Before GPS devices and online mapping services became available and affordable, planning a cross-country trip involved the meticulous study of maps and decisions about choosing the best route. I remember my dad sitting for hours looking at map after map trying to find a path that would give us the best vacation experience. Finding lodging along the chosen route entailed making call after call to determine who had the best accommodations and rates.
“Time is the Most Valuable Thing a Man Can Spend.”
The Greek philosopher Theophrastus advised that wasting time is an extravagant, cost expense. The GPS devices and online mapping services that we rely on today seem like the big brothers of automatic routing applications that we find in PCB design software. While manual routing gives a designer a certain amount of satisfaction and “feel” when visualizing and building a complex layout, automatic routing saves time by making decisions about tracks and vias.
Automatic routing operates through numerically intense processes and depends on the ability of the designer to locate components and nets from a schematic. When a designer establishes the design rules for automatic routing software, the software responds with a route based on maximum reliability and signal strength. The designer retains control by selecting the nets and reviewing the results of the automatic router actions. Examples of electrical and design rules that impact automatic routing include width, clearance, layers and layer direction, routing and rule priorities, and SMD fanout control. Based on those behavior parameters, an automatic router 1) sets the direction and route of each net on a PCB, 2) provides a routing sequence for the nets and 3) uses the routing sequence to route each net.
A designer who utilizes accurate parameters gives automatic routing the ability to properly calculate routes. Given the input of design rule parameters, component spacing, and well-designed paths, an automatic router completes the PCB layout with the trace lengths and the placement of vias needed to mitigate signal crosstalk. In addition, the automatic router takes care of tedious tasks such as pin-to-pin routing for the nets.
Navigating a dense circuit board can become tedious both for slow machines and busy designers.
Although a designer or design team controls the placement of components, automatic routers work to make connections between pins and components, helping to ensure proper heat dissipation, control of electromagnetic interference, reliability, and signal integrity.
Routing Potential: Options, Options, Options!
Automatic routing methods include the grid-based, topology-based, and shape-based solutions--and a newer approach called interactive routing. Grid routers split the design into grid segments based on coordinates while specifying the starting and ending pins for each net, and recognizing any obstacles. During routing, the software follows design rules to avoid the obstacles. The adherence to design rules also allows grid-based automatic routers to perform sweeps and passes for any trace rule violations. If a rule violation exists, the router will route, check, and reroute until the design passes the rule or the software exhausts its options.
Topology-based routing uses spatial analysis to map the board space without relying on polygonal shapes or coordinates. The mapping tools define a routing path and then use routing algorithms to convert the recommended path to a precise route. Spatial analysis forms a triangulated map that the algorithms use to avoid obstacles and build point-to-point connections.
Shape-based automatic routing uses a gridless approach that identifies the real shape of obstacles, defines the space between obstacles as a series of rectangles, and avoids the obstacles when generating a path. The software sees each shape as polygonal geometry with no associaton to any specific routing grid. Because shape-based routing only occurs at right angles and by coordinates, the routing paths associated with each contiguous shape only extend in vertical and horizontal directions. While the routing path follows the edges of rectangles, the software relies on adaptive routing algrorithms that use conflict reduction to discover the natural flow of a net.
Interactive routing takes everything about routing to a different level by addressing a single net, a differential pair of nets, or a set of selected nets. The process of interactive routing begins with transferring a design from the schematic into a workspace. Then, a designer establishes conflict resolution rules and uses an interactive routing command to select a pad, connection line, via, a partially completed track, or an object that has a net attribute. Interactivity occurs as the software follows design rules and finds the closest electrical object and then weaves around any objects as it attempts to define a route path from their to the cursor location. Design teams can adjust the routing track width, change layers, add vias, or change via sizes while the software performs its routing assignment.
Humans Still Needed!
Automatic routers model the routing process based on human input and then attempt to replicate the routing process automatically. As a result, limitations exist. If the software does not sense that manual routing will not work for an area of the PCB, automatic routing will not provide a solution. Once the automatic routing software has completed a task, a designer must confirm that the routing results align with the desired results. Another check involves determining if the routing results follow rules established for critical nets.
We haven’t completely automated circuit design, yet.
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