_GPOUT2_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_GPOUT2_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
	static_assert(CLOCKS_CLK_GPOUT3_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_GPOUT3_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
	static_assert(CLOCKS_CLK_REF_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_REF_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
	static_assert(CLOCKS_CLK_SYS_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_SYS_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
	static_assert(CLOCKS_CLK_PERI_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_PERI_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
	static_assert(CLOCKS_CLK_USB_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_USB_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
	static_assert(CLOCKS_CLK_ADC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_ADC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");
	static_assert(CLOCKS_CLK_RTC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN1 == (CLOCKS_CLK_RTC_CTRL_AUXSRC_VALUE_CLKSRC_GPIN0 + 1), "hw mismatch");

	bool clock_configure_gpin(enum clock_index clk_index, uint gpio, uint32_t src_freq, uint32_t freq) {
	// Configure a clock to run from a GPIO input
	uint gpin = 0;
	if (gpio == 20) gpin = 0;
	else if (gpio == 22) gpin = 1;
	else {
	invalid_params_if(CLOCKS, true);
	}

	// Work out sources. GPIN is always an auxsrc
	uint src = 0;

	// GPIN1 == GPIN0 + 1
	uint auxsrc = gpin0_src[clk_index] + gpin;

	if (has_glitchless_mux(clk_index)) {
	// AUX src is always 1
	src = 1;
	}

	// Set the GPIO function
	gpio_set_function(gpio, GPIO_FUNC_GPCK);

	// Now we have the src, auxsrc, and configured the gpio input
	// call clock configure to run the clock from a gpio
	return clock_configure(clk_index, src, auxsrc, src_freq, freq);
	}