{"id":1756,"date":"2015-06-14T01:18:40","date_gmt":"2015-06-14T08:18:40","guid":{"rendered":"http:\/\/micromouseusa.com\/?p=1756"},"modified":"2015-09-21T17:58:27","modified_gmt":"2015-09-22T00:58:27","slug":"how-to-calculate-the-actual-load-capacitance-for-crystal-on-a-mcu","status":"publish","type":"post","link":"http:\/\/micromouseusa.com\/?p=1756","title":{"rendered":"How to calculate the actual load capacitance for crystal on a MCU."},"content":{"rendered":"<div class=\"usertext-body may-blank-within md-container \">\n<div class=\"md\">\n<p><a href=\"http:\/\/micromouseusa.com\/wp-content\/uploads\/2015\/06\/xtal.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\" size-full wp-image-1763 aligncenter\" src=\"http:\/\/micromouseusa.com\/wp-content\/uploads\/2015\/06\/xtal.jpg\" alt=\"xtal\" width=\"262\" height=\"187\" \/><\/a>Crystal is on the most often used clock source for micro-controller. It requires 2 capacitor with same value(typically) connect between the 2 terminals of the crystal and the ground\u00a0 in order to match the load capacitance of the crystal that you are using.<\/p>\n<p>Here is the how you calculate the values of load capacitors used for crystal:<\/p>\n<p><span style=\"color: #ff00ff;\"><strong>Load Capacitance = [(Cd x Cg)\/(Cd+Cg)]+Cic+\u25b3C<\/strong> <\/span><\/p>\n<p>where Cd and Cg are the 2 capacitor you placed on each side of the crystal. Cic is the capacitance of IC, usually 3-5pF experimentally. and I usually ignore \u25b3C as the capacitance on PCB since no high speed device is involved.<\/p>\n<p>Since Cd ss same as Cg most likely, we can simplify the equation to:<\/p>\n<p><strong><span style=\"color: #ff00ff;\">Load Capacitance = [(Cd x Cd)\/(Cd+Cd)]+Cic = Cd\/2+Cic<\/span><\/strong><\/p>\n<p>therefore Cd = (LC &#8211; Cic) x 2<\/p>\n<p>If Lc is 8pF, I say Cic is 3-5pF, Cd will be 6-10pF<\/p>\n<p>most of the CL for crystal running at 8MHz(normally used for stm32) I ever used are 18pf, in this case, the Cd will be 26-30pF. However, it can run with lower capacitance depends on the quality of crystal since I made it worked at 18pF capacitors before. If you don&#8217;t have the exact value of capacitor needed for crystal, higher is better than lower value just in case it&#8217;s too low to make the crystal oscillate.<\/p>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>Crystal is on the most often used clock source for micro-controller. It requires 2 capacitor with same value(typically) connect between the 2 terminals of the crystal and the ground\u00a0 in order to match the load capacitance of the crystal that &hellip; <a href=\"http:\/\/micromouseusa.com\/?p=1756\">Continue reading <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","_links_to":"","_links_to_target":""},"categories":[8,10],"tags":[],"jetpack_featured_media_url":"","_links":{"self":[{"href":"http:\/\/micromouseusa.com\/index.php?rest_route=\/wp\/v2\/posts\/1756"}],"collection":[{"href":"http:\/\/micromouseusa.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/micromouseusa.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/micromouseusa.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/micromouseusa.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1756"}],"version-history":[{"count":7,"href":"http:\/\/micromouseusa.com\/index.php?rest_route=\/wp\/v2\/posts\/1756\/revisions"}],"predecessor-version":[{"id":1768,"href":"http:\/\/micromouseusa.com\/index.php?rest_route=\/wp\/v2\/posts\/1756\/revisions\/1768"}],"wp:attachment":[{"href":"http:\/\/micromouseusa.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1756"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/micromouseusa.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1756"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/micromouseusa.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1756"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}