AM486DE2 datasheet(39/52 Pages) AMD

39 / 52 page

Am486DE2 Microprocessor

ELECTRICAL DATA

The following sections describe recommended electri-

cal connections for the Am486DE2 microprocessor, and

electrical specifications.

Power Connections

The Am486DE2 microprocessor has modest power re-

quirements. However, the high clock-frequency output

buffers can cause power surges as multiple output buff-

ers drive new signal levels simultaneously. For clean,

on-chip power distribution at high frequency, 23 VCC pins

and 28 VSS pins feed the microprocessor in the 168-pin

PGA package. The 208-lead SQFP package includes

53 VCC pins and 38 VSS pins.

Power and ground connections must be made to all ex-

ternal 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 Am486DE2 microprocessor requires only 3.3 V as

input power. Unlike other 3-V 486 processors, the

Am486DE2 microprocessor does

not require a V

CC5 in-

put of 5 V to indicate the presence of 5-V I/O devices on

the system motherboard. For socket compatibility, this

pin is INC (Internal No Connect), allowing the

Am486DE2 CPU to operate in 3-V sockets in systems

that use 5-V I/O.

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.

Other Connection Recommendations

For reliable operation, always connect unused inputs to

an appropriate signal level. Active Low inputs should be

connected to VCC 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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