Esd; Usb Selective Suspend - Intel 855GM Design Manual

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1. A part must be chosen with the impedance value that provides the required noise attenuation. This
is a function of the electrical and mechanical characteristics of the part chosen and the frequency
and strength of the noise present on the USB traces that you are trying to suppress.
2. Once you have a part that gives passing EMI results the second step is to test the effect this part
has on signal quality. Higher impedance common-mode chokes generally have a greater damaging
effect on signal quality, so care must be used when increasing the impedance without doing
thorough testing. Thorough testing means that the signal quality must be checked for Low-speed,
Full-speed and High-speed USB operation.
11.4.5.

ESD

Classic USB (1.0/1.1) provided ESD suppression using in line ferrites and capacitors that formed a low
pass filter. This technique doesn't work for USB 2.0 due to the much higher signal rate of high-speed
data. A device that has been tested successfully is based on spark gap technology. Proper placement of
any ESD protection device is on the data lines between the common-mode choke and the USB connector
data pins as shown in Figure 103. Other types of low-capacitance ESD protection devices may work as
well but were not investigated. As with the common mode choke solution, it is recommended to include
footprints for some type of ESD protection device as a stuffing option in case it is needed to pass ESD
testing.
11.4.6.

USB Selective Suspend

The USB Specification states that the maximum current consumption on the "USB bus" is 500 mA for
normal operation, 2.5mA for suspend power when remote wakeup is supported, and 500 uA otherwise.
However, some Bluetooth devices may require more current in suspend state than specified in the USB
specification. Therefore, the system designers should ensure that, on their particular system
implementation, there is enough current supplied to the Bluetooth device during suspend state in order
for selective suspend to function properly.
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Intel 855GM/855GME Chipset Platform Design Guide
I/O Subsystem
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