Datasheet
IBM PowerPC 750GX RISC Microprocessor
DD1.X
5.6 Output Buffer DC Impedance
The 750GX 60x drivers were characterized over various process, voltage, and temperature conditions. To
measure driver impedance, an external resistor is connected to the chip pad, either to OVDD or GND. Then
the value of such resistor is varied until the pad voltage is OVDD/2 (see Figure 5-5).
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 high, SW1 is closed (SW2 is open), and RN is trimmed until
Pad = OVDD/2; RN then becomes the resistance of the pull-up devices. When Data is held low, SW2 is closed
(SW1 is open), and RP is trimmed until Pad = OVDD/2; RP then becomes the resistance of the pull-down
devices. With a properly designed driver, RP and RN are close to each other in value; then driver impedance
equals (RP + RN)/2.
Figure 5-5. Driver Impedance Measurement
OVDD
RP
SW2
Pad
Data
SW1
RN
GND
750GX_ds_body.fm SA14-2765-02
September 2, 2005
System Design Information
Page 55 of 73
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