Issue link: https://iconnect007.uberflip.com/i/1153097
AUGUST 2019 I DESIGN007 MAGAZINE 81 With DRC's customizable properties, design- ers can easily define rule parameters, such as target impedance on the traces and minimum and maximum allowable trace length. Because the specific differential traces that are being tested have a 90-ohm differential impedance, the target impedance parameter should be de- fined as 90-ohm before running the rule. DRC provides the flexibility to customize each rule for individual design parameters, thereby re- ducing false violations and giving designers more control. Checking Proper Placement of Decoupling Capacitors With DRC One of the many challenges that engineers face on a regular basis is ensuring that their de- signs meet the defined specifications for each component. For example, the datasheet for the BeagleBone Black's TI Sitara processor defines the maximum allowable distance for bypass capacitors on the VDDS DDR net as 400 mils. If decoupling capacitors are not placed proper- ly, there is a high chance that the DDR3 inter- face could malfunction. Manually checking the placement of decou- pling capacitors on each net is a tedious and time-consuming process. Alternatively, the decoupling capacitor placement rule in DRC will locate all improperly placed, and non - existent, decoupling capacitors for a defined net or component. With the information ob- tained from the TI Sitara's datasheet, the 400- mil placement radius on the VDDS DDR net can be specified within the decoupling capac- itor rule. After running the rule and selecting a specific instance of the violation, the tool will highlight where the error occurs, as well as indicate the designated 400-mil test radius (Figure 6). Additional Rules Available With DRC Routing a trace from one layer to another is a common design practice used to accom- modate today's densely packed PCB layouts; however, care must be taken to reduce the risk of common-mode radiation. Frequent- ly, stitching capacitors or stitching vias are placed near the net when a plane change oc- curs to allow for a continuous current return Figure 6: The DRC tool can locate each instance of improper decoupling capacitor placement while showing the defined test radius.