Fresh import of the PX4 firmware sources.

1 files changed, 801 insertions, 0 deletions

diff --git a/nuttx/configs/px4fmu/src/up_hrt.c b/nuttx/configs/px4fmu/src/up_hrt.c
new file mode 100644
index 000000000..35aecfe08
--- /dev/null
+++ b/nuttx/configs/px4fmu/src/up_hrt.c
@@ -0,0 +1,801 @@
+/****************************************************************************
+ *
+ *   Copyright (C) 2012 PX4 Development Team. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name PX4 nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+ 
+/**
+ * @file High-resolution timer callouts and timekeeping.
+ *
+ * This can use any general or advanced STM32 timer.
+ *
+ * Note that really, this could use systick too, but that's
+ * monopolised by NuttX and stealing it would just be awkward.
+ *
+ * We don't use the NuttX STM32 driver per se; rather, we 
+ * claim the timer and then drive it directly.
+ */
+
+#include <nuttx/config.h>
+#include <nuttx/arch.h>
+#include <nuttx/irq.h>
+
+#include <sys/types.h>
+#include <stdbool.h>
+
+#include <assert.h>
+#include <debug.h>
+#include <time.h>
+#include <queue.h>
+#include <errno.h>
+#include <string.h>
+
+#include <arch/board/board.h>
+#include <arch/board/up_hrt.h>
+
+#include "chip.h"
+#include "up_internal.h"
+#include "up_arch.h"
+
+#include "stm32_internal.h"
+#include "stm32_gpio.h"
+#include "stm32_tim.h"
+
+#ifdef CONFIG_HRT_TIMER
+
+/* HRT configuration */
+#if   HRT_TIMER == 1
+# define HRT_TIMER_BASE		STM32_TIM1_BASE
+# define HRT_TIMER_POWER_REG	STM32_RCC_APB2ENR
+# define HRT_TIMER_POWER_BIT	RCC_APB2ENR_TIM1EN
+# define HRT_TIMER_VECTOR	STM32_IRQ_TIM1CC
+# define HRT_TIMER_CLOCK	STM32_APB2_TIM1_CLKIN
+# if CONFIG_STM32_TIM1
+#  error must not set CONFIG_STM32_TIM1=y and HRT_TIMER=1
+# endif
+#elif HRT_TIMER == 2
+# define HRT_TIMER_BASE		STM32_TIM2_BASE
+# define HRT_TIMER_POWER_REG	STM32_RCC_APB1ENR
+# define HRT_TIMER_POWER_BIT	RCC_APB2ENR_TIM2EN
+# define HRT_TIMER_VECTOR	STM32_IRQ_TIM2
+# define HRT_TIMER_CLOCK	STM32_APB1_TIM2_CLKIN
+# if CONFIG_STM32_TIM2
+#  error must not set CONFIG_STM32_TIM2=y and HRT_TIMER=2
+# endif
+#elif HRT_TIMER == 3
+# define HRT_TIMER_BASE		STM32_TIM3_BASE
+# define HRT_TIMER_POWER_REG	STM32_RCC_APB1ENR
+# define HRT_TIMER_POWER_BIT	RCC_APB2ENR_TIM3EN
+# define HRT_TIMER_VECTOR	STM32_IRQ_TIM3
+# define HRT_TIMER_CLOCK	STM32_APB1_TIM3_CLKIN
+# if CONFIG_STM32_TIM3
+#  error must not set CONFIG_STM32_TIM3=y and HRT_TIMER=3
+# endif
