Page 106 - Embedded Microprocessor Systems Real World Design

let EMC considerations completely drive the design, but if it matters little oth-
erwise, you might think about EMC in choosing a processor.

ESD Protection
Protecting a system against ESD involves many of the same techniques used for pre-
venting RF emissions problems. ESD interference often takes the form of RF
energy, and the same things that keep RF in a box tend to keep it out.
Shielding. Shield the system and use shielded cables where appropriate.
Filters. EM1 filters on signal lines will help keep ESD off the processor board.
Grounding. Make sure ESD energy has a low-impedance path from the
discharge point (usually on the chassis or other operator-accessible areas) to
ground. If the lowest-impedance path for ESD is through the ground plane on
your board, that is where it will go. Avoid having ground loops through your
board wherever possible. Do not ground the CPU board to the chassis
through the mounting standoffs. Instead, have a single wire return to the DC
power supply and have a single point connection to chassis there.
Interfaces. Unfortunately, the embedded system often must talk to other
devices. The interfaces often require a ground reference at both ends to
operate correctly as, for example, RS232. Consequently, you are forced to
design in ground loops just because of the interface requirements. You may
need ferrite beads or EMC filters where the interface signals enter and leave
the board. This will attenuate the high-frequency energy of an ESD pulse.
Watchdog timer. Add a watchdog timer to your circuit so that, if ESD corrupts
program execution, the system can recover.
Isolation. Use optical isolation where it makes sense, especially if you are
connecting to a system with more severe ESD requirements than your own.

Other EM1 Considerations
Ground Loops It is increasingly common for embedded systems to be controlled
from an external computer. If the external computer is connected to a different
AC power source than the embedded system (such as a 120V computer connected
to a 280V, three-phase machine), you may get ground loops between the two
systems. Be sure the grounds are common. If you cannot make the grounds the
same (maybe because the customer controls where the computer plugs in), opti-
cally isolate the interface.

Differential Interfaces Differential interfaces, like W85 or LVDS, can reduce
susceptibility to ground noise and other types of electrical noise. However, while

88 Embedded Micrcpocessm Systems