80960JA Datasheet, PDF (40/77 Pages) Intel Corporation – EMBEDDED 32-BIT MICROPROCESSOR

80960JA/JF/JD/JT 3.3 V Microprocessor
4.3
Connection Recommendations
For clean on-chip power distribution, V_CC and V_SS pins separately feed the device’s functional units.
Power and ground connections must be made to all 80960Jx power and ground pins. On the circuit board,
every V_CC pin should connect to a power plane and every V_SS pin should connect to a ground plane. Place
liberal decoupling capacitance near the 80960Jx, since the processor can cause transient power surges.
Pay special attention to the Test Reset (TRST) pin. It is essential that the JTAG Boundary Scan Test Access
Port (TAP) controller initializes to a known state whether it will be used or not. If the JTAG Boundary Scan
function will be used, connect a pulldown resistor between the TRST pin and V_SS. If the JTAG Boundary
Scan function will not be used (even for board-level testing), connect the TRST pin to V_SS.
Do not connect the TDI, TDO, and TCK pins if the TAP Controller will not be used.
Note: Pins identified as NC must not be connected in the system.
4.4
Figure 8.
VCC5 Pin Requirements (VDIFF)
In 3.3 V only systems where the 80960Jx input pins are driven from 3.3 V logic, connect the VCC5
pin directly to the 3.3 V V_CC plane.
In mixed voltage systems where the processor is powered by 3.3 V and interfaces with 5 V
components, VCC5 must be connected to 5 V. This allows proper 5 V tolerant buffer operation,
and prevents damage to the input pins. The voltage differential between the 80960Jx VCC5 pin and
its 3.3 V V_CC pins must not exceed 2.25 V. If this requirement is not met, current flow through the
pin may exceed the value at which the processor is damaged. Instances when the voltage can
exceed 2.25 V is during power up or power down, where one source reaches its level faster than the
other, briefly causing an excess voltage differential. Another instance is during steady-state
operation, where the differential voltage of the regulator (provided a regulator is used) cannot be
maintained within 2.25 V. Two methods are possible to prevent this from happening:
• Use a regulator that is designed to prevent the voltage differential from exceeding 2.25 V, or,
• As shown in Figure 8, place a 100 Ω resistor in series with the VCC5 pin to limit the current
through VCC5.
VCC5 Current-Limiting Resistor
+5 V (±0.25 V)
VCC5 Pin
100 Ω
(±5%, 0.5 W)
Table 20.
If the regulator cannot prevent the 2.25 V differential, the addition of the resistor is a simple and
reliable method for limiting current. The resistor can also prevent damage in the case of a power
failure, where the 5 V supply remains on and the 3.3 V supply goes to zero.
VDIFF Parameters
Symbol Parameter Min Max Units
Notes
VDIFF
VCC5-V_CC
Difference
VCC5 input should not exceed V_CC by more than 2.25 V
2.25
V during power-up and power-down, or during
steady-state operation.
40
Advance Information Datasheet