+#elif HRT_TIMER == 4
+# define HRT_TIMER_BASE		STM32_TIM4_BASE
+# define HRT_TIMER_POWER_REG	STM32_RCC_APB1ENR
+# define HRT_TIMER_POWER_BIT	RCC_APB2ENR_TIM4EN
+# define HRT_TIMER_VECTOR	STM32_IRQ_TIM4
+# define HRT_TIMER_CLOCK	STM32_APB1_TIM4_CLKIN
+# if CONFIG_STM32_TIM4
+#  error must not set CONFIG_STM32_TIM4=y and HRT_TIMER=4
+# endif
+#elif HRT_TIMER == 5
+# define HRT_TIMER_BASE		STM32_TIM5_BASE
+# define HRT_TIMER_POWER_REG	STM32_RCC_APB1ENR
+# define HRT_TIMER_POWER_BIT	RCC_APB2ENR_TIM5EN
+# define HRT_TIMER_VECTOR	STM32_IRQ_TIM5
+# define HRT_TIMER_CLOCK	STM32_APB1_TIM5_CLKIN
+# if CONFIG_STM32_TIM5
+#  error must not set CONFIG_STM32_TIM5=y and HRT_TIMER=5
+# endif
+#elif HRT_TIMER == 8
+# define HRT_TIMER_BASE		STM32_TIM8_BASE
+# define HRT_TIMER_POWER_REG	STM32_RCC_APB2ENR
+# define HRT_TIMER_POWER_BIT	RCC_APB2ENR_TIM8EN
+# define HRT_TIMER_VECTOR	STM32_IRQ_TIM8CC
+# define HRT_TIMER_CLOCK	STM32_APB2_TIM8_CLKIN
+# if CONFIG_STM32_TIM8
+#  error must not set CONFIG_STM32_TIM8=y and HRT_TIMER=6
+# endif
+#elif HRT_TIMER == 9
+# define HRT_TIMER_BASE		STM32_TIM9_BASE
+# define HRT_TIMER_POWER_REG	STM32_RCC_APB1ENR
+# define HRT_TIMER_POWER_BIT	RCC_APB2ENR_TIM9EN
+# define HRT_TIMER_VECTOR	STM32_IRQ_TIM1BRK
+# define HRT_TIMER_CLOCK	STM32_APB1_TIM9_CLKIN
+# if CONFIG_STM32_TIM9
+#  error must not set CONFIG_STM32_TIM9=y and HRT_TIMER=9
+# endif
+#elif HRT_TIMER == 10
+# define HRT_TIMER_BASE		STM32_TIM10_BASE
+# define HRT_TIMER_POWER_REG	STM32_RCC_APB1ENR
+# define HRT_TIMER_POWER_BIT	RCC_APB2ENR_TIM10EN
+# define HRT_TIMER_VECTOR	STM32_IRQ_TIM1UP
+# define HRT_TIMER_CLOCK	STM32_APB1_TIM10_CLKIN
+# if CONFIG_STM32_TIM10
+#  error must not set CONFIG_STM32_TIM11=y and HRT_TIMER=10
+# endif
+#elif HRT_TIMER == 11
+# define HRT_TIMER_BASE		STM32_TIM11_BASE
+# define HRT_TIMER_POWER_REG	STM32_RCC_APB1ENR
+# define HRT_TIMER_POWER_BIT	RCC_APB2ENR_TIM11EN
+# define HRT_TIMER_VECTOR	STM32_IRQ_TIM1TRGCOM
+# define HRT_TIMER_CLOCK	STM32_APB1_TIM11_CLKIN
+# if CONFIG_STM32_TIM11
+#  error must not set CONFIG_STM32_TIM11=y and HRT_TIMER=11
+# endif
+#else
+# error HRT_TIMER must be set in board.h if CONFIG_HRT_TIMER=y
+#endif
+
+/*
+ * HRT clock must be a multiple of 1MHz greater than 1MHz
+ */
+#if (HRT_TIMER_CLOCK % 1000000) != 0
+# error HRT_TIMER_CLOCK must be a multiple of 1MHz
+#endif
+#if HRT_TIMER_CLOCK <= 1000000
+# error HRT_TIMER_CLOCK must be greater than 1MHz
+#endif
+
+/*
+ * Minimum/maximum deadlines.
