{"id":17972,"date":"2017-09-25T08:00:29","date_gmt":"2017-09-25T15:00:29","guid":{"rendered":"http:\/\/www.autodesk.com\/products\/eagle\/blog\/?p=1691"},"modified":"2023-09-25T12:00:13","modified_gmt":"2023-09-25T19:00:13","slug":"length-match-high-speed-nets","status":"publish","type":"post","link":"https:\/\/www.autodesk.com\/products\/fusion-360\/blog\/length-match-high-speed-nets\/","title":{"rendered":"How to Length Match High Speed Nets"},"content":{"rendered":"

How to Length Match High Speed Nets in Autodesk EAGLE<\/span><\/h1>\n\n\n

In our <\/span>previous blog<\/span><\/a>, we showed you how to take advantage of the latest technologies like HDMI, USB 3.0, or DDR by routing differential pairs in Autodesk EAGLE. Now we need to ensure your signals arrive at the same time with the help of length matching. While this technique works great for coupled traces like differential pairs, you might also length match single nets within a bus or chip whose signal timing needs to be exact. We\u2019ll be looking at both scenarios and show you how to use the Meander tool in Autodesk EAGLE to get the job done.<\/span><\/p>\n\n\n

Why Match the Lengths of Traces?<\/h2>\n\n\n

If you\u2019re planning to design a PCB layout that includes Double-Data Rate (DDR) memory, then you\u2019ll be working with not one but two data bit transmissions in a single clock cycle. Big deal, they\u2019ll both arrive at the same time automatically, right? Not necessarily. Because the speed of electrons flying down your circuit is always at a fraction of the speed of light, if you have two signals traveling down two traces of different lengths, then they\u2019ll arrive at different times. When that happens, you\u2019ve got data corruption and signal timing issues.<\/span><\/p>\n\n\n\n

Whether you\u2019re routing differential pairs or an entire bus for your high speed memory, it\u2019s always critical that the length of your traces are properly matched. You can accomplish this in Autodesk EAGLE by adding delays in your traces with the Meander tool. There are three situations where you\u2019ll typically use length matching:<\/span><\/p>\n\n\n\n

    \n
  1. For single-ended nets<\/b>. You\u2019ll length match single-ended nets at their terminating connections that are part of a bus or chip. <\/span><\/li>\n\n\n\n
  2. For differential pairs<\/b>. You\u2019ll length match both nets in a differential pair to keep them tightly coupled along their path of travel. <\/span><\/li>\n\n\n\n
  3. For single nets and differential pairs.<\/b> You\u2019ll length match both single-ended and differential pairs when they\u2019re all part of the same memory module or bus to ensure every signal arrives at the same time.<\/span><\/li>\n<\/ol>\n\n\n\n

    Let\u2019s look at how to use the Meander tool in Autodesk EAGLE to length match your nets. This process is largely the same whether you\u2019re matching single-ended nets or differential pairs. <\/span><\/p>\n\n\n

    Step 1 – Determine Your Longest Net Length<\/h2>\n\n\n

    We\u2019re going to assume that you already have your single-ended nets or <\/span>differential pairs routed<\/span><\/a>. Now you\u2019ll need to determine which of your nets within your bus or chip has the longest length and then matches the remaining nets to this length. There are a couple of ways to determine the length of a net, including:<\/span><\/p>\n\n\n

    The Info Tool<\/h3>\n\n\n
      \n
    1. Select the <\/span>Info \"info-icon\" tool on the left-hand side of your interface. <\/span><\/li>\n\n\n\n
    2. Left-click on a net to open the <\/span>Properties dialog<\/b>.<\/span><\/li>\n\n\n\n
    3. Make a note of the value in the Length field<\/b>.<\/li>\n<\/ol>\n\n\n

