pw_chrono#

Pigweed’s chrono module provides facilities for applications to deal with time, leveraging many pieces of STL’s the std::chrono library but with a focus on portability for constrained embedded devices and maintaining correctness.

Note

This module is still under construction, the API is not yet stable.

duration and time_point#

Pigweed’s time primitives rely on C++’s <chrono> library to enable users to express intents with strongly typed real time units through std::chrono::duration and std::chrono::time_point .

What are they?#

At a high level, durations and time_points at run time are tick counts which are wrapped in templated metadata which is only used at compile time.

The STL’s std::chrono::duration class template represents a time interval. It consists of a count of ticks of type rep and a tick period, where the tick period is a std::ratio representing the time in seconds from one tick to the next.

The only data stored in a duration is a tick count of type rep. The period is included as part of the duration’s type, and is only used when converting between different durations.

Similarly, the STL’s std::chrono::time_point class template represents a point in time (i.e. timestamp). It consists of a value of type duration which represents the time interval from the start of the clock’s epoch.

The duration and time_point class templates can be represented with the following simplified model, ignoring most of their member functions:

namespace std::chrono {

template<class Rep, class Period = std::ratio<1, 1>>
class duration {
 public:
  using rep = Rep;
  using period = Period;

  constexpr rep count() const { return tick_count_; }

  static constexpr duration zero() noexcept {
    return duration(0);
  }

  // Other member functions...

 private:
  rep tick_count_;
};

template<class Clock, class Duration = typename Clock::duration>
class time_point {
 public:
  using duration = Duration;
  using rep = Duration::rep;
  using period = Duration::period;
  using clock = Clock;

  constexpr duration time_since_epoch() const { return time_since_epoch_; }

  // Other member functions...

 private:
  duration time_since_epoch_;
};

}  // namespace std::chrono

What rep type should be used?#

The duration’s rep, or tick count type, can be a floating point or a signed integer. For most applications, this is a signed integer just as how one may represent the number of ticks for an RTOS API or the number of nanoseconds in POSIX.

The rep should be able to represent the durations of time necessary for the application. Pigweed recommends that the duration’s rep used for a clock use int64_t in order to trivially avoid integer underflow and overflow risks by covering a range of at least ±292 years. This matches the STL’s requirements for the duration helper types which are relevant for a clock’s tick period:

  • std::chrono::nanoseconds  duration</*signed integer type of at least 64 bits*/, std::nano>

  • std::chrono::microseconds duration</*signed integer type of at least 55 bits*/, std::micro>

  • std::chrono::milliseconds duration</*signed integer type of at least 45 bits*/, std::milli>

  • std::chrono::seconds  duration</*signed integer type of at least 35 bits*/>

With this guidance one can avoid common pitfalls like uint32_t millisecond tick rollover bugs when using RTOSes every 49.7 days.

Warning

We recommend avoiding the duration<>::min() and duration<>::max() helper member functions where possible as they exceed the ±292 years duration limit assumption. There’s an immediate risk of integer underflow or overflow for any arithmetic operations. Consider using std::optional instead of priming a variable with a value at the limit.

Helper duration types and literals#

The STL’s <chrono> library includes a set of helper types based on actual time units, including the following (and more):

  • std::chrono::nanoseconds

  • std::chrono::microseconds

  • std::chrono::milliseconds

  • std::chrono::seconds

  • std::chrono::minutes

  • std::chrono::hours

As an example you can use these as follows:

#include <chrono>

void Foo() {
  Bar(std::chrono::milliseconds(42));
}

In addition, the inline namespace std::literals::chrono_literals includes:

  • operator""ns for std::chrono::nanoseconds

  • operator""us for std::chrono::microseconds

  • operator""ms for std::chrono::milliseconds

  • operator""s for std::chrono::seconds

  • operator""min for std::chrono::minutes

  • operator""h for std::chrono::hours

As an example you can use these as follows:

using std::literals::chrono_literals::ms;
// Or if you want them all: using namespace std::chrono_literals;

void Foo() {
  Bar(42ms);
}

For these helper duration types to be compatible with API’s that take a SystemClock::duration either an implicit or explicit lossy conversion must be done.

