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mbed-os/targets/TARGET_NORDIC/TARGET_MCU_NRF51822/us_ticker.c

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/* mbed Microcontroller Library
* Copyright (c) 2013 Nordic Semiconductor
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include <stddef.h>
#include <stdbool.h>
#include "us_ticker_api.h"
#include "cmsis.h"
#include "PeripheralNames.h"
#include "nrf_delay.h"
#include "mbed_toolchain.h"
#include "mbed_critical.h"
/*
* Note: The micro-second timer API on the nRF51 platform is implemented using
* the RTC counter run at 32kHz (sourced from an external oscillator). This is
* a trade-off between precision and power. Running a normal 32-bit MCU counter
* at high frequency causes the average power consumption to rise to a few
* hundred micro-amps, which is prohibitive for typical low-power BLE
* applications.
* A 32kHz clock doesn't offer the precision needed for keeping u-second time,
* but we're assuming that this will not be a problem for the average user.
*/
#define MAX_RTC_COUNTER_VAL 0x00FFFFFF /**< Maximum value of the RTC counter. */
#define RTC_CLOCK_FREQ (uint32_t)(32768)
#define RTC1_IRQ_PRI 3 /**< Priority of the RTC1 interrupt (used
* for checking for timeouts and executing
* timeout handlers). This must be the same
* as APP_IRQ_PRIORITY_LOW; taken from the
* Nordic SDK. */
#define MAX_RTC_TASKS_DELAY 47 /**< Maximum delay until an RTC task is executed. */
#define FUZZY_RTC_TICKS 2 /* RTC COMPARE occurs when a CC register is N and the RTC
* COUNTER value transitions from N-1 to N. If we're trying to
* setup a callback for a time which will arrive very shortly,
* there are limits to how short the callback interval may be for us
* to rely upon the RTC Compare trigger. If the COUNTER is N,
* writing N+2 to a CC register is guaranteed to trigger a COMPARE
* event at N+2. */
#define RTC_UNITS_TO_MICROSECONDS(RTC_UNITS) (((RTC_UNITS) * (uint64_t)1000000) / RTC_CLOCK_FREQ)
#define MICROSECONDS_TO_RTC_UNITS(MICROS) ((((uint64_t)(MICROS) * RTC_CLOCK_FREQ) + 999999) / 1000000)
#define US_TICKER_SW_IRQ_MASK 0x1
static bool us_ticker_inited = false;
static volatile uint32_t overflowCount; /**< The number of times the 24-bit RTC counter has overflowed. */
static volatile bool us_ticker_callbackPending = false;
static uint32_t us_ticker_callbackTimestamp;
static bool os_tick_started = false; /**< flag indicating if the os_tick has started */
/**
* The value previously set in the capture compare register of channel 1
*/
static uint32_t previous_tick_cc_value = 0;
// us ticker fire interrupt flag for IRQ handler
volatile uint8_t m_common_sw_irq_flag = 0;
/*
RTX provide the following definitions which are used by the tick code:
* os_trv: The number (minus 1) of clock cycle between two tick.
* os_clockrate: Time duration between two ticks (in us).
* OS_Tick_Handler: The function which handle a tick event.
This function is special because it never returns.
Those definitions are used by the code which handle the os tick.
To allow compilation of us_ticker programs without RTOS, those symbols are
exported from this module as weak ones.
*/
MBED_WEAK uint32_t const os_trv;
MBED_WEAK uint32_t const os_clockrate;
MBED_WEAK void OS_Tick_Handler() { }
static inline void rtc1_enableCompareInterrupt(void)
{
NRF_RTC1->EVTENCLR = RTC_EVTEN_COMPARE0_Msk;
NRF_RTC1->INTENSET = RTC_INTENSET_COMPARE0_Msk;
}
static inline void rtc1_disableCompareInterrupt(void)
{
NRF_RTC1->INTENCLR = RTC_INTENSET_COMPARE0_Msk;
NRF_RTC1->EVTENCLR = RTC_EVTEN_COMPARE0_Msk;
}
static inline void rtc1_enableOverflowInterrupt(void)
{
NRF_RTC1->EVTENCLR = RTC_EVTEN_OVRFLW_Msk;
NRF_RTC1->INTENSET = RTC_INTENSET_OVRFLW_Msk;
}
static inline void rtc1_disableOverflowInterrupt(void)
{
NRF_RTC1->INTENCLR = RTC_INTENSET_OVRFLW_Msk;
NRF_RTC1->EVTENCLR = RTC_EVTEN_OVRFLW_Msk;
}
static inline void invokeCallback(void)
{
us_ticker_callbackPending = false;
rtc1_disableCompareInterrupt();
us_ticker_irq_handler();
}
/**
* @brief Function for starting the RTC1 timer. The RTC timer is expected to
* keep running--some interrupts may be disabled temporarily.
