DD 2.X
PowerPC 750FX RISC Microprocessor
Preliminary
5.4 Output Buffer DC Impedance
The 750FX 60x drivers were characterized over various process, voltage, and temperature conditions. To
measure Z , an external resistor is connected to the chip pad, either to OV or GND. Then the value of such
0
DD
resistor is varied until the pad voltage is OV /2 (see Figure 5-6).
DD
The output impedance is actually the average of two resistances: the resistance of the pull-up and the resis-
tance of pull-down devices. When Data is held low, SW1 is closed (SW2 is open), and R is trimmed until
N
Pad = OV /2. R then becomes the resistance of the pull-down devices. When Data is held high, SW2 is
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N
closed (SW1 is open), and R is trimmed until Pad = OV /2. R then becomes the resistance of the pull-up
P
DD
P
devices. With a properly designed driver R and R are close to each other in value, then Z = (R + R )/2.
P
N
0
P
N
Figure 5-6. Driver Impedance Measurement
OVDD
RP
SW2
Pad
Data
SW1
RN
GND
5. System Design Information
Page 42 of 63
Body_750FX_DS_DD2.X.fm.2.0
June 9, 2003
IBM [ IBM ]
