MPC755 datasheet(40/56 Pages) MOTOROLA

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MPC755 RISC Microprocessor Hardware Specifications, Rev. 6.1

Freescale Semiconductor

System Design Information

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.

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.

(L2)OVDD

OGND

Pad

Data

SW1

SW2

(L2)OVDD

MPC755 датащи(PDF) 40 Page - Motorola, Inc