Intel 80960HD - Manual (Page 38)

80960HA/HD/HT
38 Datasheet
4.3 Recommended Connections
Power and ground connections must be made to multiple V
CC
and V
SS
(GND) pins. Every
80960Hx-based circuit board should include power (V
CC
) and ground (V
SS
) planes for power
distribution. Every V
CC
pin must be connected to the power plane; every V
SS
pin must be
connected to the ground plane. Pins identified as “NC” —no connect pins—must not be connected
in the system.
Liberal decoupling capacitance should be placed near the 80960Hx. The processor may cause
transient power surges when its output buffers transition, particularly when connected to large
capacitive loads.
Low inductance capacitors and interconnects are recommended for best high-frequency electrical
performance. Inductance may be reduced by shortening the board traces between the processor and
decoupling capacitors as much as possible. Capacitors specifically designed for PGA packages
offer the lowest possible inductance.
For reliable operation, always connect unused inputs to an appropriate signal level. In particular,
any unused interrupt (XINT7:0, NMI) input should be connected to V
CC
through a pull-up resistor,
as should BTERM when not used. Pull-up resistors should be in the range of 20 K
Ω
for each pin
tied high. When READY or HOLD are not used, the unused input should be connected to ground.
N.C. pins must always remain unconnected.
4.4 VCC5 Pin Requirements (V
DIFF
)
In mixed-voltage systems that drive 80960Hx processor inputs in excess of 3.3 V, the VCC5 pin
must be connected to the system’s 5 V supply. To limit current flow into the VCC5 pin, there is a
limit to the voltage differential between the VCC5 pin and the other V
CC
pins. The voltage
differential between the 80960Hx VCC5 pin and its 3.3 V V
CC
pins should never exceed 2.25 V.
This limit applies to power-up, power-down, and steady-state operation. Table 21 outlines this
requirement.
Meeting this requirement ensures proper operation and ensures that the current draw into the VCC5
pin does not exceed the I
CC5
specification.
When the voltage difference requirements cannot be met due to system design limitations, an
alternate solution may be employed. As shown in
Figure 7 , a minimum of 100
Ω
series resistor
may be used to limit the current into the VCC5 pin. This resistor ensures that current drawn by the
VCC5 pin does not exceed the maximum rating for this pin.
This resistor is not necessary in systems that may ensure the V
DIFF
specification.
In 3.3 V-only systems and systems that drive 80960Hx pins from 3.3 V logic, connect the VCC5
pin directly to the 3.3 V V
CC
plane.
Figure 7. VCC5 Current-Limiting Resistor
+5 V (±0.25 V) VCC5 Pin
100 Ω
(±5%, 0.5 W)