System Design Information
OV /2. R then becomes the resistance of the pull-up devices. R and R are designed to be close to each
DD
P
P
N
other in value. Then, Z = (R + R ) ÷ 2.
0
P
N
OVDD
RN
SW2
SW1
Pad
RP
Data
OGND
Figure 44. Driver Impedance Measurement
Two measurements give the value of this resistance and the strength of the driver current source. First, the
output voltage is measured while driving logic 1 without an external differential termination resistor. The
measured voltage is V = R
× I
. Second, the output voltage is measured while driving logic 1
1
source
source
with an external precision differential termination resistor of value R . The measured voltage is
term
V = (1 ÷ (1/R + 1/R )) × I
. Solving for the output impedance gives R
= R
× (V ÷ V – 1).
2
1
2
source
source
term 1 2
The drive current is then I
= V ÷ R
.
source
1
source
Table 69 summarizes the signal impedance targets. The driver impedance are targeted at minimum V
,
DD
nominal OV , 105°C.
DD
Table 69. Impedance Characteristics
Local Bus, Ethernet,
PCI Signals
(Not Including PCI
Output Clocks)
PCI Output Clocks
(Including
PCI_SYNC_OUT)
DUART, Control,
Configuration, Power
Management
Impedance
DDR DRAM Symbol
Unit
R
R
42 Target
42 Target
NA
25 Target
25 Target
NA
42 Target
42 Target
NA
20 Target
20 Target
NA
Z0
Z0
W
W
W
N
P
Differential
ZDIFF
Note: Nominal supply voltages. See Table 1, Tj = 105°C.
21.6 Configuration Pin Multiplexing
The MPC8347EA power-on configuration options can be set through external pull-up or pull-down
resistors of 4.7 kΩ on certain output pins (see the customer-visible configuration pins). These pins are used
as output only pins in normal operation.
MPC8347EA PowerQUICC II Pro Integrated Host Processor Hardware Specifications, Rev. 12
Freescale Semiconductor
93
FREESCALE [ Freescale ]
