title: STM32F4 - Using a timer to control non-adjacent timer
body: <p>My processor is an STM32F437ZGT6 and I wish to count two different pulse trains (RPM). The range is quite wide, I may have an engine that idles at 150 rpm and we get a pulse from the cam, so 0.5 pulses per revolution, or 1.25 pulses per second. At the other extreme I may need to count 460 flywheel teeth at 3000 rpm, 23000 pulses per second. I have a prescaler available so I can divide the external event by up to 8 but even so this become too intense at higher speeds because every event or eight event causes an interrupt. </p>
  
  <p>One alternative I am considering would be to have one timer use the external event as the clock and it would just count events within a time window. My difficulty comes from determining how to use another timer to control the window by setting and clearing CEN or some similar action. </p>
  
  <p>In RM0090, section 18.3.15 Timer synchronization the example shows one timer controlling another, timer 1 controlling timer two. I thought that may be useable but althought I did not read otherwise I don't see that any two timers could be paired. The signal I am interested in actually feeds two timers. TIM1 ch1 and TIM9 ch1.</p>
  
  <p>Any suggestions would be appreciated as I don't want to cobble up some Rube Goldberg scheme where one timer fires off an ISR and then the ISR opens and closes the time window. </p>
  
  <p>I should have noted that a lookup table is provided that provides the expected engine speed and the number of pulses per revolution.</p>
  
  <p>Thanks,
  jh</p>