PowerPC 750 SCM RISC Microprocessor
PID8p-750
Preliminary Copy
L2CLKOUTB network could also be used depending on the loading, frequency, and number of SRAMs.
Output Buffer DC Impedance
The PID8p-750 60x and L2 I/O drivers were characterized over process, voltage, and temperature. To mea-
sure Z , an external resistor is connected to the chip pad, either to OVDD or GND. Then the value of such
0
resistor is varied until the pad voltage is OVDD/2; see Figure 17, “Driver Impedance Measurement,” below.
The output impedance is actually the average of two components, the resistances of the pull-up and pull-
down devices. When Data is held low, SW1 is closed (SW2 is open), and RN is trimmed until Pad = OVDD/2. RN
then becomes the resistance of the pull-down devices. When Data is held high, SW2 is closed (SW1 is open),
and RP is trimmed until Pad = OVDD/2. RP then becomes the resistance of the pull-up devices. With a properly
designed driver RP and RN are close to each other in value, then Z0 = (RP + RN)/2.
Figure 17. Driver Impedance Measurement
OVDD
RN
SW2
Pad
Data
SW1
RP
GND
The following table summarizes the impedance a board designer would design to for a typical process. These
values were derived by simulation at 65°C. As the process improves, the output impedance will be lower by
several ohms than this typical value.
Impedance Characteristics
VDD = 1.9VDC, L2OVDD=OVDD = 3.3VDC, T = 65°C
J
Process
Typical
60x
43
L2
38
Symbol
Z0
Unit
Ω
Page 32
Version 2.0
Datasheet
9/30/99
IBM [ IBM ]