Converting between time units and clock durations#

So why go through all of this trouble instead of just using ticks or instead just using one time unit such as nanoseconds? For example, imagine that you have a 1kHz RTOS tick period and you would like to express a timeout duration:

// Instead of using ticks which are not portable between RTOS configurations,
// as the tick period may be different:
constexpr uint32_t kFooNotificationTimeoutTicks = 42;
bool TryGetNotificationFor(uint32_t ticks);

// And instead of using a time unit which is prone to accidental conversion
// errors as all variables must maintain the time units:
constexpr uint32_t kFooNotificationTimeoutMs = 42;
bool TryGetNotificationFor(uint32_t milliseconds);

// We can instead use a defined clock and its duration for the kernel and rely
// on implicit lossless conversions:
#include <chrono>
#include "pw_chrono/system_clock.h"
constexpr SystemClock::duration kFooNotificationTimeout =
    std::chrono::milliseconds(42);
bool TryGetNotificationFor(SystemClock::duration timeout);

void MaybeProcessNotification() {
  if (TryGetNotificationFor(kFooNotificationTimeout)) {
    ProcessNotification();
  }
}

Implicit lossless conversions#

Wait, but how does this work? Is there a hidden cost? The duration type comes with built in implicit lossless conversion support which is evaluated at compile time where possible.

If you rely on implicit conversions then the worst case cost is multiplication, there is no risk of a division operation.

If the implicit conversion cannot be guaranteed at compile time to be lossless for all possible tick count values, then it will fail to compile.

As an example you can always convert from std::chrono::seconds to std::chrono::milliseconds in a lossless manner. However, you cannot guarantee for all tick count values that std::chrono::milliseconds can be losslessly converted to std::chrono::seconds, even though it may work for some values like 0, 1000, etc.

#include <chrono>

constexpr std::chrono::milliseconds this_compiles =
    std::chrono::seconds(42);

// This cannot compile, because for some duration values it is lossy even
// though this particular value can be in theory converted to whole seconds.
// constexpr std::chrono::seconds this_does_not_compile =
//    std::chrono::milliseconds(1000);

Explicit lossy conversions#

Although we recommend sticking to implicit lossless conversions, what if for some reason a lossy conversion is required? For example what if we’re using a 128Hz RTOS tick clock?

The 128Hz period can be perfectly represented with a std::ratio<1,128>. However you will not be able to implicitly convert any real time unit durations to this duration type. Instead explicit lossy conversions must be used. Pigweed recommends explicitly using:

  • std::chrono::floor to round down.

  • std::chrono::round to round to the nearest, rounding to even in halfway cases.

  • std::chrono::ceil to round up.

  • pw::chrono::SystemClock::for_at_least to round up using the SystemClock::period, as a more explicit form of std::chrono::ceil.

Note

Pigweed does not recommend using std::chrono::duration_cast<> which truncates dowards zero like static_cast. This is typically not the desired rounding behavior when dealing with time units. Instead, where possible we recommend the more explicit, self-documenting std::chrono::floor, std::chrono::round, and std::chrono::ceil.

Now knowing this, the previous example could be portably and correctly handled as follows:

#include <chrono>

#include "pw_chrono/system_clock.h"

// We want to round up to ensure we block for at least the specified duration,
// instead of rounding down. Imagine for example the extreme case where you
// may round down to zero or one, you would definitely want to at least block.
constexpr SystemClock::duration kFooNotificationTimeout =
    std::chrono::ceil(std::chrono::milliseconds(42));
bool TryGetNotificationFor(SystemClock::duration timeout);

void MaybeProcessNotification() {
  if (TryGetNotificationFor(kFooNotificationTimeout)) {
    ProcessNotification();
  }
}

This code is lossless if the clock period is 1kHz and it’s correct using a division which rounds up when the clock period is 128Hz.

Note

When using pw::chrono::SystemClock::duration for timeouts we recommend using its SystemClock::for_at_least() to round up timeouts in a more explicit, self documenting manner which uses std::chrono::ceil internally.

