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[oss-web.git] / in / rgblamp / timer.md

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-title: RGB Lamp Timers
-linktitle: rgb-timer
-parent: rgb-code
-ctime: 2012-03-24
-
-# Timers
-
-Most microcontroller applications must respond to temporal events.  The timers
-subsystem allows an application to have multiple timers running concurrently,
-each with a different duration before expiration.  This implementation is very
-simple, in that timers can be started, stoped, and polled for expiration
-(fired).  By integrating with tasks, a timer expiration can post an event.
-
-Timers are identified by a timer id.  A timer started will fire at the end of
-the time period specified in the timer start call.  A timer may also be started
-via startPeriodic(), which will fire the timer periodically according to the
-timer period specified in the call.
-
-# Implementation
-
-The timer subsystem is implemented through the use of the PIC16 TMR0.  It is
-configured to trigger a hardware interrupt every 32.768 milliseconds.  Within
-each interrupt, the status of the currently active timers is updated.  This is a
-very simplistic approach.  A better solution would be to use a timer compare
-feature, which would dramatically reduce the number of ISR events and therefore
-servicing overhead for the timer subsystem.
-
-Periodically the application can poll for timer fired by the tmr_fired() call.
-A good time to do this is in the ISR, and timers that fire can post tasks.  The
-comments in tmr.h gives a complete example.  Check the
-[repository](http://oss/gitweb?p=rgblamp.git;a=summary).
-
-# Timer and low power modes
-
-Unfortunately, the PIC16 has limitations when it comes to using timers in sleep.
-While it is true that Timer 1 can run when the PIC16 is in sleep using an
-external 32 kHz crystal, no timer compare feature is possible in sleep.
-Therefore, the only timer inerrupt that can wake the micro is the overflow.
-This overflow happens every 2 seconds.  In fact, the lamp code uses this
-behavior to handle the long term timer events via the tmr32 module.
-Specifically, this is used for the auto-off and auto-on events.
-
-It is possible to get different overflow periods by programmatically setting the
-Timer 1 register.  Since this is a 16 bit value implemented in 2 8-bit
-registers, there are race conditions that challenge this use.  But the ability
-to support variable timer periods in sleep is possible.
-
-When it comes to managing temporal events and deep sleep, the MSP430, the EFM32,
-and the STM32L are superior processors.
-