Watt's the wattage when used on a PIC?

[Tabsoft]

Tony,

I did not see where you indicated which pin on the PIC you are using for input.
As you probably already know that which pin you use matters in the answer to your question about what voltage will be recognized as a 1 (high) or 0 (low) by the PIC.

The PIC you are using has 3 types of digital input pins, each with different thresholds for low and high. Standard Inputs, TTL Buffered inputs and Schmitt Trigger inputs.

Table 15.4 of the DS has the Min/Max for Input Low and Input High for each of the pin types.

You probably already knew this, but just in case.

Regards,

[AvionicsMaster1]

No. You don't need to make the resistor bigger if you keep the same ratios previously stated.

Yes. IMSWO by looking at the datasheet if you're using a TTL input 2.5 V is above the level supposed to be recognized as a high. Why you don't design it for more voltage than that is curious. I'm assuming you'll have a 12v signal you'll have to schmooze so why not make it 5ish volts and not worry about ambiguity. According the datasheet, if I understand it right, with a Schmitt trigger input 2.5 v is way over a recognizable high. So pick a pin type and design for it.

[ERMEGM]

I was being conservative for any over voltages from the vehicle, hence the Zener. I just found another circuit with the same #2 VD setup as above. 2.2k to ground and 33k to the output yields 1.7 volts when 13.8 volts is applied. No cap and no Zener and on a 16F628A, RB0.

This is all new to me as I've been using opto's. Never looked at the datasheet that way as I had never intended to connect it directly. Didn't think you could, but now I'm finding out that you can. Looking to explore it further. Right now, my pins are RB0, RB1 and RB2.

[MichelJasmin]

This is all new to me as I've been using opto's. Never looked at the datasheet that way as I had never intended to connect it directly.

OK, let's start from the beginning... You said you had some opto that failed you. Optos are sensitive to reverse voltage and forward current must be limited too. Your circuit will be on a car so you must protect your opto. So, please forget about those resistor divider and/or zener and use an opto the proper way:



The maximum continous current for the PC817 is 60mA. With a 1k resistor it's about 60V continous. Note the 1N4004 to protect the LED from any reverse voltage (spikes can occur). Trust me, do not try to connect your PIC to a vehicule voltage without an opto. At work I maintain our GPS tracking system which has I/O to monitor some things (sorry, don't have time to explain) and some I/O's are tied almost directly to the CPU and guess what? They fail some times!

[ERMEGM]

That is the exact setup I have. I am using the ILD205 instead for the opto and the LS4148 for the reverse polarity protection. I have a diode on the positive input as well. The design of the board and wires prevents a reverse polarity connection. These optos are failing right out of the packaging. A bad batch? Maybe, but nonetheless, very time consuming to repair. Either way, if the opto fails, or the resistor fails, blowing the pic, the board is coming back to me.

At this point, my time is worth more than the cost of the pic or opto. I designed it this way because of the features of the opto, however, I've seen the other setup work time and time again without any problems. I was going to add the cap and Zener to boost up the protection a little bit on the VD setup. I'm not looking for cheap. I am more interested in reliability. The opto served me well until this last batch. Now I'm skeptical.

[MichelJasmin]

FIY, this is how our GPS units are designed:



On the "ignition" input the TVS is a big 1.5KE33 and none of the ~480 units failed me. The other "conventionnal" I/O are either "ground inputs" (the ones that fails often) or optoisolated. I had a few units who failed on those opto inputs but the TVS is a 0805 SMT. If I came across a failed one I'll do an autopsy to find out what failed...

HTH