*/
static void rtc1_start()
{
NRF_RTC1->PRESCALER = 0; /* for no pre-scaling. */
rtc1_enableOverflowInterrupt();
NVIC_SetPriority(RTC1_IRQn, RTC1_IRQ_PRI);
NVIC_ClearPendingIRQ(RTC1_IRQn);
NVIC_EnableIRQ(RTC1_IRQn);
NRF_RTC1->TASKS_START = 1;
nrf_delay_us(MAX_RTC_TASKS_DELAY);
}
/**
* @brief Function for stopping the RTC1 timer. We don't expect to call this.
*/
void rtc1_stop(void)
{
// If the os tick has been started, RTC1 shouldn't be stopped
// In that case, us ticker and overflow interrupt are disabled.
if (os_tick_started) {
rtc1_disableCompareInterrupt();
rtc1_disableOverflowInterrupt();
} else {
NVIC_DisableIRQ(RTC1_IRQn);
rtc1_disableCompareInterrupt();
rtc1_disableOverflowInterrupt();
NRF_RTC1->TASKS_STOP = 1;
nrf_delay_us(MAX_RTC_TASKS_DELAY);
NRF_RTC1->TASKS_CLEAR = 1;
nrf_delay_us(MAX_RTC_TASKS_DELAY);
}
}
/**
* @brief Function for returning the current value of the RTC1 counter.
*
* @return Current RTC1 counter as a 64-bit value with 56-bit precision (even
* though the underlying counter is 24-bit)
*/
static inline uint64_t rtc1_getCounter64(void)
{
if (NRF_RTC1->EVENTS_OVRFLW) {
overflowCount++;
NRF_RTC1->EVENTS_OVRFLW = 0;
NRF_RTC1->EVTENCLR = RTC_EVTEN_OVRFLW_Msk;
}
return ((uint64_t)overflowCount << 24) | NRF_RTC1->COUNTER;
}
/**
* @brief Function for returning the current value of the RTC1 counter.
*
* @return Current RTC1 counter as a 32-bit value (even though the underlying counter is 24-bit)
*/
static inline uint32_t rtc1_getCounter(void)
{
return rtc1_getCounter64();
}
/**
* @brief Function for handling the RTC1 interrupt for us ticker (capture compare channel 0 and overflow).
*
* @details Checks for timeouts, and executes timeout handlers for expired timers.
*/
void us_ticker_handler(void)
{
if (m_common_sw_irq_flag & US_TICKER_SW_IRQ_MASK) {
m_common_sw_irq_flag &= ~US_TICKER_SW_IRQ_MASK;
us_ticker_irq_handler();
}
if (NRF_RTC1->EVENTS_OVRFLW) {
overflowCount++;
NRF_RTC1->EVENTS_OVRFLW = 0;
NRF_RTC1->EVTENCLR = RTC_EVTEN_OVRFLW_Msk;
}
if (NRF_RTC1->EVENTS_COMPARE[0]) {
NRF_RTC1->EVENTS_COMPARE[0] = 0;
NRF_RTC1->EVTENCLR = RTC_EVTEN_COMPARE0_Msk;
if (us_ticker_callbackPending && ((int)(us_ticker_callbackTimestamp - rtc1_getCounter()) <= 0))
invokeCallback();
}
}
void us_ticker_init(void)
{
if (us_ticker_inited) {
return;
}
rtc1_start();
us_ticker_inited = true;
}
uint32_t us_ticker_read()
{
if (!us_ticker_inited) {
us_ticker_init();
}
/* Return a pseudo microsecond counter value. This is only as precise as the
* 32khz low-freq clock source, but could be adequate.*/
return RTC_UNITS_TO_MICROSECONDS(rtc1_getCounter64());
}
/**
* Setup the us_ticker callback interrupt to go at the given timestamp.
*
* @Note: Only one callback is pending at any time.
*
* @Note: If a callback is pending, and this function is called again, the new
* callback-time overrides the existing callback setting. It is the caller's
* responsibility to ensure that this function is called to setup a callback for
* the earliest timeout.