      \"net-properties-length\"<\/h2>\n\n

      <\/h3>\n\n

      ULP Script<\/h3>\n\n\n

      There\u2019s also a handy ULP script that will display basic information for every net on your PCB including lengths, theoretical currents, etc. To run this script type <\/span>\u201c<\/b>run length-freq-ri<\/span>\u201d<\/b> in the <\/span>EAGLE Command Line<\/b> and press <\/span>Enter<\/b>. This will open the <\/span>Eagle: Wire length of Layout dialog <\/b>showing you all of your traces. <\/span><\/p>\n\n\n\n

      \"wire-length-ulp\"\/<\/figure>\n\n\n\n

      If you need to obtain a specific impedance for your lines then, the length you need isn\u2019t necessarily determined by the longest for your routed traces. In these cases, your length will be determined through calculations or from a signal integrity simulation.<\/span><\/p>\n\n\n

      Step 2 – Meander Your Net(s)<\/h2>\n\n\n

      Got your longest net length? Good, now we can meander all of our nets to this length with the Meander tool. Let\u2019s first look at an example with two single-ended, high-speed signals terminating at our memory chip:<\/span><\/p>\n\n\n\n

      \"high-speed-single-nets\"\/<\/figure>\n\n\n\n

      MMC1_DATA0 is our longest net with a length of 455 mils. We can match net MMC1_DATA1 to this length with the following steps:<\/span><\/p>\n\n\n\n

        \n
      1. Select the <\/span>Meander \"meander-icon\" tool <\/b>on the left-hand side of your interface. <\/span><\/li>\n\n\n\n
      2. Enter your desired net length in the <\/span>EAGLE Command Line<\/b> and press enter. <\/span><\/li>\n\n\n\n
      3. Left-click<\/b> on the net you want to length tune, then left-click again to confirm the selection and begin the meandering process.<\/span><\/li>\n\n\n\n
      4. Drag your cursor to the left or right to add meanders to your net. You\u2019ll notice a dialog box that changes percentage as you move your meander. Your nets are perfectly matched when this reaches 100%.<\/span><\/li>\n\n\n\n
      5. Once you\u2019ve reached your desired net length, left-click to finalize the meandering. <\/span><\/li>\n<\/ol>\n\n\n\n
        \"single-net-length-tuning\"\/<\/figure>\n\n\n\n

        Meandering a pair of differential pairs works the same. Below we have two coupled differential pairs which have been length tuned using the <\/span>Meander tool \"meander-icon\"<\/b>.<\/span><\/p>\n\n\n\n

        \"differential-pairs-length-matched\"\/<\/figure>\n\n\n\n

        Note that when length matching differential pairs the Meander tool will open a dialog that displays two percentages. The top percentage is the primary trace that you selected with the Meander tool, and the bottom is the coupled secondary trace. Both will be matched to the specified length above the two percentages.<\/span><\/p>\n\n\n\n

        \"length-matching-percentages\"\/<\/figure>\n\n\n

        Match Those Lengths<\/h2>\n\n\n

        Precise signal timing is essential in any high speed application and ensures that your data arrives as intended at its destination. To make this happen, use the Meandering tool Autodesk EAGLE to balance the lengths of single-ended nets, differential pairs, and groups of nets in a bus or memory block. With all of your nets precisely matched you\u2019ll get to enjoy the inherited benefits, like control impedances and reduced risk of data corruption due to timing.<\/span><\/p>\n\n\n\n

        Ready to start length matching nets in your own high speed design project? <\/span>Try Autodesk EAGLE for free today!<\/span><\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

        Got clock cycles to worry about in your high speed design? You\u2019ll want to know how to length match your high speed nets to keep everything in sync! Learn how inside. <\/p>\n","protected":false},"author":2425,"featured_media":440,"menu_order":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":""},"categories":[434],"tags":[],"coauthors":[],"class_list":["post-17972","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-eagle","dhig-theme--light"],"acf":[],"yoast_head":"\nHow to Length Match High Speed Nets | EAGLE | Blog<\/title>\n<meta name=\"description\" content=\"Learn how to length match high speed nets in Autodesk EAGLE to meet critical timing requirements in high speed PCB design applications like DDR.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.autodesk.com\/products\/fusion-360\/blog\/length-match-high-speed-nets\/\" \/>\n<meta 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