+ *
+ * These are suitable for use with a 16-bit timer/counter clocked
+ * at 1MHz.  The high-resolution timer need only guarantee that it
+ * not wrap more than once in the 50ms period for absolute time to
+ * be consistently maintained.
+ *
+ * The minimum deadline must be such that the time taken between
+ * reading a time and writing a deadline to the timer cannot
+ * result in missing the deadline.
+ */
+#define HRT_INTERVAL_MIN	50
+#define HRT_INTERVAL_MAX	50000
+
+/*
+ * Period of the free-running counter, in microseconds.
+ */
+#define HRT_COUNTER_PERIOD	65536
+
+/*
+ * Scaling factor(s) for the free-running counter; convert an input
+ * in counts to a time in microseconds.
+ */
+#define HRT_COUNTER_SCALE(_c)	(_c)
+
+/*
+ * Timer register accessors
+ */
+#define REG(_reg)	(*(volatile uint32_t *)(HRT_TIMER_BASE + _reg))
+
+#define rCR1     	REG(STM32_GTIM_CR1_OFFSET)
+#define rCR2     	REG(STM32_GTIM_CR2_OFFSET)
+#define rSMCR    	REG(STM32_GTIM_SMCR_OFFSET)
+#define rDIER    	REG(STM32_GTIM_DIER_OFFSET)
+#define rSR      	REG(STM32_GTIM_SR_OFFSET)
+#define rEGR     	REG(STM32_GTIM_EGR_OFFSET)
+#define rCCMR1   	REG(STM32_GTIM_CCMR1_OFFSET)
+#define rCCMR2   	REG(STM32_GTIM_CCMR2_OFFSET)
+#define rCCER    	REG(STM32_GTIM_CCER_OFFSET)
+#define rCNT     	REG(STM32_GTIM_CNT_OFFSET)
+#define rPSC     	REG(STM32_GTIM_PSC_OFFSET)
+#define rARR     	REG(STM32_GTIM_ARR_OFFSET)
+#define rCCR1    	REG(STM32_GTIM_CCR1_OFFSET)
+#define rCCR2    	REG(STM32_GTIM_CCR2_OFFSET)
+#define rCCR3    	REG(STM32_GTIM_CCR3_OFFSET)
+#define rCCR4    	REG(STM32_GTIM_CCR4_OFFSET)
+#define rDCR     	REG(STM32_GTIM_DCR_OFFSET)
+#define rDMAR    	REG(STM32_GTIM_DMAR_OFFSET)
+
+/*
+ * Specific registers and bits used by HRT sub-functions
+ */
+#if HRT_TIMER_CHANNEL == 1
+# define rCCR_HRT	rCCR1			/* compare register for HRT */
+# define DIER_HRT	GTIM_DIER_CC1IE		/* interrupt enable for HRT */
+# define SR_INT_HRT	GTIM_SR_CC1IF		/* interrupt status for HRT */
+#elif HRT_TIMER_CHANNEL == 2
+# define rCCR_HRT	rCCR2			/* compare register for HRT */
+# define DIER_HRT	GTIM_DIER_CC2IE		/* interrupt enable for HRT */
+# define SR_INT_HRT	GTIM_SR_CC2IF		/* interrupt status for HRT */
+#elif HRT_TIMER_CHANNEL == 3
+# define rCCR_HRT	rCCR3			/* compare register for HRT */
+# define DIER_HRT	GTIM_DIER_CC3IE		/* interrupt enable for HRT */
+# define SR_INT_HRT	GTIM_SR_CC3IF		/* interrupt status for HRT */
+#elif HRT_TIMER_CHANNEL == 4
+# define rCCR_HRT	rCCR4			/* compare register for HRT */
+# define DIER_HRT	GTIM_DIER_CC4IE		/* interrupt enable for HRT */
+# define SR_INT_HRT	GTIM_SR_CC4IF		/* interrupt status for HRT */
+#else
+# error HRT_TIMER_CHANNEL must be a value between 1 and 4
+#endif
+
+/*
+ * Queue of callout entries.
