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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 I 2 C).
To measure Z 0 for the single-ended drivers, an external resistor is connected from the chip pad to OV DD
or GND. Then, the value of each resistor is varied until the pad voltage is OV DD /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 R P is trimmed until the voltage at the pad equals
OV DD /2. R P then becomes the resistance of the pull-up devices. R P and R N are designed to be close to each
other in value. Then, Z 0 = (R P + R N )/2.
OV DD
R N
SW2
Data
Pad
SW1
R P
OGND
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 V 1 = R source × I source . Second, the output voltage is measured
while driving logic 1 with an external precision differential termination resistor of value R term . The
measured voltage is V 2 = (1/(1/R 1 + 1/R 2 )) × I source . Solving for the output impedance gives
R source = R term × (V 1 /V 2 – 1). The drive current is then I source = V 1 /R source .
MPC8309 PowerQUICC II Pro Integrated Communications Processor Family Hardware Specifications, Rev. 3
Freescale Semiconductor
77
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相关PDF资料
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