MPC755 datasheet(40/56 Pages) NXP

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MPC755 RISC Microprocessor Hardware Specifications, Rev. 8

Freescale Semiconductor

System Design Information

Figure 22 describes the driver impedance measurement circuit described above.

Figure 22. Driver Impedance Measurement Circuit

Alternately, the following is another method to determine the output impedance of the MPC755. A voltage

source, Vforce, is connected to the output of the MPC755 as shown in Figure 23. 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.

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

Figure 23 describes the alternate driver impedance measurement circuit.

Figure 23. Alternate Driver Impedance Measurement Circuit

(L2)OVDD

OGND

Pad

Data

SW1

SW2

(L2)OVDD

BGA

Data

Vforce

OGND

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