*
* @Note: If this function is used to setup an interrupt which is immediately
* pending--such as for 'now' or a time in the past,--then the callback is
* invoked a few ticks later.
*/
void us_ticker_set_interrupt(timestamp_t timestamp)
{
if (!us_ticker_inited) {
us_ticker_init();
}
/*
* The argument to this function is a 32-bit microsecond timestamp for when
* a callback should be invoked. On the nRF51, we use an RTC timer running
* at 32kHz to implement a low-power us-ticker. This results in a problem
* based on the fact that 1000000 is not a multiple of 32768.
*
* Going from a micro-second based timestamp to a 32kHz based RTC-time is a
* linear mapping; but this mapping doesn't preserve wraparounds--i.e. when
* the 32-bit micro-second timestamp wraps around unfortunately the
* underlying RTC counter doesn't. The result is that timestamp expiry
* checks on micro-second timestamps don't yield the same result when
* applied on the corresponding RTC timestamp values.
*
* One solution is to translate the incoming 32-bit timestamp into a virtual
* 64-bit timestamp based on the knowledge of system-uptime, and then use
* this wraparound-free 64-bit value to do a linear mapping to RTC time.
* System uptime on an nRF is maintained using the 24-bit RTC counter. We
* track the overflow count to extend the 24-bit hardware counter by an
* additional 32 bits. RTC_UNITS_TO_MICROSECONDS() converts this into
* microsecond units (in 64-bits).
*/
const uint64_t currentTime64 = RTC_UNITS_TO_MICROSECONDS(rtc1_getCounter64());
uint64_t timestamp64 = (currentTime64 & ~(uint64_t)0xFFFFFFFFULL) + timestamp;
if (((uint32_t)currentTime64 > 0x80000000) && (timestamp < 0x80000000)) {
timestamp64 += (uint64_t)0x100000000ULL;
}
uint32_t newCallbackTime = MICROSECONDS_TO_RTC_UNITS(timestamp64);
/* Check for repeat setup of an existing callback. This is actually not
* important; the following code should work even without this check. */
if (us_ticker_callbackPending && (newCallbackTime == us_ticker_callbackTimestamp)) {
return;
}
/* Check for callbacks which are immediately (or will *very* shortly become) pending.
* Even if they are immediately pending, they are scheduled to trigger a few
* ticks later. This keeps things simple by invoking the callback from an
* independent interrupt context. */
if ((int)(newCallbackTime - rtc1_getCounter()) <= (int)FUZZY_RTC_TICKS) {
newCallbackTime = rtc1_getCounter() + FUZZY_RTC_TICKS;
}
NRF_RTC1->CC[0] = newCallbackTime & MAX_RTC_COUNTER_VAL;
us_ticker_callbackTimestamp = newCallbackTime;
if (!us_ticker_callbackPending) {
us_ticker_callbackPending = true;
rtc1_enableCompareInterrupt();
}
}
void us_ticker_fire_interrupt(void)
{
core_util_critical_section_enter();
m_common_sw_irq_flag |= US_TICKER_SW_IRQ_MASK;
NVIC_SetPendingIRQ(RTC1_IRQn);
core_util_critical_section_exit();
}
void us_ticker_disable_interrupt(void)
{
if (us_ticker_callbackPending) {
rtc1_disableCompareInterrupt();
us_ticker_callbackPending = false;
}
}
void us_ticker_clear_interrupt(void)
{
NRF_RTC1->EVENTS_OVRFLW = 0;
NRF_RTC1->EVENTS_COMPARE[0] = 0;
}
void us_ticker_free(void)
{
}
#if defined (__CC_ARM) /* ARMCC Compiler */
__asm void RTC1_IRQHandler(void)
{
IMPORT OS_Tick_Handler
IMPORT us_ticker_handler
/**
* Chanel 1 of RTC1 is used by RTX as a systick.
* If the compare event on channel 1 is set, then branch to OS_Tick_Handler.
* Otherwise, just execute us_ticker_handler.
* This function has to be written in assembly and tagged as naked because OS_Tick_Handler
* will never return.
* A c function would put lr on the stack before calling OS_Tick_Handler and this value
* would never been dequeued.
*
* \code
* void RTC1_IRQHandler(void) {
if(NRF_RTC1->EVENTS_COMPARE[1]) {
// never return...
