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diff --git a/docs/source/timers.rst b/docs/source/timers.rst index e0e57cb..56dd686 100644 --- a/docs/source/timers.rst +++ b/docs/source/timers.rst @@ -2,42 +2,73 @@ .. _timers: -======== - Timers -======== +Timers +====== There are four general purpose timers in the Maple microcontroller that can be configured to generate periodic or delayed events with minimal work done by the microcontroller. For example, the :ref:`PWM -<pwm>` channels, once enabled, generate regular square-wave signals on -specific output pins that will continue even if the microcontroller is -busy crunching numbers or handling communications interrupts. By -attaching interrupt handlers to these channels (instead of just -changing the voltage on an external pin), more complex events like -printing to a serial port, updating a variable, or emitting a whale -mating call can be scheduled. You can even modify the configuration of -the timer itself at a regular interval; the possibilities are endless! - -The four timers each have four separate compare channels. Each timer -is a single 16-bit counter that can be configured with both a -prescaler and an overflow value. The prescaler acts as a divider of -the 72MHz system clock; without prescaling the counter would get up to -65536 (2 to the 16th power) and roll over more than a thousand times a -second; even with the full prescaler it rolls over about once a -minute. The overflow value is the maximum value the counter will go up -to. It defaults to the full 65535; smaller values will cause the -counter to reset to zero more frequently. +<pwm>` channels can generate regular square-wave signals on specific +output pins without consuming extra clock cycles. By attaching +interrupt handlers to these channels (instead of just changing the +voltage on an external pin), more complex events can be scheduled. + +.. contents:: Contents + :local: + +Introduction +------------ + +.. _timers-prescale: + +The four timers each have four separate compare channels. Each channel +has an associated 16-bit counter that can be configured with a 16-bit +prescaler and a 16-bit overflow value. The prescaler determines how +fast the counter changes, while the overflow value determines when it +gets reset. + +The prescaler acts as a divider of the 72MHz system clock. That is, +with a prescaler of 1, the channel's counter increments with a +frequency of 72MHz, rolling over (passing the maximum 16-bit unsigned +integer value of 65,535) more than a thousand times a second. With a +prescaler of 7200, it has a frequency of (72/7200) MHz = 10 KHz, +rolling over approximately every 6.55 seconds. + +The overflow value is the maximum value the counter will go up to. It +defaults to the full 65,535; smaller values will cause the counter to +reset to zero more frequently. + +Whenever a channel's counter reaches its overflow value, an "update +event" interrupt is generated. You can configure the Maple to notify +you when this takes place, by registering an interrupt handler, which +is a function that will be called when the update event occurs. + +libmaple Reference +------------------ +The libmaple API for interacting with timers is documented at the +:ref:`HardwareTimer reference <lang-hardwaretimer>`. Caveats ------- +.. _timers-pwm-conflicts: + **PWM Conflicts:** Because PWM functionality on a given pin depends on the configuration of the timer and channel, you must chose your channels carefully if you want to use both timer interrupts and PWM in -the same program. Refer to the full :ref:`pin mapping table -<pin-mapping-mega-table>` to match up timer channels and Maple header -pin numbers. +the same program. Refer to the following table to match up timer +channels and Maple header pin numbers: + +.. _timers-pin-channel-map: + +.. csv-table:: + :header: Timer, Ch. 1 pin, Ch. 2 pin, Ch. 3 pin, Ch. 4 pin + + ``Timer1``, 6, 7, 8, -- + ``Timer2``, 2, 3, 1, 0 + ``Timer3``, 12, 11, 27, 28 + ``Timer4``, 5, 9, 14, 24 **Overhead:** there is some overhead associated with function and interrupt calls (loading and unloading the stack, preparing state, @@ -50,7 +81,7 @@ configuration being the same. **Jitter:** other interrupts (USB, Serial, SysTick, or other timers) can and will get called before or during the timer - interrupt