Path: utzoo!attcan!telly!problem!compus!lethe!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!van-bc!mdivax1!moss Newsgroups: sci.electronics Subject: Crstal oscillator load capacitance Message-ID: <1990Dec21.193438.4172@mdivax1.uucp> Date: 21 Dec 90 19:34:38 GMT Reply-To: mdivax1!moss () Organization: Mobile Data International, Richmond, B.C., Canada Lines: 60 Return-Path: Apparently-To: van-bc!rnews When designing a parallel mode oscillator like those used on most microcontrollers (80C31, 68HC11, etc.), it is necessary to specifiy a parallel crystal with a given load capacitance. I am trying to come up with a formula for determining the load capacitance. |\ | \ -----| >O------ | | / | | |/ | | | | | | | ----- | | | | | | | | +--| | | |--+ | | | | | | | | ------ | | | XTAL | | | | | C1 ------- ------- C2 ------- ------- | | | | | | ------- ------- ----- ----- --- --- For the circuit above, the load capacitance should be Cload = C1 + C2 + 2 * pin capacitance Now most design examples use C1 and C2 in the range of 20 to 33 pF, and pin capacitance is approx. 10 pF for most ICs, so Cload should be in the range of 60 to 100 pF. However, the standard Cload values for stock crystals (and the design examples I've seen) usually have Cload in the range of 20 to 40 pF. Has anyone explored this issue or seen a good explanantion of this discrepancy? I will be using crystals in large enough volumes that I can specify Cload to the manufacturer; however, I feel that I might be missing something in my analysis. Any help would be greatly appreciated. Barry Moss P.S. I already know that parallel and series resistors have the same internal composition, but that they resonate at slightly different frequencies when connnected series or parallel. -- ------------------------------------------------------- Barry Moss (604) 277-1511 Mobile Data International, 11411 Number Five Road Richmond, BC, Canada V7A 4Z3