Use of count() and time_since_epoch()#

It’s easy to escape the typesafe chrono types through the use of duration<>::count() and time_point<>::time_since_epoch(), however this increases the risk of accidentally introduce conversion and arithmetic errors.

For this reason, we recommend avoiding these two escape hatches until it’s absolutely necessary due to I/O such as RPCs or writing to non-volatile storage.

Discrete Timeouts#

We briefly want to mention a common pitfall when working with discrete representations of time durations for timeouts (ticks and real time units) on systems with a continously running clock which is backed by discrete time intervals (i.e. whole integer constant tick periods).

Imagine an RTOS system where we have a constant tick interval. If we attempt to sleep for 1 tick, how long will the kernel actually let us sleep?

In most kernels you will end up sleeping somewhere between 0 and 1 tick periods inclusively, i.e. [0, 1], if we ignore scheduling latency and preemption. This means it can randomly be non-blocking vs blocking!

This is because internally kernels use a decrementing timeout counter or a deadline without taking the current current progression through the existing tick period into account.

For this reason all of Pigweed’s time bound APIs will internally add an extra tick to timeout intents when needed to guarantee that we will block for at least the specified timeout.

This same risk exists if a continuously running hardware timer is used for a software timer service.

Note

When calculating deadlines based on a timeout when using pw::chrono::SystemClock::timeout, we recommend using its SystemClock::TimePointAfterAtLeast() which adds an extra tick for you internally.

Clocks#

We do not recomend using the clocks provided by <chrono> including but not limited to the std::chrono::system_clock, std::chrono::steady_clock, and std::chrono::high_resolution_clock. These clocks typically do not work on embedded systems, as they are not backed by any actual clocks although they often do compile. In addition, their APIs miss guarantees and parameters which make them difficult and risky to use on embedded systems.

In addition, the STL time bound APIs heavily rely on templating to permit different clocks and durations to be used. We believe this level of template metaprogramming and the indirection that comes with that can be confusing. On top of this, accidental use of the wrong clock and/or conversions between them is a frequent source of bugs. For example using a real time clock which is not monotonic for a timeout or deadline can wreak havoc when the clock is adjusted.

For this reason Pigweed’s timeout and deadline APIs will not permit arbitrary clock and duration selection. Outside of small templated helpers, all APIs will require a specific clock’s duration and/or time-point. For almost all of Pigweed this means that the pw::chrono::SystemClock is used which is usually backed by the kernel’s clock.

PigweedClock Requirements#

pw_chrono extends the C++ named Clock and TrivialClock requirements with the PigweedClock Requirements to make clocks more friendly for embedded systems.

This permits the clock compatibility to be verified through static_assert at compile time which the STL’s requirements do not address. For example whether the clock continues to tick while interrupts are masked or whether the clock is monotonic even if the clock period may not be steady due to the use of low power sleep modes.

For a type PWC to meet the PigweedClock Requirements:

  • The type PWC must meet C++14’s Clock and TrivialClock requirements.

  • The PWC::rep must be int64_t to ensure that there cannot be any overflow risk regardless of the PWC::period configuration. This is done because we do not expect any clocks with periods coarser than seconds which already require 35 bits.

  • const bool PWC::is_monotonic must return true if and only if the clock can never move backwards. This effectively allows one to describe an unsteady but monotonic clock by combining the C++14’s Clock requirement’s const bool PWC::is_steady.

  • const bool PWC::is_free_running must return true if and only if the clock continues to move forward, without risk of overflow, regardless of whether global interrupts are disabled or whether one is in a critical section or even non maskable interrupt.

  • const bool PWC::is_always_enabled must return true if the clock is always enabled and available. If false, the clock must:

    • Ensure the const bool is_{steady,monotonic,free_running} attributes are all valid while the clock is not enabled to ensure they properly meet the previously stated requirements.

    • Meet C++14’s BasicLockable requirements (i.e. provide void lock() & void unlock()) in order to provide std::scoped_lock support to enable a user to enable the clock.

