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Timebase calculation and Clock sources

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Neon22 edited this page Feb 5, 2014 · 3 revisions

###Clock sources Timers get their clock source from External pins or Internal timer sources.

####External External = pins: TI1 or TI2 or ETR

  1. set pin to be used:
  • in TIMx_CCMR1 reg - set pin to be used by writing CCxS bits
  1. select polarity of input
  • in TIMx_CCER reg - write CCxP and CCxNP to select rising, falling or both edges
  1. select external clock mode1
  • in TIMx_SMCR reg - write SMS=111
  1. optionally configure filter and prescaler
  • in TIMx_CCMR1 - write ICxF[3:0] bits
  1. optionally set trigger input source as Timer TIx
  • in TIMx_SMCR reg - write TS bits
  1. Enable channel
  • in TIMx_CCER reg - set CCEx bit

If ETR is to be input clock:

  1. set clock mode2
  • in TIMx_SMCR reg - set ECE=1
  1. configure prescaler, filter, polarity
  • in TIMx_SMCR reg - set ETPS[1:0], ETF[3:0], ETP

####Internal One Timer can be used as the prescaler for another.

The first timer update_event, or output_compare signal is used as clock for the second. Uses TRGI to map. Counter mode is set using the TIMx_CR1 reg and CMS bits as indicated in the example below. The counter mode sets whether the update_event occurs on overflow and/or underflow of the Timer.

##Time Base generation A Timer can be used as a Time base generator. A 16bit timer can generate update_events from nanoseconds up to a few minutes.

The calculation is:

  • update_event = TIM_CLK / (( PSC + 1) * (APR + 1) * (RCR + 1))

where:

  • TIM_CLK is the timer clock input
  • PSC is the 16bit prescaler reg
  • ARR is the 16/32bit autoreload reg
  • RCR is the 16bit repetition counter

####Examples: Basic single update_event

  • where TIM_CLK=72MHz, Prescaler=1, Autoreload =65535, and no repetition
  • update_event = 72 * 10e6 / ((1+1) (65535+1) (1)) = 549.3 Hz

####External clock mode 1 TIM_CLK is replaced by T1x_CLK - an ext freq applied to pins TI1 or 2

  • where TIx_CLK = 50 kHz, Prescaler=1, Autoreload =255, Repetition =2
  • update-event = 50000 / ((1+1) (255+1) (2+1)) = 32.55 Hz

####External clock mode 2 TIM_CLK is replaced by ETR_CLK. ETR_PSC is also factored in as follows:

  • update_event = ETR_CLK / ((ETR_PSC + 1) * ( PSC + 1) * (APR + 1) * (RCR + 1))
  • where ETR_CLK =100kHz, Prescaler=1, ETR_PSC=2, Autoreload=255, and Repetition=2
  • update_event = 100 * 10e3 / ((2+1) (1+1) (255+1) (2+1)) = 21.7 Hz

####Internal trigger clock mode 1 (ITRx) TIM_CLK is replaced by ITRx_CLK which is the internal trigger freq mapped to timer Trigger input TRGI.

  • where ITRx_CLK = 8kHz, Prescaler=1, Autoreload=255, Repetition =1
  • update_event = 8000 / ((1+1) (255+1) (1+1)) = 7.8Hz

The counter mode indicates if the update_event is generated:

  • on overflow - if mode = up counting, the DIR bit is reset in TIMx_CR1
  • on underlfow - if mode = down counting, the DIR bit is set in TIMx_CR1
  • both - if mode is center aligned, the CMS bits are non zero

The update_event is also generated by:

  • software if the UG bit (Update Generation) is set in TIM_EGR reg.
  • update generation through the slave mode controller

refer to Timer app note: DM00042534.pdf

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