Optimal component placement goes a long way towards minimizing crossover connection lines. However, crossovers can never be completely avoided. Learn how you can simplify your routing process and avoid wasting time with crossover connections.
Optimal component placement goes a long way towards minimizing crossover connection lines. However, crossovers can never be completely avoided. A large number of crossover connections make routing a PCB extremely challenging and time-consuming to complete. So how can you simplify your routing process and avoid wasting time with crossover connections? The key lies in pin, part and differential pair swapping.
Optimizing Your Crossover Connections with Part and Pin Swapping
It is common for PCB designers too, wherever electrically possible, swap net assignments from one device pin to another eligible device pin. Similarly, sub-parts within a common package can be swapped to reduce crossover connections.
Pin swapping relies on the fact that nets of two different physical pins can be swapped without having any negative impact on the electrical functionality of the design. A basic example would be the two pins of a resistor. Because a resistor’s pin does not have a unique polarity, you can freely swap the pins to eliminate a crossover, and yet still function as intended.
Another practical example would be a high pin-count connector, where there is not a strict requirement for a specific signal assignment to each and every pin. With the flexibility to swap many pins on a connector, several crossover connections could potentially be eliminated. Perhaps the most eligible component type of pin swapping is an FPGA device, where its user definable I/O pins, within applicable voltage banks, let you freely re-assigned pins as needed.
With sub-part swapping, similar parts within a common package are swapped. For example, an LM6154 Quad Op Amp IC has four separate and identical op amps within a single package. Thus, you could swap op amp C (pins 8, 9, and 10) with op amp A (pins 2, 3, and 1) to eliminate crossover connections lines while maintaining the same functionality. Sub-part swapping is sometimes called “gate swapping”, implying that the 4 individual gates within an SN74S02N Quad NOR gate package can be freely swapped.
Device pin and sub-part swapping greatly help in reducing the overall number of crossover connections in a PCB. To successfully implement device pin or sub-part swaps, you must define up front which pins are eligible to be swapped. Furthermore, once pin or part swaps are made within the PCB design, the schematic must be updated to reflect the changes and remain synchronized with the PCB layout. Failing to keep them in synch can lead to disastrous errors.
Part and Pin Swapping in Altium Designer
Declaring which pins or sub-parts can be swapped within a project or symbol library creates many opportunities to eliminate crossovers. Using interactive or automatic swapping capabilities greatly reduces the number of crossover connection within a design.
Pin or part swapping is accomplished in three general steps: configuring the swap data, performing the pin or part swaps, and finally, synchronizing schematics with the swap updates. Learn how to use this technology in Altium Designer by downloading a free white paper.
About the Author
With an emphasis on microprocessor systems design, John earned his Bachelor of Science degree from SUNY Polytechnic Institute. He initially worked as a design engineer in the Defense industry developing diagnostic test programs for complex PCBs. Subsequently, John has worked as a senior application engineer in the EDA industry supporting a wide range of ASIC, FPGA, and PCB design and verification solutions.More Content by John Magyar