AM486DX2 datasheet(57/67 Pages) AMD

57 / 67 page

Am5X86 Microprocessor

AMD

PRELIMINARY

Electrical Data

The following sections describe recommended electri-

cal connections for the Am5X86 microprocessors and

electrical specifications.

11.1 Power and Grounding

11.1.1 Power Connections

Am5X86 microprocessors with 16 Kbytes of cache have

modest power requirements. However, the high clock

frequency output buffers can cause power surges as

multiple output buffers drive new signal levels simulta-

neously. For clean, on-chip power distribution at high

frequency, 23 V

CC pins and 28 VSS pins feed the micro-

processor in the 168-pin PGA package. The 208-pin

SQFP package includes 53 V

CC pins and 38 VSS pins.

Power and ground connections must be made to all

external V

CC and VSS pins of the microprocessors. On a

circuit board, all V

CC pins must connect to a VCC plane.

Likewise, all V

SS pins must connect to a common GND

plane.

The Am5X86 microprocessor family requires only 3.3 V

as input power. Unlike other 3-V processors, the

Am5X86 microprocessor family does not require a

VCC5 input of 5 V to indicate the presence of 5-V I/O

devices on the system motherboard. For socket com-

patibility, this pin is INC, allowing the Am5X86 CPU to

operate in 3-V sockets in systems that use 5-V I/O.

11.1.2 Power Decoupling Recommendations

Liberal decoupling capacitance should be placed near

the microprocessor. The microprocessor, driving its 32-

bit parallel address and data buses at high frequencies,

can cause transient power surges, particularly when

driving large capacitive loads.

Low inductance capacitors and interconnects are rec-

ommended for best high-frequency electrical perfor-

mance. Inductance can be reduced by shortening circuit

board traces between the microprocessor and the de-

coupling capacitors. Capacitors designed specifically

for use with PGA packages are commercially available.

11.1.3 Other Connection Recommendations

For reliable operation, always connect unused inputs to

an appropriate signal level. Active Low inputs should be

connected to V

CC through a pull-up resistor. Pull-ups in

the range of 20 K

Ω are recommended. Active High in-

puts should be connected to GND.

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