MPC7410 datasheet(38/56 Pages) FREESCALE

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MPC7410 RISC Microprocessor Hardware Specifications, Rev. 6.1

Freescale Semiconductor

System Design Information

8.4 Connection Recommendations

To ensure reliable operation, it is highly recommended to connect unused inputs to an appropriate signal level

through a resistor. Unused active low inputs should be tied to OVDD. Unused active high inputs should be connected

to GND. All NC (no connect) signals must remain unconnected.

Power and ground connections must be made to all external VDD, OVDD, L2OVDD, and GND pins of the MPC7410.

Note that power must be supplied to L2OVDD even if the L2 interface of the MPC7410 will not be used; the

remainder of the L2 interface may be left unterminated.

8.5 Output Buffer DC Impedance

The MPC7410 60x and L2 I/O drivers are characterized over process, voltage, and temperature. To measure Z0, an

external resistor is connected from the chip pad to OVDD or GND. Then, the value of each resistor is varied until

the pad voltage is OVDD/2 (see Figure 23).

The output impedance is the average of two components, the resistances of the pull-up and pull-down devices. When

data is held low, SW2 is closed (SW1 is open), and RN is trimmed until the voltage at the pad equals (L2)OVDD/2.

RN then becomes the resistance of the pull-down devices. When data is held high, SW1 is closed (SW2 is open), and

RP is trimmed until the voltage at the pad equals (L2)OVDD/2. RP then becomes the resistance of the pull-up devices.

RP and RN are designed to be close to each other in value. Then, Z0 = (RP + RN)/2.

Figure 23 describes the driver impedance measurement circuit described above.

Figure 23. Driver Impedance Measurement Circuit

Alternately, the following is another method to determine the output impedance of the MPC7410. A voltage source,

Vforce, is connected to the output of the MPC7410, as in Figure 24. Data is held low, the voltage source is set to a

value that is equal to (L2)OVDD/2, and the current sourced by Vforce is measured. The voltage drop across the

pull-down device, which is equal to (L2)OVDD/2, is divided by the measured current to determine the output

impedance of the pull-down device, RN. Similarly, the impedance of the pull-up device is determined by dividing

the voltage drop of the pull-up, (L2)OVDD/2, by the current sank by the pull-up when the data is high and Vforce is

equal to (L2)OVDD/2. This method can be employed with either empirical data from a test setup or with data from

simulation models, such as IBIS.

OVDD

OGND

Pad

Data

SW1

SW2

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