---------- From: pic microcontroller discussion list on behalf of Scott Horton Sent: Tuesday, December 03, 1996 11:16 AM To: Multiple recipients of list PICLIST Subject: Hardware Question 16C84 Please excuse my weak electronics abilities as I am an ME struggling my way into programming these little critters. I've read (and are still reading) lots of the PIC resources, ECH, FAQ archives, etc. I have a couple of questions I'm sure someone (probably everyone but me ) can help with: 1. Can someone tell me (in laymen terms?) what a Schmitt Trigger is/does/is used for (as shown in the 16C84 notes)? 2. One of the App Notes from MC - re RS232 interfacing, showed a schematic with a component called ZVN104 which was used as part of an interface to a RS232 level. I plan on using a MAX chip or one of the other driver chips but I am still curious as to exactly what that component was - for my (continuing) education - I was unable to find it with any of my electronics suppliers, or at least didn't know what I was looking for :). Thanks in advance, Scott Scott.Horton1@bridge.bst.bls.com - Simplest explanation I can think of for what a schmidt trigger does is that it takes an analog waveform and makes it digital. In simple terms it looks at the input voltage waveform and when it reaches a set threshold voltage, say 3 volts, it triggers its output high. The output will not follow input. Simply speaking the input can continue to rise or fall after the output has been set but it would have no effect on the output-the output would remain high. Another use might be to debounce a mechanical switch used at the input. Again simply speaking most digital IC circuits have very fast response times. A mechanical switch has a point within its movement when the contacts are so close together that any vibrations can cause the circuit path to be closed and opened very rapidly. Digital circuits could interpret this to be many (but very fast) open and closes of the switch and then react as though the switch had been operated very many times. If you are designing a circuit that you want to be predictable, as in the push of a button causes the circuit to behave a certain way, then key bounce can introduce some very unpredictable results in your circuit. The schmidt trigger can be used to eliminate this because you can choose values that create certain time constants (as in the schmidt trigger will not respond to any changes at its input for a certain period of time after it has changed its output state). Another application for it would be to shape a waveform from sine wave to square wave. This is common in clock circuits such as what a frequency counter is. The schmidt trigger would be used at the input of the frequency counter to shape the input wave form and make it easier for the counting circuits to act upon the input in a predictable and reliable way. (as in digital circuits have a very specific window of voltages that they read as a digital 1 or digital 0 and the schmidt trigger eliminates the "gray" area of voltages that could be read as either because the voltage is at the threshold point of being either a 1 or a zero) Hope this helps. -Mike