MPC8315CVRADDA datasheet(102/106 Pages) FREESCALE

102 / 106 page

MPC8315E PowerQUICC II Pro Processor Hardware Specifications, Rev. 2

102

Freescale Semiconductor

System Design Information

Power and ground connections must be made to all external VDD, GVDD, LVDD, NVDD, and GND pins

of the device.

26.5

Output Buffer DC Impedance

The MPC8315E drivers are characterized over process, voltage, and temperature. For all buses, the driver

is a push-pull single-ended driver type (open drain for I2C).

To measure Z0 for the single-ended drivers, an external resistor is connected from the chip pad to NVDD

or GND. Then, the value of each resistor is varied until the pad voltage is NVDD/2 (see Figure 64). The

output impedance is the average of two components, the resistances of the pull-up and 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

NVDD/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 64. Driver Impedance Measurement

The value of this resistance and the strength of the driver’s current source can be found by making two

measurements. First, the output voltage is measured while driving logic 1 without an external differential

termination resistor. The measured voltage is V1 = Rsource  Isource. Second, the output voltage is measured

while driving logic 1 with an external precision differential termination resistor of value Rterm. The

measured voltage is V2 =(1/(1/R1 +1/R2))  Isource. Solving for the output impedance gives Rsource =

Rterm  (V1/V2 – 1). The drive current is then Isource =V1/Rsource.

NVDD

OGND

Pad

Data

SW1

SW2

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