OS_Tick_Handler();
} else {
us_ticker_handler();
}
}
* \endcode
*/
ldr r0,=0x40011144
ldr r1, [r0, #0]
cmp r1, #0
beq US_TICKER_HANDLER
bl OS_Tick_Handler
US_TICKER_HANDLER
push {r3, lr}
bl us_ticker_handler
pop {r3, pc}
nop /* padding */
}
#elif defined (__GNUC__) /* GNU Compiler */
__attribute__((naked)) void RTC1_IRQHandler(void)
{
/**
* Chanel 1 of RTC1 is used by RTX as a systick.
* If the compare event on channel 1 is set, then branch to OS_Tick_Handler.
* Otherwise, just execute us_ticker_handler.
* This function has to be written in assembly and tagged as naked because OS_Tick_Handler
* will never return.
* A c function would put lr on the stack before calling OS_Tick_Handler and this value
* would never been dequeued.
*
* \code
* void RTC1_IRQHandler(void) {
if(NRF_RTC1->EVENTS_COMPARE[1]) {
// never return...
OS_Tick_Handler();
} else {
us_ticker_handler();
}
}
* \endcode
*/
__asm__ (
"ldr r0,=0x40011144\n"
"ldr r1, [r0, #0]\n"
"cmp r1, #0\n"
"beq US_TICKER_HANDLER\n"
"bl OS_Tick_Handler\n"
"US_TICKER_HANDLER:\n"
"push {r3, lr}\n"
"bl us_ticker_handler\n"
"pop {r3, pc}\n"
"nop"
);
}
#else
#error Compiler not supported.
#error Provide a definition of RTC1_IRQHandler.
/*
* Chanel 1 of RTC1 is used by RTX as a systick.
* If the compare event on channel 1 is set, then branch to OS_Tick_Handler.
* Otherwise, just execute us_ticker_handler.
* This function has to be written in assembly and tagged as naked because OS_Tick_Handler
* will never return.
* A c function would put lr on the stack before calling OS_Tick_Handler and this value
* will never been dequeued. After a certain time a stack overflow will happen.
*
* \code
* void RTC1_IRQHandler(void) {
if(NRF_RTC1->EVENTS_COMPARE[1]) {
// never return...
OS_Tick_Handler();
} else {
us_ticker_handler();
}
}
* \endcode
*/
#endif
/**
* Return the next number of clock cycle needed for the next tick.
* @note This function has been carrefuly optimized for a systick occuring every 1000us.
*/
static uint32_t get_next_tick_cc_delta() {
uint32_t delta = 0;
if (os_clockrate != 1000) {
// In RTX, by default SYSTICK is is used.
// A tick event is generated every os_trv + 1 clock cycles of the system timer.
delta = os_trv + 1;
} else {
// If the clockrate is set to 1000us then 1000 tick should happen every second.
// Unfortunatelly, when clockrate is set to 1000, os_trv is equal to 31.
// If (os_trv + 1) is used as the delta value between two ticks, 1000 ticks will be
// generated in 32000 clock cycle instead of 32768 clock cycles.
// As a result, if a user schedule an OS timer to start in 100s, the timer will start
// instead after 97.656s
// The code below fix this issue, a clock rate of 1000s will generate 1000 ticks in 32768
// clock cycles.
// The strategy is simple, for 1000 ticks:
// * 768 ticks will occur 33 clock cycles after the previous tick
// * 232 ticks will occur 32 clock cycles after the previous tick
// By default every delta is equal to 33.
// Every five ticks (20%, 200 delta in one second), the delta is equal to 32
// The remaining (32) deltas equal to 32 are distributed using primes numbers.
static uint32_t counter = 0;
if ((counter % 5) == 0 || (counter % 31) == 0 || (counter % 139) == 0 || (counter == 503)) {
delta = 32;
} else {
delta = 33;
}
++counter;
if (counter == 1000) {
counter = 0;
}
}
return delta;
}
static inline void clear_tick_interrupt() {
NRF_RTC1->EVENTS_COMPARE[1] = 0;
NRF_RTC1->EVTENCLR = (1 << 17);
}
/**
* Indicate if a value is included in a range which can be wrapped.
* @param begin start of the range
* @param end end of the range
* @param val value to check
* @return true if the value is included in the range and false otherwise.
