intial attempt

Timer-capture-mode.md

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+Here the Timer can be used to measure an external signal and determine the timing period.
+
+1. Select active input
+  * in CCMRx reg - set CCxS bits to non-zero.
+    * if zero the CCRx reg will be read-only.
+2. Program filter and prescaler if required
+  * filter: in CCMRx reg - write IC1F[3:0] bits
+  * prescaler: write IC1PSC[1:0] bits
+3. Program polarity
+  * write CCxNP or CCxP bits to select between rising, falling, or both edges
+
+You need two consecutive captures to determine timing of signal
+Period is calculated by subtracting these two values where polarity_index = 1 if rising or falling edge is used and 2 id both.
+
+```  Period = Capture(1) / (TIMx_CLK * (PSC+1) * (ICxPSC) * polarity_index(2))  ```
+
+capture diff of CCRx_tn and CCRx_tn+1 is:
+```
+	if CCRx_tn < CCRx_tn+1:
+		capture = CCRx_tn+1 - CCRx_tn
+	else: capture = (ARR_max - CCRx_tn) + CCRx_tn+1
+```
+To facilitate input capture - we reset the timer counter after each rising edge by
+* select TIxFPx as input trigger - in TMCR reg - set TS bits
+* select reset mode as slave mode - in SMCCR reg - config SMS bits
+
+So when an edge is detected, the counter is reset and period of ext signal is given by CCRx register. (ICPSC reg is not considered).
+
+Period is given by:
+
+```  Period = CCRx / (TIMx_CLK * (PSC+1)* polarity_index(1))```
+
+###Timer output compare mode
+Several output compare modes exist. You can use them to control an output waveform or to indicate when a period of time has elapsed.
+* output compare timing
+  * comparing CCRx and CNT has no effect on output.
+  * used to generate a timing base
+* output compare active
+  * set channel output to active level on match
+  * OCxRef signal is forced high when CNT matches CCRx (the capture/compare reg)
+* output compare inactive
+  * set channel to inactive level on match
+  * OCxRef signal is forced low when CNT matches CCRx
+* output compare toggle
+  * OCxRef toggles when CNT matches CCRx
+* output compare forced active/inactive
+  * OCxRef is forced high(active mnode) or low (inactivemode) independently of CNT value
+
+
+To configure Timer into one of these modes:
+
+1. select clock source
+2. in ARR and CCRx reg - write desired data
+3. Configure output mode:
+  * select output compare mode: timing/active/inactive/toggle
+  * if not timing - in CCER reg - select polarity by writing CCxP
+  * disable preload for CCx - in CCMRx reg - write OCxPE
+  * enable capture/compare - in CCMRx - write CCxE
+4. Enable counter:
+  * in TIMx_CR1 reg - set CEN bit
+5. If DMA/Interrupt is required - set CCxIE or CCxDE bit
+
+
+Calculate update rate - compare timing / toggle of Timer output
+
+```   CCx update rate = TIMx_Counter_CLK / CCRx```
+
+where:
+* if internal clock: ```TIMx_Counter_CLK = TIM_CLK / (PSC+1)```
+* if external clock mode 1 or mode 2 ```  TIMx_Counter_CLK = ETR_CLK / ((ETR_PSC)*(PSC+1))```
+
+
+Calculate delay - compare active/inactive Timer output
+* ```  CCx delay = CCRx / TIMx_Counter_CLK```
+where:
+* if internal clock: ```TIMx_Counter_CLK = TIM_CLK / (PSC+1)```
+* if external clock mode 1 or mode 2 ```  TIMx_Counter_CLK = ETR_CLK / ((ETR_PSC)*(PSC+1))```
+* if internal trigger clock: ```  TIMx_Counter_CLK = ITRx_CLK / (PSC+1)```