    • Provide const bool is_{steady,monotonic,free_running}_while_enabled attributes which meet the attributes only while the clock is enabled.

  • const bool PWC::is_stopped_in_halting_debug_mode must return true if and only if the clock halts, without further modification, during halting debug mode , for example during a breakpoint while a hardware debugger is used.

  • const Epoch PWC::epoch must return the epoch type of the clock, the Epoch enumeration is defined in pw_chrono/epoch.h.

  • The function time_point PWC::now() noexcept must always be thread and interrupt safe, but not necessarily non-masking and bare-metal interrupt safe.

  • const bool PWC::is_non_masking_interrupt_safe must return true if and only if the clock is safe to use from non-masking and bare-metal interrupts.

The PigweedClock requirement will not require now() to be a static function, however the upstream façades will follow this approach.

SystemClock facade#

The pw::chrono::SystemClock is meant to serve as the clock used for time bound operations such as thread sleeping, waiting on mutexes/semaphores, etc. The SystemClock always uses a signed 64 bit as the underlying type for time points and durations. This means users do not have to worry about clock overflow risk as long as rational durations and time points as used, i.e. within a range of ±292 years.

The SystemClock represents an unsteady, monotonic clock.

The epoch of this clock is unspecified and may not be related to wall time (for example, it can be time since boot). The time between ticks of this clock may vary due to sleep modes and potential interrupt handling. SystemClock meets the requirements of C++’s TrivialClock and Pigweed’s PigweedClock.

This clock is used for expressing timeout and deadline semantics with the scheduler in Pigweed including pw_sync, pw_thread, etc.

C++#

class pw::chrono::SystemClock#
using rep = int64_t;#
using period = std::ratio<PW_CHRONO_SYSTEM_CLOCK_PERIOD_SECONDS_NUMBERATOR, PW_CHRONO_SYSTEM_CLOCK_PERIOD_SECONDS_DENOMINATOR>;#

The period is specified by the backend.

using duration = std::chrono::duration<rep, period>;#
using time_point = std::chrono::time_point<SystemClock>;#
static constexpr Epoch epoch = backend::kSystemClockEpoch;#

The epoch must be provided by the backend.

static constexpr bool is_monotonic = true;#

The time points of this clock cannot decrease.

static constexpr bool is_steady = false;#

However, the time between ticks of this clock may slightly vary due to sleep modes. The duration during sleep may be ignored or backfilled with another clock.

static constexpr bool is_free_running = backend::kSystemClockFreeRunning;#

The now() function may not move forward while in a critical section or interrupt. This must be provided by the backend.

static constexpr bool is_stopped_in_halting_debug_mode = true;#

The clock must stop while in halting debug mode.

static constexpr bool is_always_enabled = true;#

The now() function can be invoked at any time.

static constexpr bool is_nmi_safe = backend::kSystemClockNmiSafe;#

The now() function may work in non-masking interrupts, depending on the backend. This must be provided by the backend.

static time_point now() noexcept;#

This is thread and IRQ safe.

template<class Rep, class Period>
static constexpr duration for_at_least(std::chrono::duration<Rep, Period> d);#

This is purely a helper, identical to directly using std::chrono::ceil, to convert a duration type which cannot be implicitly converted where the result is rounded up.

static time_point TimePointAfterAtLeast(duration after_at_least);#

Computes the nearest time_point after the specified duration has elapsed.

This is useful for translating delay or timeout durations into deadlines.

The time_point is computed based on now() plus the specified duration where a singular clock tick is added to handle partial ticks. This ensures that a duration of at least 1 tick does not result in [0,1] ticks and instead in [1,2] ticks.

