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Datasheet

Electrical Specifications

2.5

Reserved, Unused, and TESTHI Pins

All RESERVED pins must remain unconnected. Connection of these pins to VCC, VSS, or to any

other signal (including each other) can result in component malfunction or incompatibility with

future processors. See Chapter 4 for a pin listing of the processor and the location of all

RESERVED pins.

For reliable operation, always connect unused inputs or bidirectional signals to an appropriate

signal level. In a system-level design, on-die termination has been included on the Celeron D

processor to allow signals to be terminated within the processor silicon. Most unused GTL+ inputs

should be left as no connects, as GTL+ termination is provided on the processor silicon. However,

see Table 2-4 for details on GTL+ signals that do not include on-die termination. Unused active

high inputs should be connected through a resistor to ground (VSS). Unused outputs can be left

unconnected; however, this may interfere with some test access port (TAP) functions, complicate

debug probing, and prevent boundary scan testing. A resistor must be used when tying

bidirectional signals to power or ground. When tying any signal to power or ground, a resistor will

also allow for system testability. For unused GTL+ input or I/O signals, use pull-up resistors of the

same value as the on-die termination resistors (RTT). See Table 2-18.

TAP, Asynchronous GTL+ inputs, and Asynchronous GTL+ outputs do not include on-die

termination. Inputs and used outputs must be terminated on the system board. Unused outputs may

be terminated on the system board or left unconnected. Note that leaving unused outputs

unterminated may interfere with some TAP functions, complicate debug probing, and prevent

boundary scan testing.

The TESTHI pins must be tied to the processor VCC using a matched resistor, where a matched

resistor has a resistance value within ±20% of the impedance of the board transmission line traces.

For example, if the trace impedance is 60

Ω, then a value between 48 Ω and 72 Ω is required.

The TESTHI pins may use individual pull-up resistors or be grouped together as detailed below. A

matched resistor must be used for each group:

• TESTHI[1:0]

• TESTHI[7:2]

• TESTHI8 – cannot be grouped with other TESTHI signals

• TESTHI9 – cannot be grouped with other TESTHI signals

• TESTHI10 – cannot be grouped with other TESTHI signals

• TESTHI11 – cannot be grouped with other TESTHI signals

• TESTHI12 – cannot be grouped with other TESTHI signals

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