115 lines
3.6 KiB
ReStructuredText
115 lines
3.6 KiB
ReStructuredText
The Timers
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==========
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The pyboard has 14 timers which each consist of an independent counter
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running at a user-defined frequency. They can be set up to run a function
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at specific intervals.
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The 14 timers are numbered 1 through 14, but 3 is reserved
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for internal use, and 5 and 6 are used for servo and ADC/DAC control.
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Avoid using these timers if possible.
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Let's create a timer object::
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>>> tim = pyb.Timer(4)
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Now let's see what we just created::
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>>> tim
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Timer(4)
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The pyboard is telling us that ``tim`` is attached to timer number 4, but
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it's not yet initialised. So let's initialise it to trigger at 10 Hz
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(that's 10 times per second)::
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>>> tim.init(freq=10)
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Now that it's initialised, we can see some information about the timer::
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>>> tim
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Timer(4, prescaler=624, period=13439, mode=UP, div=1)
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The information means that this timer is set to run at the peripheral
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clock speed divided by 624+1, and it will count from 0 up to 13439, at which
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point it triggers an interrupt, and then starts counting again from 0. These
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numbers are set to make the timer trigger at 10 Hz: the source frequency
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of the timer is 84MHz (found by running ``tim.source_freq()``) so we
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get 84MHz / 625 / 13440 = 10Hz.
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Timer counter
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-------------
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So what can we do with our timer? The most basic thing is to get the
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current value of its counter::
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>>> tim.counter()
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21504
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This counter will continuously change, and counts up.
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Timer callbacks
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---------------
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The next thing we can do is register a callback function for the timer to
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execute when it triggers (see the [switch tutorial](tut-switch) for an
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introduction to callback functions)::
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>>> tim.callback(lambda t:pyb.LED(1).toggle())
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This should start the red LED flashing right away. It will be flashing
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at 5 Hz (2 toggle's are needed for 1 flash, so toggling at 10 Hz makes
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it flash at 5 Hz). You can change the frequency by re-initialising the
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timer::
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>>> tim.init(freq=20)
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You can disable the callback by passing it the value ``None``::
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>>> tim.callback(None)
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The function that you pass to callback must take 1 argument, which is
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the timer object that triggered. This allows you to control the timer
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from within the callback function.
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We can create 2 timers and run them independently::
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>>> tim4 = pyb.Timer(4, freq=10)
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>>> tim7 = pyb.Timer(7, freq=20)
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>>> tim4.callback(lambda t: pyb.LED(1).toggle())
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>>> tim7.callback(lambda t: pyb.LED(2).toggle())
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Because the callbacks are proper hardware interrupts, we can continue
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to use the pyboard for other things while these timers are running.
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Making a microsecond counter
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----------------------------
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You can use a timer to create a microsecond counter, which might be
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useful when you are doing something which requires accurate timing.
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We will use timer 2 for this, since timer 2 has a 32-bit counter (so
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does timer 5, but if you use timer 5 then you can't use the Servo
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driver at the same time).
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We set up timer 2 as follows::
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>>> micros = pyb.Timer(2, prescaler=83, period=0x3fffffff)
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The prescaler is set at 83, which makes this timer count at 1 MHz.
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This is because the CPU clock, running at 168 MHz, is divided by
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2 and then by prescaler+1, giving a frequency of 168 MHz/2/(83+1)=1 MHz
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for timer 2. The period is set to a large number so that the timer
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can count up to a large number before wrapping back around to zero.
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In this case it will take about 17 minutes before it cycles back to
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zero.
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To use this timer, it's best to first reset it to 0::
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>>> micros.counter(0)
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and then perform your timing::
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>>> start_micros = micros.counter()
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... do some stuff ...
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>>> end_micros = micros.counter()
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