+ */
+static struct sq_queue_s	callout_queue;
+
+/* timer-specific functions */
+static void		hrt_tim_init(void);
+static int		hrt_tim_isr(int irq, void *context);
+
+/* callout list manipulation */
+static void		hrt_call_internal(struct hrt_call *entry,
+					  hrt_abstime deadline,
+					  hrt_abstime interval,
+					  hrt_callout callout,
+					  void *arg);
+static void		hrt_call_enter(struct hrt_call *entry);
+static void		hrt_call_reschedule(void);
+static void		hrt_call_invoke(void);
+
+/*
+ * Specific registers and bits used by PPM sub-functions
+ */
+#ifdef CONFIG_HRT_PPM
+# if HRT_PPM_CHANNEL == 1
+#  define rCCR_PPM	rCCR1			/* capture register for PPM */
+#  define DIER_PPM	GTIM_DIER_CC1IE		/* capture interrupt (non-DMA mode) */
+#  define SR_INT_PPM	GTIM_SR_CC1IF		/* capture interrupt (non-DMA mode) */
+#  define SR_OVF_PPM	GTIM_SR_CC1OF		/* capture overflow (non-DMA mode) */
+#  define CCMR1_PPM	1			/* not on TI1/TI2 */
+#  define CCMR2_PPM	0			/* on TI3, not on TI4 */
+#  define CCER_PPM	(GTIM_CCER_CC1E | GTIM_CCER_CC1P | GTIM_CCER_CC1NP) /* CC1, both edges */
+# elif HRT_PPM_CHANNEL == 2
+#  define rCCR_PPM	rCCR2			/* capture register for PPM */
+#  define DIER_PPM	GTIM_DIER_CC2IE		/* capture interrupt (non-DMA mode) */
+#  define SR_INT_PPM	GTIM_SR_CC2IF		/* capture interrupt (non-DMA mode) */
+#  define SR_OVF_PPM	GTIM_SR_CC2OF		/* capture overflow (non-DMA mode) */
+#  define CCMR1_PPM	2			/* not on TI1/TI2 */
+#  define CCMR2_PPM	0			/* on TI3, not on TI4 */
+#  define CCER_PPM	(GTIM_CCER_CC2E | GTIM_CCER_CC2P | GTIM_CCER_CC2NP) /* CC2, both edges */
+# elif HRT_PPM_CHANNEL == 3
+#  define rCCR_PPM	rCCR3			/* capture register for PPM */
+#  define DIER_PPM	GTIM_DIER_CC3IE		/* capture interrupt (non-DMA mode) */
+#  define SR_INT_PPM	GTIM_SR_CC3IF		/* capture interrupt (non-DMA mode) */
+#  define SR_OVF_PPM	GTIM_SR_CC3OF		/* capture overflow (non-DMA mode) */
+#  define CCMR1_PPM	0			/* not on TI1/TI2 */
+#  define CCMR2_PPM	1			/* on TI3, not on TI4 */
+#  define CCER_PPM	(GTIM_CCER_CC3E | GTIM_CCER_CC3P | GTIM_CCER_CC3NP) /* CC3, both edges */
+# elif HRT_PPM_CHANNEL == 4
+#  define rCCR_PPM	rCCR4			/* capture register for PPM */
+#  define DIER_PPM	GTIM_DIER_CC4IE		/* capture interrupt (non-DMA mode) */
+#  define SR_INT_PPM	GTIM_SR_CC4IF		/* capture interrupt (non-DMA mode) */
+#  define SR_OVF_PPM	GTIM_SR_CC4OF		/* capture overflow (non-DMA mode) */
+#  define CCMR1_PPM	0			/* not on TI1/TI2 */
+#  define CCMR2_PPM	2			/* on TI3, not on TI4 */
+#  define CCER_PPM	(GTIM_CCER_CC4E | GTIM_CCER_CC4P | GTIM_CCER_CC4NP) /* CC4, both edges */
+# else
