MPC8309EC datasheet(78/82 Pages) FREESCALE

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MPC8309 PowerQUICC II Pro Integrated Communications Processor Family Hardware Specifications, Rev. 0

Freescale Semiconductor

System Design Information

to minimize inductance. Suggested bulk capacitors—100 to 330 µF (AVX TPS tantalum or Sanyo

OSCON).

25.4

Output Buffer DC Impedance

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 OVDD

or GND. Then, the value of each resistor is varied until the pad voltage is OVDD/2 (see Figure 47). 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

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 47. 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.

OVDD

OGND

Pad

Data

SW1

SW2

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