Polled timers

Polled timers can be used to measure elapsed time intervals or to check for timeouts within code loops. Traditionally one might do this as follows:

const unsigned TIMEOUT_MS = 100;

unsigned start = millis();
while(millis() - start < TIMEOUT_MS) {
   // do some stuff
}
unsigned elapsed = millis() - start;
Serial.print("Elapsed time: ");
Serial.print(elapsed);
Serial.println("ms");

Note

A common source of bugs when hand-coding such loops can be illustrated by this alternative:

unsigned timeout = millis() + TIMEOUT_MS;
while(millis() < timeout) {
   // do some stuff
}

At first glance this looks better than the first approach as we don’t have a subtraction within the loop. However, when millis() exceeds MAXINT - TIMEOUT_MS the timeout calculation will wrap and the loop will never get executed. This takes a little under 25 days, but with microseconds it’ll happen in less than an hour. This may not be the desired behaviour.

It’s generally safer and easier to use a PolledTimer:

OneShotFastMs timer(TIMEOUT_MS);
while(!timer.expired()) {
   // do some stuff
}
auto elapsed = timer.elapsedTime(); // Returns a `NanoTime::Time` object, value with time units
Serial.print("Elapsed time: ");
Serial.println(elapsed.toString()); // Includes units in the elapsed time interval
// Show time rounded to nearest whole seconds
Serial.println(elapsed.as<NanoTime::Seconds>().toString());

If you prefer to use microseconds, use OneShotFastUs instead or specify the units directly:

OneShotElapseTimer<NanoTime::Microseconds> timer;

Another advantage of polled timers is speed. Every call to millis (or micros) requires a calculation from clock ticks into milliseconds (or microseconds). It’s also a function call.

Polled timers measure time using hardware clock ticks, and query the hardware timer register directly without any function calls or calculations. This makes them a much better choice for tight timing loops.

Here’s the output from the BenchmarkPolledTimer module:

How many loop iterations can we achieve in 100 ms ?
Using millis(), managed 55984 iterations, average loop time = 1786ns (143 CPU cycles)
Using micros(), managed 145441 iterations, average loop time = 688ns (55 CPU cycles)
Using PolledTimer, managed 266653 iterations, average loop time = 375ns (30 CPU cycles)

API Documentation

group polled_timer

Polled interval timers.

Unnamed Group

template<NanoTime::Unit unit> using OneShotElapseTimer = PolledTimer::OneShot<PolledTimerClock, unit>

template<NanoTime::Unit unit> using PeriodicElapseTimer = PolledTimer::Periodic<PolledTimerClock, unit>

using OneShotFastMs = OneShotElapseTimer<NanoTime::Milliseconds>

using PeriodicFastMs = PeriodicElapseTimer<NanoTime::Milliseconds>

using OneShotFastUs = OneShotElapseTimer<NanoTime::Microseconds>

using PeriodicFastUs = PeriodicElapseTimer<NanoTime::Microseconds>

using ElapseTimer = OneShotFastUs 

template<NanoTime::Unit units> using OneShotCpuCycleTimer = PolledTimer::OneShot<CpuCycleClockNormal, units>

template<NanoTime::Unit units> using PeriodicCpuCycleTimer = PolledTimer::Periodic<CpuCycleClockNormal, units>

template<NanoTime::Unit units> using OneShotCpuCycleTimerFast = PolledTimer::OneShot<CpuCycleClockFast, units>

template<NanoTime::Unit units> using PeriodicCpuCycleTimerFast = PolledTimer::Periodic<CpuCycleClockFast, units>

using CpuCycleTimer = OneShotCpuCycleTimer<NanoTime::Nanoseconds>

using CpuCycleTimerFast = OneShotCpuCycleTimerFast<NanoTime::Nanoseconds>

typedef OneShotFastMs oneShotFastMs

typedef PeriodicFastMs periodicFastMs

typedef OneShotFastUs oneShotFastUs

typedef PeriodicFastUs periodicFastUs

Defines

POLLED_TIMER_MARGIN_US: Timer intervals are limited to the maximum clock time, minus this safety margin.

Note

Specified in microseconds, this is the minimum timer poll interval to ensure no missed polls across the full timer range. Larger margin means smaller time range.

namespace PolledTimer

Typedefs

template<typename Clock, NanoTime::Unit unit> using OneShot = Timer<Clock, unit, false, uint32_t>

template<typename Clock, NanoTime::Unit unit> using Periodic = Timer<Clock, unit, true, uint32_t>

template<class Clock, NanoTime::Unit unit_, bool IsPeriodic, typename TimeType> class Timer : public NanoTime::TimeSource<Clock, unit_, TimeType>

#include <PolledTimer.h>

Template class to implement a polled timer.

If the interval is set to 0, the timer will expire immediately, and remain so: canWait() will return false. Call cancel() to prevent the timer from ever expiring: canExpire() will return false. Call start() to set the timer going with the previously set interval. Call reset() to set the timer with a new interval.

Note

Intervals and expiry use ‘tick’ values which are very efficient, whereas ‘time’ values must be computed so are very slow (~100+ cycles for uint32_t, considerably more for uint64_t). The class keeps a note of a ‘start’ time (in ticks) used to determine the elapsed() return value. An ‘interval’ value may be given is specified, then the expired() method returns true.

tparam Clock: Clock source
tparam unit: Time unit for tick/time conversions
tparam IsPeriodic
tparam TimeType: Variable type to use (uint32_t or uint64_t)

A periodic timer will automatically restart when expired. A one-shot timer will remain expired until reset.

Public Functions

inline Timer(const TimeType &timeInterval = 0)

Create a Timer with optional expiry time.

Parameters: timeInterval – Relative time until expiry

inline void start(): Start the timer.

template<uint64_t timeInterval> inline void reset()

Start the timer with a new expiry interval.

Template Parameters: timeInterval – Time to expire after last call to start()

template<uint64_t timeInterval> inline constexpr uint32_t checkTime()

Check the given time interval is valid and return the corresponding tick count.

Note

If time interval is invalid fails compilation

Template Parameters: timeInterval –
Returns: uint32_t –

inline bool reset(const TimeType &timeInterval)

Start the timer with a new expiry interval.

See: See resetTicks()

Parameters: timeInterval – Time to expire after last call to start()
Returns: bool – true on success, false on failure

inline bool resetTicks(const TimeType &interval)

Start the timer with a new expiry interval.

Note

If time interval is 0, timer will expire immediately, and if it exceeds the maximum interval the timer will never expire.

Parameters: interval – Clock ticks to expire after last call to start()
Returns: bool – true on success, false if interval is out of range

inline void cancel(): Cancelling a timer means it will never expire.

inline TickType elapsedTicks() const: Get elapsed ticks since start() was last called.

inline NanoTime::Time<TimeType> elapsedTime() const: Get elapsed time since start() was last called.

inline TickType remainingTicks() const: Get ticks remaining until expiry.

inline NanoTime::Time<TimeType> remainingTime() const: Get time remaining until expiry.

inline bool expired()

Determine if timer has expired.

Returns: bool –