+#  error HRT_PPM_CHANNEL must be a value between 1 and 4 if CONFIG_HRT_PPM is set
+# endif
+
+/*
+ * PPM decoder tuning parameters
+ */
+# define PPM_MAX_PULSE_WIDTH	500		/* maximum width of a pulse */
+# define PPM_MIN_CHANNEL_VALUE	750		/* shortest valid channel signal */
+# define PPM_MAX_CHANNEL_VALUE	2400		/* longest valid channel signal */
+# define PPM_MIN_START		5000		/* shortest valid start gap */
+
+/* decoded PPM buffer */
+#define PPM_MAX_CHANNELS	12
+uint16_t ppm_buffer[PPM_MAX_CHANNELS];
+unsigned ppm_decoded_channels;
+
+/* PPM edge history */
+uint16_t ppm_edge_history[32];
+unsigned ppm_edge_next;
+
+/* PPM pulse history */
+uint16_t ppm_pulse_history[32];
+unsigned ppm_pulse_next;
+
+static uint16_t ppm_temp_buffer[PPM_MAX_CHANNELS];
+
+/* PPM decoder state machine */
+struct {
+	uint16_t	last_edge;	/* last capture time */
+	uint16_t	last_mark;	/* last significant edge */
+	unsigned	next_channel;
+	enum {
+		UNSYNCH = 0,
+		ARM,
+		ACTIVE,
+		INACTIVE
+	} phase;
+} ppm;
+
+static void	hrt_ppm_decode(uint32_t status);
+
+#else
+/* disable the PPM configuration */
+# define rCCR_PPM	0
+# define DIER_PPM	0
+# define SR_INT_PPM	0
+# define SR_OVF_PPM	0
+# define CCMR1_PPM	0
+# define CCMR2_PPM	0
+# define CCER_PPM	0
+#endif /* CONFIG_HRT_PPM */
+
+/*
+ * Initialise the timer we are going to use.
+ *
+ * We expect that we'll own one of the reduced-function STM32 general
+ * timers, and that we can use channel 1 in compare mode.
+ */
+static void
+hrt_tim_init(void)
+{
+	/* clock/power on our timer */
+        modifyreg32(HRT_TIMER_POWER_REG, 0, HRT_TIMER_POWER_BIT);
+
+        /* claim our interrupt vector */
+        irq_attach(HRT_TIMER_VECTOR, hrt_tim_isr);
+
+        /* disable and configure the timer */
+        rCR1 = 0;
+        rCR2 = 0;
+        rSMCR = 0;
+        rDIER = DIER_HRT | DIER_PPM;
+        rCCER = 0;		/* unlock CCMR* registers */
+        rCCMR1 = CCMR1_PPM;		
+        rCCMR2 = CCMR2_PPM;
+        rCCER = CCER_PPM;
+        rDCR = 0;
+
+        /* configure the timer to free-run at 1MHz */
+        rPSC = (HRT_TIMER_CLOCK / 1000000) - 1;	/* this really only works for whole-MHz clocks */
+
+        /* run the full span of the counter */
+        rARR = 0xffff;
+
+        /* set an initial capture a little ways off */
+        rCCR_HRT = 1000;
+
+        /* generate an update event; reloads the counter, all registers */
+        rEGR = GTIM_EGR_UG;
+
+        /* enable the timer */
+        rCR1 = GTIM_CR1_CEN;
+
+        /* enable interrupts */
+        up_enable_irq(HRT_TIMER_VECTOR);
+}
+
+#ifdef CONFIG_HRT_PPM
+/*
+ * Handle the PPM decoder state machine.
