Looking for a bit of help understanding why my circuit behaves the way it does. I've built an electronic speedo which uses a hall effect sensor and outputs the speed via a multiplexed 7 segment arrangement. There is also an LCD in the system for other functions.
What I have found is that unless I put series resistors on the power pins of the 18F4420 (270 ohm), then int2 fires constantly. int2 is where my hall effect sensor is connected. This results in the system giving me a huge speed reading all of the time. The reason I took out the resistors were twofold: Firstly, I've been trying to get the segments reasonably bright, and secondly I've never come across any circuit diagram for a PIC showing the need to have resistors on the power pins like that. I put them there in the first place because the chip was running too hot long before I got to the point in the project of using 7 seg displays. But I think the reason the chip was getting hot back then was because I was driving LEDs off the output pins without any current limiting resistors (which now of course I have).
I've found that when I connect my scope to try and understand what is going on, the problem goes away as soon as I connect the fly lead to ground i.e. the speed returns to zero, but I still can't get a reading via the hall effect sensor when that happens. But if I put the resistors back on the +5v pins (there are two sets of power pins on this chip) then everything works as it should.
Can anyone explain what electrical properties are causing my problem and what I should do? Afraid I'm not able to post a full schematic at the moment.
What I have found is that unless I put series resistors on the power pins of the 18F4420 (270 ohm), then int2 fires constantly. int2 is where my hall effect sensor is connected. This results in the system giving me a huge speed reading all of the time. The reason I took out the resistors were twofold: Firstly, I've been trying to get the segments reasonably bright, and secondly I've never come across any circuit diagram for a PIC showing the need to have resistors on the power pins like that. I put them there in the first place because the chip was running too hot long before I got to the point in the project of using 7 seg displays. But I think the reason the chip was getting hot back then was because I was driving LEDs off the output pins without any current limiting resistors (which now of course I have).
I've found that when I connect my scope to try and understand what is going on, the problem goes away as soon as I connect the fly lead to ground i.e. the speed returns to zero, but I still can't get a reading via the hall effect sensor when that happens. But if I put the resistors back on the +5v pins (there are two sets of power pins on this chip) then everything works as it should.
Can anyone explain what electrical properties are causing my problem and what I should do? Afraid I'm not able to post a full schematic at the moment.