Preliminary
PowerPC 750FL RISC Microprocessor
5.4 Output Buffer dc Impedance
The 750FL 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
DD
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
750flds60.fm.6.0
April 27, 2007
System Design Information
Page 49 of 65
IBM [ IBM ]