+ */
+static void
+hrt_ppm_decode(uint32_t status) 
+{
+	uint16_t count = rCCR_PPM;
+	uint16_t width;
+	uint16_t interval;
+	unsigned i;
+
+	/* if we missed an edge, we have to give up */
+	if (status & SR_OVF_PPM)
+		goto error;
+
+	/* how long since the last edge? */
+	width = count - ppm.last_edge;
+	ppm.last_edge = count;
+
+	ppm_edge_history[ppm_edge_next++] = width;
+	if (ppm_edge_next >= 32)
+		ppm_edge_next = 0;
+
+	/* 
+	 * if this looks like a start pulse, then push the last set of values
+	 * and reset the state machine
+	 */
+	if (width >= PPM_MIN_START) {
+
+		/* export the last set of samples if we got something sensible */
+		if (ppm.next_channel > 4) {
+			for (i = 0; i < ppm.next_channel && i < PPM_MAX_CHANNELS; i++)
+				ppm_buffer[i] = ppm_temp_buffer[i];
+			ppm_decoded_channels = i;
+		}
+
+		/* reset for the next frame */
+		ppm.next_channel = 0;
+
+		/* next edge is the reference for the first channel */
+		ppm.phase = ARM;
+
+		return;
+	}
+
+	switch (ppm.phase) {
+	case UNSYNCH:
+		/* we are waiting for a start pulse - nothing useful to do here */
+		return;
+
+	case ARM:
+		/* we expect a pulse giving us the first mark */
+		if (width > PPM_MAX_PULSE_WIDTH)
+			goto error;		/* pulse was too long */
+	
+		/* record the mark timing, expect an inactive edge */
+		ppm.last_mark = count;
+		ppm.phase = INACTIVE;
+		return;
+
+	case INACTIVE:
+		/* this edge is not interesting, but now we are ready for the next mark */
+		ppm.phase = ACTIVE;
+		return;
+
+	case ACTIVE:
+		/* determine the interval from the last mark */
+		interval = count - ppm.last_mark;
+		ppm.last_mark = count;
+
+		ppm_pulse_history[ppm_pulse_next++] = interval;
+		if (ppm_pulse_next >= 32)
+			ppm_pulse_next = 0;
+
+		/* if the mark-mark timing is out of bounds, abandon the frame */
+		if ((interval < PPM_MIN_CHANNEL_VALUE) || (interval > PPM_MAX_CHANNEL_VALUE))
+			goto error;
+
+		/* if we have room to store the value, do so */
+		if (ppm.next_channel < PPM_MAX_CHANNELS)
+			ppm_temp_buffer[ppm.next_channel++] = interval;
+
+		ppm.phase = INACTIVE;
+		return;		
+
+	}
+
+	/* the state machine is corrupted; reset it */
+
+error:
+	/* we don't like the state of the decoder, reset it and try again */
+	ppm.phase = UNSYNCH;
+}
+#endif /* CONFIG_HRT_PPM */
+
+/*
+ * Handle the compare interupt by calling the callout dispatcher
+ * and then re-scheduling the next deadline.
+ */
+static int
+hrt_tim_isr(int irq, void *context)
+{
+	uint32_t status;
+
+	/* copy interrupt status */
+	status = rSR;
+
+	/* ack the interrupts we just read */
+	rSR = ~status;
+
+#ifdef CONFIG_HRT_PPM
+	/* was this a PPM edge? */
+	if (status & (SR_INT_PPM | SR_OVF_PPM))
+		hrt_ppm_decode(status);
+#endif
+
+	/* was this a timer tick? */
+	if (status & SR_INT_HRT) {
+		/* run any callouts that have met their deadline */
+		hrt_call_invoke();
+
+		/* and schedule the next interrupt */
+		hrt_call_reschedule();
+	}
+
+	return OK;
+}
+
+/*
+ * Fetch a never-wrapping absolute time value in microseconds from
+ * some arbitrary epoch shortly after system start.
+ */
+hrt_abstime
+hrt_absolute_time(void)
+{
+	hrt_abstime	abstime;
+	uint32_t	count;
+	uint32_t	flags;
+
+	/*
+	 * Counter state.  Marked volatile as they may change
+	 * inside this routine but outside the irqsave/restore
+	 * pair.  Discourage the compiler from moving loads/stores
+	 * to these outside of the protected range.
