Electronic – RPM counter using 8051 microcontroller

If possible I'd suggest selecting a microcontroller that supports a counter operation using the timer inputs; rather than manually incrementing a counter inside an ISR (which at high frequencies quickly ends up saturating the microcontroller activity) you allow the hardware to handle the counting. At this point your code simply becomes a matter of waiting for your periodic interrupt then calculating the frequency.

To extend the range and make the frequency counter more generalised (removing the need for multiple ranges at the expense of a little more work for the MCU) you could use the following technique.

Select a periodic interrupt rate that allows for measurement accuracy at the highest input frequency; this should take into account your counter size (you need to select the timer period such that the timer counter will not overflow at the maximum input frequency). For this example I'll assume that the input counter value can be read from the variable "timer_input_ctr".

Include a variable for counting periodic interrupts (should be initialised to 0 at startup); for this example I'll refer to this variable as "isr_count". The interrupt period is contained in the constant "isr_period".

Your periodic interrupt should be implemented as (C pseudo-code):

void timer_isr()
{
  isr_count++;
  if (timer_input_ctr > 0)
  {
    frequency = timer_input_ctr / (isr_count * isr_period).
    timer_input_ctr = 0;
    isr_count = 0;
  }
}

Obviously this rough example relies on some floating point math that may not be compatible for low-end microcontrollers, there are techniques to overcome this but they are outside of the scope of this answer.

There are two different common methods to measure frequency with this PIC. Which one is appropriate depends on things you haven't told us, like the minimum and maximum frequency.

The conceptually simple way is to do a frequency counter. Hardware can assist here in that you can have a external signal increment timer 0 or timer 1. Then the firmware reads the timer at known intervals, and the rest is simple math. Note that you do NOT want to reset the timer after each read. Simply do a unsigned integer subtract of the new reading minus the previous and you get the counts in the last interval.

You have to keep timer overflow in mind. Timer 0 is 8 bits and timer 1 16 bits. If using timer 0, for example, you have to be sure that the sampling period is short enough so that there are less than 256 cycles per period at the highest input frequency. With timer 1 you get a lot more room since it's 16 bits wide. You can therefore allow up to 65535 cycles per sampling period.

The other way to measure frequency is to actually measure the period of individual cycles or groups of cycles. This is best done by using one of the CCP modules in capture mode. Timer 1 is set up as a free running timer, and the CCP module takes a snapshot of it every 1, 4, or 16 periods of the external signal. In the CCP interrupt, you subtract the new capture reading minus the previous to get the time duration of the external period (or 4 or 16 periods), and the rest is math.

The CCP capture method is usually more accurate and responsive, but more tricky to implement and to deal with all the corner cases. With this method you have to worry about timer overflow for low input frequencies, not high. I usually deal with this by grabbing the timer value periodically faster than it can wrap, and effectively extending the timer to a arbitrary number of bits in firmware.