Echelon Series 6000 Chip databook - Manual (Page 91)

Achieving High Immunity
Achieving good immunity to ESD and other types of network transients requires good layout
of the power, ground, and other device circuitry. In general, an ESD current will return to
Earth ground or to other nearby metal structures. The device’s ground scheme must be able
to pass this ESD current between the network connection and the device’s external ground
connection without generating significant voltage gradients across the device.
To achieve high immunity, ensure that your design conforms to the following general
guidelines:
• Use a star-ground configuration for your device layout
• Limit entry points in the device for ESD current
• Provide ground guarding for switching power supply control loops
• Provide good decoupling for V
DD33
and V
DD18
inputs
• Maintain separation between digital circuitry and cabling for the network and power
In a star-ground configuration, the power supply, network coupling circuit, and any I/O
circuitry are distributed on the PCB in the form of a star, with the respective connectors and
any chassis ground connections forming the center of the star. The host microprocessor and
other sensitive circuitry should be located away from the center of the star. The goal of the
star-ground configuration is to conduct transients that enter the device on one cable out of
the device through the other cables, with minimal disruption to other functional areas of the
device. If the device has a metal chassis, ESD and other transients generally return to that
chassis by way of the star-ground center point. If the device's logic ground is connected to
this chassis ground, you should only connect it at this single point, with a short standoff, in
the center of the star. Keep noisy digital lines (such as host microprocessor memory array
lines) away from the metal enclosure walls. If a device is housed in a plastic enclosure and is
powered with an isolating transformer, an explicit Earth ground or chassis ground might not
be available. In this case, it is still important for the network connector and power supply
connector to be located near the center of the star.
Switching power supply control loops can pick up radio-frequency (RF) noise and rectify it.
RF immunity depends on limiting the pickup of such RF noise, so you need to provide
sufficient ground guarding for the switching power supply control loops.
To provide good decoupling for V
DD33
and V
DD18
inputs, you should distribute V
DD33
and V
DD18
through low inductance traces and planes in the same manner as ground. All of the ground
pins on an FT 6000 Smart Transceiver or Neuron 6000 Processor should be connected with
either a ground plane (for PCBs with at least 4 layers) or a ground pad directly underneath
the FT 6000 Smart Transceiver or Neuron 6000 Processor on the bottom of the board (for
PCBs with 2 layers). Place one SMT decoupling capacitor per power pin between the FT
6000 Smart Transceiver or Neuron 6000 Processor and ground. Place each decoupling
capacitor on the top side of the PCB, with the connection to its V
DD33
or V
DD18
pin as short as
possible.
Maintaining separation between digital circuitry and cabling for the network and power
limits RF crosstalk to any traces associated with the network or power supply (and any I/O
lines that leave the device).
Electrostatic Discharge
Electronic systems in industrial and commercial environments frequently encounter
electrostatic discharge (ESD). An ESD event is a momentary electric current that flows
Series 6000 Chip Data Book 79