49.8 Designing for High-Speed Peripherals

The PIC32C family of devices have peripherals that operate at frequencies much higher than typical for an embedded environment. Due to these high-speed peripheral signals, it is important to consider several factors when designing a product that uses these peripherals, and the PCB on which these components will be placed. Adhering to these recommendations will help achieve the following goals:

  • Minimize the effects of electromagnetic interference for the proper operation of the product.
  • Run all PCB high-speed signals first on component side of PCB.
  • Ensure signals arrive at their intended destination at the same time by matching critical trace lengths on the PCB.
  • Minimize crosstalk. Insure continuous ground under all high-speed signals.
  • Maintain signal integrity by the use of termination resistors in the 30-50 ohm range.
  • Reduce system noise by using bulk and high frequency decoupling caps and inductors on power rails.
  • Minimize ground bounce and power sag. Use a dedicated ground plane if possible or at a minimum a star ground configuration. Do not daisy chain ground and power traces to components.

System Design

Impedance Matching

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, hence place a series resistor at the load to create the matching impedance, see figure below for an example.

Figure 49-12. Series Resistor

PCB Layout Recommendations

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 must 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 of 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

EMI/EMC/EFT (IEC 61000-4-4 and IEC 61000-4-2) Suppression Considerations

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 must evaluate the use of Pie-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 that inductor used is of a shielded type.