Pulse capture signal basic conditioning

Hello,

So i'm working on a digital dashboard and everything is working very nicely and now its time to knock out the tach and wheelspeed sensors part. For this i will use my PIC18F4480's pulse capture mode.

The signals from the car are 12V in amplitude. I can't find a maximum input voltage spec on this chip, but i assume thats on the high side. Should i use a voltage divider to knock em down to 5Vish? or should i use an opamp which might also help as a buffer. Are there any input impedence issues or anything else i might need to consider?

Thanks

Just to throw another 1 into the mix, you may also consider an opto isolator, this will buffer the PIC from any nasty noise on the 12V signal coming in.

The PIC has a high input impedence so a simple resistor divider and zener diode (or low voltage MOV) should do the job, but an opto may just be more reliable in the long run Take your pick (no pun intended!).

Hope this helps.

Vmax is Vdd + 0.3v. There is a shunt diode which will conduct current from that pin to the Vdd pin if it gets above that. If the source has a high impedance it can be ok.
You can:
1. Use a voltage divider
2. Just use a 10k resistor, that makes only 0.7mA across the shunt resistor and the pin voltage won't go about Vdd+0.3v at this level.
3. Use an optoisolator.
4. Use a 5.1v zener in parallel with the pin to shunt it to ground and again use a resistor to limit current. Really since the tolerance of these is variable, esp with temp, it won't do this job well.

Some tach signals are AC. The ground is similarly protected by a shunt diode which forward biases when the voltage is below Vss-0.3.

Just a quick reminder, some of the pins on a PIC don't have shunt diodes to VDD, so if you are using the old resistor trick (which btw works a treat for bit-banging RS232, with a 10K) you need to keep this in mind.

I know that the pin you are intending to use has a shunt diode, but just thought I would make you aware that it's not a fool-proof method for every pin, using this on a 16f62x on pin 4, (mclr set as input) could very well put the PIC into program mode!

Hope this helps.