Example in C++#

#include <chrono>

#include "pw_chrono/system_clock.h"

void Foo() {
  const SystemClock::time_point before = SystemClock::now();
  TakesALongTime();
  const SystemClock::duration time_taken = SystemClock::now() - before;
  bool took_way_too_long = false;
  if (time_taken > std::chrono::seconds(42)) {
    took_way_too_long = true;
  }
}

Protobuf#

Sometimes it’s desirable to communicate high resolution time points and durations from one device to another. For this, pw_chrono provides protobuf representations of clock parameters (pw.chrono.ClockParameters) and time points (pw.chrono.TimePoint). These types are less succinct than simple single-purpose fields like ms_since_boot or unix_timestamp, but allow timestamps to be communicated in terms of the tick rate of a device, potentially providing significantly higher resolution. Logging, tracing, and system state snapshots are use cases that benefit from this additional resolution.

This module provides an overlay proto (pw.chrono.SnapshotTimestamps) for usage with pw_snapshot to encourage capture of high resolution timestamps in device snapshots. Simplified capture utilies and host-side tooling to interpret this data are not yet provided by pw_chrono.

Software Timers#

SystemTimer facade#

The SystemTimer facade enables deferring execution of a callback until a later time. For example, enabling low power mode after a period of inactivity.

The base SystemTimer only supports a one-shot style timer with a callback. A periodic timer can be implemented by rescheduling the timer in the callback through InvokeAt(kDesiredPeriod + expired_deadline).

When implementing a periodic layer on top, the user should be mindful of handling missed periodic callbacks. They could opt to invoke the callback multiple times with the expected expired_deadline values or instead saturate and invoke the callback only once with the latest expired_deadline.

The entire API is thread safe, however it is NOT always IRQ safe.

The ExpiryCallback is either invoked from a high priority thread or an interrupt. Ergo ExpiryCallbacks should be treated as if they are executed by an interrupt, meaning:

  • Processing inside of the callback should be kept to a minimum.

  • Callbacks should never attempt to block.

  • APIs which are not interrupt safe such as pw::sync::Mutex should not be used!

C++#

class pw::chrono::SystemTimer#
SystemTimer(ExpiryCallback callback)#

Constructs the SystemTimer based on the user provided pw::Function<void(SystemClock::time_point expired_deadline)>. Note that The ExpiryCallback is either invoked from a high priority thread or an interrupt.

Note

For a given timer instance, its ExpiryCallback will not preempt itself. This makes it appear like there is a single executor of a timer instance’s ExpiryCallback.

~SystemTimer()#

Cancels the timer and blocks if necssary if the callback is already being processed.

Postcondition: The expiry callback is not in progress and will not be called in the future.

void InvokeAfter(chrono::SystemClock::duration delay)#

Invokes the expiry callback as soon as possible after at least the specified duration.

Scheduling a callback cancels the existing callback (if pending). If the callback is already being executed while you reschedule it, it will finish callback execution to completion. You are responsible for any critical section locks which may be needed for timer coordination.

This is thread safe, it may not be IRQ safe.

void InvokeAt(chrono::SystemClock::time_point timestamp)#

Invokes the expiry callback as soon as possible starting at the specified time_point.

Scheduling a callback cancels the existing callback (if pending). If the callback is already being executed while you reschedule it, it will finish callback execution to completion. You are responsible for any critical section locks which may be needed for timer coordination.

This is thread safe, it may not be IRQ safe.

void Cancel()#

Cancels the software timer expiry callback if pending.

Canceling a timer which isn’t scheduled does nothing.

If the callback is already being executed while you cancel it, it will finish callback execution to completion. You are responsible for any synchronization which is needed for thread safety.

This is thread safe, it may not be IRQ safe.

Safe to use in context

Thread

Interrupt

NMI

SystemTimer::SystemTimer

SystemTimer::~SystemTimer

void SystemTimer::InvokeAfter

void SystemTimer::InvokeAt

void SystemTimer::Cancel

Example in C++#

#include "pw_chrono/system_clock.h"
#include "pw_chrono/system_timer.h"
#include "pw_log/log.h"

using namespace std::chrono_literals;

void DoFoo(pw::chrono::SystemClock::time_point expired_deadline) {
  PW_LOG_INFO("Timer callback invoked!");
}

pw::chrono::SystemTimer foo_timer(DoFoo);

void DoFooLater() {
  foo_timer.InvokeAfter(42ms);  // DoFoo will be invoked after 42ms.
}