Software Real Time Clock (RTC) – two methods
Started by oPossum , Mar 22 2012 12:28 PM
10 replies to this topic
Posted 22 March 2012 – 12:28 PM
The Launchpad comes with a 32.768 kHz crystal that can be used when timing more precise than the DCO is needed. The crystal frequency is useful for simple timing tasks, such as a RTC, because it is a power of 2. That allows a simple cascade of 15 flip-flops to create a 1 pulse per second time base. It is common for microcontroller timers to have prescalers, postscalers and/or preset divisors that are simply a tap on the flip-flip chain. The watchdog timer in the MSP430 is an example of a timer with power of 2 presets (64, 512, 8192, 32768). TimerA units have a prescaler with power of 2 presets (1, 2, 4, 8).
The sample code in this post uses the watchdog timer to create an interrupt that occurs once per second. TimerA can also be used to do this. One of the advantages of using TimerA is the ability to change the preset +/- 1 every N cycles to allow for calibration and temperature compensation of the crystal. I chose to use the watchdog timer so that TimerA was free to be used for alarm tones or PWM backlight control.