+	 */
+	static volatile hrt_abstime base_time;
+	static volatile uint32_t last_count;
+
+	/* prevent re-entry */
+	flags = irqsave();
+
+	/* get the current counter value */
+	count = rCNT;
+
+	/*
+	 * Determine whether the counter has wrapped since the
+	 * last time we're called.
+	 *
+	 * This simple test is sufficient due to the guarantee that
+	 * we are always called at least once per counter period.
+	 */
+	if (count < last_count)
+		base_time += HRT_COUNTER_PERIOD;
+
+	/* save the count for next time */
+	last_count = count;
+
+	/* compute the current time */
+	abstime = HRT_COUNTER_SCALE(base_time + count);
+
+	irqrestore(flags);
+
+	return abstime;
+}
+
+/*
+ * Convert a timespec to absolute time
+ */
+hrt_abstime
+ts_to_abstime(struct timespec *ts)
+{
+	hrt_abstime	result;
+
+	result = (hrt_abstime)(ts->tv_sec) * 1000000;
+	result += ts->tv_nsec / 1000;
+
+	return result;
+}
+
+/*
+ * Convert absolute time to a timespec.
+ */
+void
+abstime_to_ts(struct timespec *ts, hrt_abstime abstime)
+{
+	ts->tv_sec = abstime / 1000000;
+	abstime -= ts->tv_sec * 1000000;
+	ts->tv_nsec = abstime * 1000;
+}
+
+/*
+ * Initalise the high-resolution timing module.
+ */
+void
+hrt_init(void)
+{
+	sq_init(&callout_queue);
+	hrt_tim_init();
+
+#ifdef CONFIG_HRT_PPM
+	/* configure the PPM input pin */
+	stm32_configgpio(GPIO_PPM_IN);
+#endif
+}
+
+/*
+ * Call callout(arg) after interval has elapsed.
+ */
+void
+hrt_call_after(struct hrt_call *entry, hrt_abstime delay, hrt_callout callout, void *arg)
+{
+	hrt_call_internal(entry, 
+			  hrt_absolute_time() + delay,
+			  0,
+			  callout,
+			  arg);
+}
+
+/*
+ * Call callout(arg) at calltime.
+ */
+void
+hrt_call_at(struct hrt_call *entry, hrt_abstime calltime, hrt_callout callout, void *arg)
+{
+	hrt_call_internal(entry, calltime, 0, callout, arg);
+}
+
+/*
+ * Call callout(arg) every period.
+ */
+void
+hrt_call_every(struct hrt_call *entry, hrt_abstime delay, hrt_abstime interval, hrt_callout callout, void *arg)
+{
+	hrt_call_internal(entry,
+			  hrt_absolute_time() + delay,
+			  interval,
+			  callout,
+			  arg);
+}
+
+static void
+hrt_call_internal(struct hrt_call *entry, hrt_abstime deadline, hrt_abstime interval, hrt_callout callout, void *arg)
+{
+	irqstate_t flags = irqsave();
+
+	/* if the entry is currently queued, remove it */
+	if (entry->deadline != 0)
+		sq_rem(&entry->link, &callout_queue);
+
+	entry->deadline = deadline;
+	entry->period = interval;
+	entry->callout = callout;
+	entry->arg = arg;
+
+	hrt_call_enter(entry);
+
+	irqrestore(flags);
+}
+
+/*
+ * If this returns true, the call has been invoked and removed from the callout list.
+ *
+ * Always returns false for repeating callouts.
+ */
+bool
+hrt_called(struct hrt_call *entry)
+{
+	return (entry->deadline == 0);
+}
+
+/*
+ * Remove the entry from the callout list.
