Low Energy Timer - AN0026 - Application Note - Energy Micro

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1 Introduction...the world's most energy friendly microcontrollersThe unique LETIMER TM , the Low Energy Timer, is a 16-bit timer that is available in energy mode EM2in addition to EM1 and EM0. Because of this, it can be used for timing and output generation when mostof the device is powered down, allowing simple tasks to be performed while the power consumption ofthe system is kept at an absolute minimum. The LETIMER runs from the LFACLK which can be clockedby the LFXO, LFRCO or HFCORECLK LE /2. If clocked by the HFCORECLK LE /2 the LETIMER and otherperipherals running from the LFACLK will not be available in EM2.The LETIMER can be used to output a variety of waveforms with minimal software intervention.Thewaveforms include PWM, pulses with the duraction of one LFACLK LETIMER period and variable frequencywaveforms. The LETIMER is also connected to the Real Time Counter (RTC), and can be configuredto start counting on compare matches from the RTC.An overview of the LETIMER module is shown in Figure 1.1 (p. 2) . The LETIMER is a16-bit down-counter with two compare registers, LETIMERn_COMP0 and LETIMERn_COMP1. TheLETIMERn_COMP0 register can optionally act as a top value for the counter. The repeat counterLETIMERn_REP0 allows the timer to count a specified number of times before it stops. Both theLETIMERn_COMP0 and LETIMERn_REP0 registers can be double buffered by the LETIMERn_COMP1and LETIMERn_REP1 registers to allow continuous operation. The timer can generate a single pinoutput, or two linked outputs.Figure 1.1. LETIMER OverviewPeripheral busLETIMER Controland StatusCOMP1(Top Buffer)=COMP1 Match(COMP1 interrupt flag)RTC eventSWLFACLK LETIMERnStartUpdateReloadCOMP0(Top)CNT (Counter)Top loadlogic= 0=COMP0 Match(COMP0 interrupt flag)PulseControlUnderflow(UF interrupt flag)pinLETn_O0ctrlStop 0PulseControlpinctrlLETn_O1REP0(Repeat)BufferWrittenREP1(Repeat Buffer)= 1UpdateRepeatload logic= 1REP0 Zero(REP0 interrupt flag)REP1 Zero(REP1 interrupt flag)2013-05-08 - an0026_Rev1.05 2 www.energymicro.com
2 LETIMER Features2.1 Compare Registers...the world's most energy friendly microcontrollersThe LETIMER has two compare match registers, LETIMERn_COMP0 and LETIMERn_COMP1.Each of these compare registers are capable of generating an interrupt when the counter valueLETIMERn_CNT becomes equal to their value. When LETIMERn_CNT becomes equal to the valueof LETIMERn_COMP0, the interrupt flag COMP0 in LETIMERn_IF is set, and when LETIMERn_CNTbecomes equal to the value of LETIMERn_COMP1, the interrupt flag COMP1 in LETIMERn_IF is set.The compare values can be set using the LETIMER_CompareSet(LETIMER_TypeDef *letimer,unsigned int comp, uint32_t value) from the emlib.2.2 Top ValueIf COMP0TOP in LETIMERn_CTRL is set, the value of LETIMERn_COMP0 acts as the top value ofthe timer, and LETIMERn_COMP0 is loaded into LETIMERn_CNT on timer underflow. Otherwise thetimer wraps around to 0xFFFF. The underflow interrupt flag UF in LETIMERn_IF is set when the timerreaches zero.If BUFTOP in LETIMERn_CTRL is set, the value of LETIMERn_COMP0 is buffered byLETIMERn_COMP1. In this mode, the value of LETIMERn_COMP1 is loaded into LETIMERn_COMP0every time LETIMERn_REP0 is about to decrement to 0.2.3 Repeat ModesBy default, the timer wraps around to the top value or 0xFFFF on each underflow, and continues counting.The repeat counters can be used to get more control of the operation of the timer, including definingthe number of times the counter should wrap around. There are four repeat modes available which aredetailed in Table 2.1 (p. 3) .Table 2.1. LETIMER Repeat ModesREPMODE Mode Description00 Free The timer runs until it is stopped01 One-shot The timer runs as long asLETIMERn_REP0 != 0.LETIMERn_REP0 is decremented ateach timer underflow.10 Buffered The timer runs as long asLETIMERn_REP0 != 0.LETIMERn_REP0 is decrementedon each timer underflow. IfLETIMERn_REP1 has been written,it is loaded into LETIMERn_REP0when LETIMERn_REP0 is about to bedecremented to 0.11 Double The timer runs as long asLETIMERn_REP0 != 0 orLETIMERn_REP1 != 0.Both LETIMERn_REP0 andLETIMERn_REP1 are decremented ateach timer underflow.The interrupt flags REP0 and REP1 in LETIMERn_IF are set whenever LETIMERn_REP0 orLETIMERn_REP1 are decremented to 0 respectively. REP0 is also set when the value ofLETIMERn_REP1 is loaded into LETIMERn_REP0 in buffered mode.2013-05-08 - an0026_Rev1.05 3 www.energymicro.com
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Low Energy Timer - AN0026 - Application Note - Energy Micro

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