routines, causing pseudo-random delays and other + interrupt routines, causing pseudorandom delays and other frustrations. Disabling the USB port (by calling ``SerialUSB.end()``, or just @@ -58,7 +89,8 @@ configuration being the same. auto-reset and communications functionality. This will require that you put your Maple into :ref:`perpetual bootloader mode <troubleshooting-perpetual-bootloader>` before uploading a new - program to it. + program to it (or somehow causing your program to re-enable serial + over USB using :ref:`SerialUSB.begin() <lang-serialusb-begin>`). Disabling SysTick with ``systick_disable()`` helps as well. However, calling this function will break the ``micros()`` and @@ -72,33 +104,6 @@ a handler isn't going to block other interrupts from firing (e.g. USB, Serial, SysTick) if those other interrupts are important for your program. -.. _timers-modes: - -General Timer Modes -------------------- - -``TIMER_DISABLED`` - - Exactly what it sounds like: the timer stops counting, interrupts - are not called, and no state changes are output. - -``TIMER_PWM`` - - This is the default mode for pins after initialization. See the - :ref:`PWM docs <pwm>` for more information on this mode. - - .. note:: - - ``Timer1.setChannel1Mode(TIMER_PWM)`` may not work as expected; - if you want PWM functionality on a channel, make sure you don't - set it to something else. - -``TIMER_OUTPUTCOMPARE`` - - In this mode, the timer counts from 0 to the overflow value - repeatedly; every time the counter value reaches one of the - channel compare values, the corresponding interrupt is fired. - SysTick Peripheral ------------------ @@ -110,133 +115,22 @@ VGA code, where the timing jitters are transformed into visual jags in the image. The SysTick peripheral can be disabled by calling ``systick_disable()``, and re-enabled using ``systick_resume()``. -Function Reference ------------------- - -For all of these functions, ``Timer1`` can be replaced with -``Timer2``, ``Timer3``, or ``Timer4``; the channel numbers also range -from 1 to 4. - -``Timer1.pause()``/\ ``Timer1.resume()`` - - These functions start and stop the counter without affecting the - rest of the configuration. These functions can be used during the - setup period to prevent interrupts from firing before they are - completely configured. Note that there is some function call - overhead with these functions, so they are not a perfect way to - align multiple timers to the same count value. - -``Timer1.setOverflow(val)`` - - Sets the overflow (or "reload") value for the whole timer; when - the counter reaches this value it resets to zero. Defaults to - 65535 (the largest unsigned 16bit integer); setting it to anything - lower will cause interrupts to be called more frequently (see the - setPeriod function below for a shortcut). This number sets the - maximum value for the channel compare values. - -``Timer1.setPrescaleFactor(val)`` - - The prescaler acts as a clock divider to slow down the rate at - which the counter increments. For example, the system clock rate - is 72MHz, so the counter will reach 65535 in (13.89 nanoseconds) * - (65535 counts) = (910.22 microseconds), or about a thousand times - a second. If you set the prescaler to 1098, then the clock rate is - effectively 65.56KHz, and the counter will reach 65536 in (15.25 - microseconds) * (65536 counts) = (0.999 seconds), or just about - once a second. Use the :ref:`setPeriod <timers-set-period>` - function below if you are allergic to math! - -.. _timers-set-period: - -``Timer1.setPeriod(val)`` - - This tricky trick will configure the prescaler and overflow values - to generate a timer reload with a period as close to val - microseconds as possible. It returns the chosen overflow value, - which you can then use to set the channel compare values - appropriately: if you just want the interrupts to fire when the - clock rolls over and you don't care about the relative "phase", - you can always set the channel compare values to 1. - - Remember: a microsecond is 1/1,000,000th of a second, or 1/1,000 - of a millisecond. The prescaler itself is 16bit, so the longest - period that can be configured is 1/(72MHz) * (2^32) = (59.65 - seconds) or about a minute. You can get around this by creating an - interrupt that increments a 32-bit variable, by using the - ``millis()`` function, or by interfacing with an external - real-time-clock chip. - -``Timer1.setCount(val)``/\ ``Timer1.getCount()`` - - These functions let you mess with the counter's brains - directly. You can probably make it not work if you try! The timer - is 16bit, so ``val`` and the return value of ``getCount`` are - ``uint16``. - -``Timer1.setChannel1Mode(MODE)`` - - This sets the given channel (here 1) of the given timer (here 1) - to the given mode. See the :ref:`list above <timers-modes>` for - possible values; for interrupts you want ``TIMER_OUTPUTCOMPARE``. - -``Timer1.setCompare1(val)`` - - Sets the compare value for the given channel; when the counter - reaches this value the interrupt for this channel will fire if the - channel is in output compare mode and an interrupt is attached. - - By default this only changes the relative offsets between events - on a single timer ("phase"); they don't control the frequency with - which they occur. However, a common trick is to increment the - compare value manually in the interrupt handler so that the event - will fire again after the increment period. There can be a - different increment value for each channel, so this trick allows - events to be programmed at 4 different rates on a single timer! - Note that function call overhead means that the smallest increment - rate is a couple microseconds. - -``Timer1.attachCompare1Interrupt(function)``/\ ``Timer1.detachCompare1Interrupt()`` - - This is how to attach or disable an interrupt handlers to timer - channels; this what will get called when the counter reaches the - compare value set with ``setCompareN(val)``. ``function`` - (sometimes referred to as an ISR: "interrupt service routine") - should be of a type that does not accept or return any values - (C/C++ programmers: ``void (function*)(void)``). They are just - like any other function in your sketch/program and must be - initialized at the top of the file and defined below. - - ``function`` should try to do what it has to do as fast as - possible. Blinking the LED, some logic, PWM updates, and Serial - writes are fine; writing to SerialUSB or waiting for user input - can take a long time and other compare interrupts won't fire. Tip: - if you have a ``delay()`` in your ISR, you're probably doing it - wrong. - - Stay vigilant here... function pointers are serious business, and - once you go down that path you'll find yourself in a `forest of - parentheses <http://mitpress.mit.edu/sicp/>`_ before you know it. - Code Examples ------------- LED blink ^^^^^^^^^ -\ :: +:: - #define LED_PIN 13 #define LED_RATE 500000 // in microseconds; should give 0.5Hz toggles void handler_led(void); - int toggle = 0; - void setup() { - // Set up the LED to blink - pinMode(LED_PIN, OUTPUT); + // Set up the LED to blink + pinMode(BOARD_LED_PIN, OUTPUT); // Setup Timer Timer2.setChannel1Mode(TIMER_OUTPUTCOMPARE); @@ -246,20 +140,17 @@ LED blink } void loop() { - // Nothing! It's all in the interrupts + // Nothing! It's all in the interrupts } void handler_led(void) { - toggle ^= 1; - digitalWrite(LED_PIN, toggle); - } + toggleLED(); + } Racing Counters ^^^^^^^^^^^^^^^ -\ :: - - #define BUTTON_PIN 38 +:: void handler_count1(void); void handler_count2(void); @@ -270,7 +161,7 @@ Racing Counters void setup() { // Set up BUT for input - pinMode(BUTTON_PIN, INPUT_PULLUP); + pinMode(BOARD_BUTTON_PIN, INPUT_PULLUP); // Setup Counting Timers Timer3.setChannel1Mode(TIMER_OUTPUTCOMPARE); @@ -282,7 +173,7 @@ Racing Counters Timer3.setOverflow(30000); Timer4.setOverflow(30000); Timer3.setCompare1(1000); // somewhere in the middle - Timer4.setCompare1(1000); + Timer4.setCompare1(1000); Timer3.attachCompare1Interrupt(handler1); Timer4.attachCompare1Interrupt(handler2); Timer3.resume(); @@ -291,14 +182,14 @@ Racing Counters void loop() { // Display the running counts - SerialUSB.print("Count 1: "); + SerialUSB.print("Count 1: "); SerialUSB.print(count1); - SerialUSB.print("\t\tCount 2: "); + SerialUSB.print("\t\tCount 2: "); SerialUSB.println(count2); // Run... while BUT is held, pause Count2 for(int i = 0; i<1000; i++) { - if(digitalRead(BUTTON_PIN)) { + if(digitalRead(BOARD_BUTTON_PIN)) { Timer4.pause(); } else { Timer4.resume(); @@ -309,7 +200,7 @@ Racing Counters void handler1(void) { count1++; - } + } void handler2(void) { count2++; - } + } |