Now that we have a signal that is well isolated and protected we need to ensure that it will be be immune to electrical noise and provide clean consistent signals to the microcontroller.
Optocoupler Pull-Up
The first step will to be to generate a logic signal across the isolated path. This is done very simply by pulling the TACH_ISO signal high with a 10kΩ resistor. This will mean that the optocoupler will be able to sink 0.33mA (3.3V/10kΩ) at most when a current is applied on the TACH_IN signal. It is important to note that at this point the signal is inverted. So when the car sends a ignition pulse (12V), this will turn on the optocoupler and thus short TACH_ISO to GND, resulting in a logic 0 signal. Remember this short is at most 0.33mA due to the limiting resistor.
Low Pass Filter
In order to prevent unwanted electrical noise, a low pass filter will be implemented. This will eliminate any signal that exceeds the cut-off frequency. From previous testing we have confirmed that the shortest pulse for this application will be 1.6ms or 625Hz. This means that our low pass filter must have a frequency greater than 625Hz to allow the signals that we actually want to propagate to the next signal conditioning stage.
For my application circuit I chose 1kΩ and 0.1µF resulting in a cut-off frequency of 1592Hz (fc = 1/2piRC). This cut-off frequency is reasonably close to the max switching frequency we expect to see during normal operation, and was mainly chosen as a Bill of Materials (BOM) optimization. If electrical noise becomes an issue, this cut-off frequency can be dialed in closer to the 625Hz. However, low frequencies between 625Hz and 1592Hz are very unlikely to occur.
Signal Inversion
Now that the signal has been isolated and filtered, it is time to do the final conditioning. For this step an NFET is used, meaning when a logic 1 is applied to the gate of the FET the RPM signal will go low. Recall that the optocoupler circuit inverts the signal, and now we are inverting the signal again. So when 12V is seen at the TACH_IN signal we see a 3.3V at the RPM signal. Another benefit of this stage is that the NFET will act as an amplifier and gurantee that the logic 0 or 1 will be well defined and any loss through the signal conditioning will have no effect on the signal.