8101 datasheet(143/172 Pages) LSI

143 / 172 page

Power Supply Decoupling

5-17

97, 135, 147, and193 of the 8101 or pins B6, D13, G14, N13, and R7 of

the 8104) be isolated from the I/O VCCs with a 2-ohm resistor between

the VCC plane and the device core VCC pins, as shown in Figure 5.2.

Figure 5.2

Decoupling Recommendations

The 2 ohm resistor will reduce the amount of noise coupling from the I/O

VCCs to the core VCCs. A resistor is recommended over a ferrite bead

because the inductance of a ferrite bead can induce noise spikes at the

device pins. Decoupling capacitors should then be placed between the

device VCC pins and GND plane, as shown in Figure 5.2 and described

as follows.

The external SerDes device that is typically connected to the 10-bit PHY

interface can also be very sensitive to noise from the VCC plane.

Recommendations from the manufacturer of the SerDes device used

should be followed. Generically, it has been found from practice that the

SerDes should be isolated from all devices on the PCB with a ferrite

bead between the VCC plane and all of the SerDes VCC pins, as shown

in Figure 5.2. In addition, it has been found from practice that the analog

VCC

[5]

Other

Devices

GND

[1]

I/O

[3]

8101/8104

GND

[1]

VCC

Core

VCC

Analog

[4]

SerDes

GND

[2]

VCC

Digital

VCC

3.3 V

Ferrite Bead

Ferrite

Bead

GND Plane

Notes:

[1]

It is recommended that a 0.1/0.001

µF pair of capacitors for every four VCCs less than 0.5 inches from

the device VCC/GND pins, evenly distributed around all four sides of the devices.

[2]

Same as Note [1] except every two VCCs.

[3]

Core VCC pins are 70, 97, 135, 147, and 193 of the 8101 or pins B6, D13, G14, N13, and R7 of the

8104. All remaining VCC pins are I/O VCCs.

[4]

These are generic recommendations for SerDes. Follow any recommendations fromthe speciﬁc

SerDes manufacturer.

[5]

This is a generic recommendation for other digital devices. Follow any recommendations from the speviﬁc

device manufacturers.

Ω

Controller

VCC Plane

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