+ */
+void
+hrt_cancel(struct hrt_call *entry)
+{
+	irqstate_t flags = irqsave();
+
+	sq_rem(&entry->link, &callout_queue);
+	entry->deadline = 0;
+
+	/* if this is a periodic call being removed by the callout, prevent it from
+	 * being re-entered when the callout returns.
+	 */
+	entry->period = 0;
+
+	irqrestore(flags);
+}
+
+static void
+hrt_call_enter(struct hrt_call *entry)
+{
+	struct hrt_call	*call, *next;
+
+	call = (struct hrt_call *)sq_peek(&callout_queue);
+
+	if ((call == NULL) || (entry->deadline < call->deadline)) {
+		sq_addfirst(&entry->link, &callout_queue);
+		//lldbg("call enter at head, reschedule\n");
+		/* we changed the next deadline, reschedule the timer event */
+		hrt_call_reschedule();
+	} else {
+		do {
+			next = (struct hrt_call *)sq_next(&call->link);
+			if ((next == NULL) || (entry->deadline < next->deadline)) {
+				//lldbg("call enter after head\n");
+				sq_addafter(&call->link, &entry->link, &callout_queue);
+				break;
+			}
+		} while ((call = next) != NULL);
+	}
+
+	//lldbg("scheduled\n");
+}
+
+static void
+hrt_call_invoke(void)
+{
+	struct hrt_call	*call;
+	hrt_abstime deadline;
+
+	while (true) {
+		/* get the current time */
+		hrt_abstime now = hrt_absolute_time();
+
+		call = (struct hrt_call *)sq_peek(&callout_queue);
+		if (call == NULL)
+			break;
+		if (call->deadline > now)
+			break;
+
+		sq_rem(&call->link, &callout_queue);
+		//lldbg("call pop\n");
+
+		/* save the intended deadline for periodic calls */
+		deadline = call->deadline; 
+
+		/* zero the deadline, as the call has occurred */
+		call->deadline = 0;
+
+		/* invoke the callout (if there is one) */
+		if (call->callout) {
+			//lldbg("call %p: %p(%p)\n", call, call->callout, call->arg);
+			call->callout(call->arg);
+		}
+
+		/* if the callout has a non-zero period, it has to be re-entered */
+		if (call->period != 0) {
+			call->deadline = deadline + call->period;
+			hrt_call_enter(call);
+		}
+	}
+}
+
+/*
+ * Reschedule the next timer interrupt.
+ *
+ * This routine must be called with interrupts disabled.
+ */
+static void
+hrt_call_reschedule()
+{
+	hrt_abstime	now = hrt_absolute_time();
+	struct hrt_call	*next = (struct hrt_call *)sq_peek(&callout_queue);
+	hrt_abstime	deadline = now + HRT_INTERVAL_MAX;
+
+	/*
+	 * Determine what the next deadline will be.
+	 *
+	 * Note that we ensure that this will be within the counter
+	 * period, so that when we truncate all but the low 16 bits
+	 * the next time the compare matches it will be the deadline
+	 * we want.
+	 *
+	 * It is important for accurate timekeeping that the compare
+	 * interrupt fires sufficiently often that the base_time update in 
+	 * hrt_absolute_time runs at least once per timer period.
+	 */
+	if (next != NULL) {
+		//lldbg("entry in queue\n");
+		if (next->deadline <= (now + HRT_INTERVAL_MIN)) {
+			//lldbg("pre-expired\n");
+			/* set a minimal deadline so that we call ASAP */
+			deadline = now + HRT_INTERVAL_MIN;
+		} else if (next->deadline < deadline) {
+			//lldbg("due soon\n");
+			deadline = next->deadline;
+		}
+	}
+	//lldbg("schedule for %u at %u\n", (unsigned)(deadline & 0xffffffff), (unsigned)(now & 0xffffffff));
+
+	/* set the new compare value */
+	rCCR_HRT = deadline & 0xffff;
+}
+
+#endif /* CONFIG_HRT_TIMER */