2.11.1 System Design

When selecting parts to place on high-speed signal bus, if the remote I/O pin impedance of the peripheral device does not match the impedance of the pins on the PIC32C device to which it is connected, signal reflections could result, thereby degrading the quality of the signal. If it is not possible to select a product that matches impedance, place a series resistor at the load to create the matching impedance. See the following figure for an example.

The following recommendations will help ensure the PCB layout will promote the goals previously listed.

• Component Placement:
  • Place bypass capacitors as close to their component power and ground pins as possible, and place them on the same side of the PCB.
  • Devices on the same bus that have larger setup times must be placed closer to the PIC32MK GPK/MCM with CAN FD family of devices.
• Power and Ground:
  • Multi-layer PCBs will allow separate power and ground planes
  • Each ground pin should be connected to the ground plane individually
  • Place bypass capacitor vias as close to the pad as possible (preferably inside the pad)
  • If power and ground planes are not used, maximize width for power and ground traces
  • Use low-ESR, surface-mount bypass capacitors
• Clocks and Oscillators:
  • Place crystals as close as possible to the PIC32C Family device XIN/XOUT pins
  • Do not route high-speed signals near the clock or oscillator
  • Avoid via usage and branches in high speed clock lines
  • Place termination resistors at the end of clock lines
• Traces:
  • Higher-priority signals must have the shortest traces
  • Avoid long run lengths on parallel traces to reduce coupling
  • Make the clock traces as straight as possible
  • Use rounded turns rather than right-angle turns
  • Have traces on different layers intersect on right angles to minimize crosstalk
  • Maximize the distance between traces, preferably no less than three times the trace width
  • Power traces should be as short and as wide as possible
  • High-speed traces must have a continuous ground beneath them

The use of LDO regulators is preferred to reduce overall system noise and provide a cleaner power source. However, when utilizing switching Buck/Boost regulators as the local power source for PIC32C devices, as well as in electrically noisy environments or test conditions required for IEC 61000-4-4 and IEC 61000-4-2, users should evaluate the use of Pi-Filters (i.e., L-C) on the power pins, as shown in the Schematic Checklist chapter. In addition to a less noisy power source, use of this type of T-Filter can greatly reduce susceptibility to EMI sources and events. Use Transient Voltage Suppressors (TVS) on power buses as well as on all external PCB signal connections. If design requirements mandate the use of a buck or boost regulator be sure inductor used is shielded type.