*/
static inline bool is_in_wrapped_range(uint32_t begin, uint32_t end, uint32_t val) {
// regular case, begin < end
// return true if begin <= val < end
if (begin < end) {
if (begin <= val && val < end) {
return true;
} else {
return false;
}
} else {
// In this case end < begin because it has wrap around the limits
// return false if end < val < begin
if (end < val && val < begin) {
return false;
} else {
return true;
}
}
}
/**
* Register the next tick.
*/
static void register_next_tick() {
previous_tick_cc_value = NRF_RTC1->CC[1];
uint32_t delta = get_next_tick_cc_delta();
uint32_t new_compare_value = (previous_tick_cc_value + delta) & MAX_RTC_COUNTER_VAL;
// Disable irq directly for few cycles,
// Validation of the new CC value against the COUNTER,
// Setting the new CC value and enabling CC IRQ should be an atomic operation
// Otherwise, there is a possibility to set an invalid CC value because
// the RTC1 keeps running.
// This code is very short 20-38 cycles in the worst case, it shouldn't
// disturb softdevice.
__disable_irq();
uint32_t current_counter = NRF_RTC1->COUNTER;
// If an overflow occur, set the next tick in COUNTER + delta clock cycles
if (is_in_wrapped_range(previous_tick_cc_value, new_compare_value, current_counter) == false) {
new_compare_value = current_counter + delta;
}
NRF_RTC1->CC[1] = new_compare_value;
// set the interrupt of CC channel 1 and reenable IRQs
NRF_RTC1->INTENSET = RTC_INTENSET_COMPARE1_Msk;
__enable_irq();
}
/**
* Initialize alternative hardware timer as RTX kernel timer
* This function is directly called by RTX.
* @note this function shouldn't be called directly.
* @return IRQ number of the alternative hardware timer
*/
int os_tick_init (void)
{
// their is no need to start the LF clock, it is already started by SystemInit.
NVIC_SetPriority(RTC1_IRQn, RTC1_IRQ_PRI);
NVIC_ClearPendingIRQ(RTC1_IRQn);
NVIC_EnableIRQ(RTC1_IRQn);
NRF_RTC1->TASKS_START = 1;
nrf_delay_us(MAX_RTC_TASKS_DELAY);
NRF_RTC1->CC[1] = 0;
clear_tick_interrupt();
register_next_tick();
os_tick_started = true;
return RTC1_IRQn;
}
/**
* Acknowledge the tick interrupt.
* This function is called by the function OS_Tick_Handler of RTX.
* @note this function shouldn't be called directly.
*/
void os_tick_irqack(void)
{
clear_tick_interrupt();
register_next_tick();
}
/**
* Returns the overflow flag of the alternative hardware timer.
* @note This function is exposed by RTX kernel.
* @return 1 if the timer has overflowed and 0 otherwise.
*/
uint32_t os_tick_ovf(void) {
uint32_t current_counter = NRF_RTC1->COUNTER;
uint32_t next_tick_cc_value = NRF_RTC1->CC[1];
return is_in_wrapped_range(previous_tick_cc_value, next_tick_cc_value, current_counter) ? 0 : 1;
}
/**
* Return the value of the alternative hardware timer.
* @note The documentation is not very clear about what is expected as a result,
* is it an ascending counter, a descending one ?
* None of this is specified.
* The default systick is a descending counter and this function return values in
* descending order, even if the internal counter used is an ascending one.
* @return the value of the alternative hardware timer.
*/
uint32_t os_tick_val(void) {
uint32_t current_counter = NRF_RTC1->COUNTER;
uint32_t next_tick_cc_value = NRF_RTC1->CC[1];
// do not use os_tick_ovf because its counter value can be different
if(is_in_wrapped_range(previous_tick_cc_value, next_tick_cc_value, current_counter)) {
if (next_tick_cc_value > previous_tick_cc_value) {
return next_tick_cc_value - current_counter;
} else if(current_counter <= next_tick_cc_value) {
return next_tick_cc_value - current_counter;
} else {
return next_tick_cc_value + (MAX_RTC_COUNTER_VAL - current_counter);
}
} else {
// use (os_trv + 1) has the base step, can be totally inacurate ...
uint32_t clock_cycles_by_tick = os_trv + 1;
// if current counter has wrap arround, add the limit to it.
if (current_counter < next_tick_cc_value) {
current_counter = current_counter + MAX_RTC_COUNTER_VAL;
}
return clock_cycles_by_tick - ((current_counter - next_tick_cc_value) % clock_cycles_by_tick);
}
}
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