Datasheet

MPC8308 PowerQUICC II Pro Processor Hardware Specification, Rev. 3
Freescale Semiconductor 79
System Design Information
Figure 55. Driver Impedance Measurement
The value of this resistance and the strength of the driver’s current source can be found by making two
measurements. First, the output voltage is measured while driving logic 1 without an external differential
termination resistor. The measured voltage is V
1
= R
source
 I
source
. Second, the output voltage is measured
while driving logic 1 with an external precision differential termination resistor of value R
term
. The
measured voltage is V
2
=(1/(1/R
1
+1/R
2
))  I
source
. Solving for the output impedance gives R
source
=
R
term
 (V
1
/V
2
– 1). The drive current is then I
source
=V
1
/R
source
.
This table summarizes the signal impedance targets. The driver impedance are targeted at minimum V
DD
,
nominal NV
DD
, 105C.
23.6 Configuration Pin Muxing
The device provides the user with power-on configuration options which can be set through the use of
external pull-up or pull-down resistors of 4.7 K on certain output pins (see customer visible
configuration pins). These pins are generally used as output only pins in normal operation.
While PORESET is asserted however, these pins are treated as inputs. The value presented on these pins
while PORESET is asserted, is latched when PORESET deasserts, at which time the input receiver is
disabled and the I/O circuit takes on its normal function. Careful board layout with stubless connections
to these pull-up/pull-down resistors coupled with the large value of the pull-up/pull-down resistor should
minimize the disruption of signal quality or speed for output pins thus configured.
Table 60. Impedance Characteristics
Impedance
Local Bus, Ethernet, DUART, Control,
Configuration, Power Management
DDR DRAM Symbol Unit
R
N
42 Target 20 Target Z
0

R
P
42 Target 20 Target Z
0

Note: Nominal supply voltages. See Tabl e 2, T
j
= 105C.
NV
DD
VSS
R
P
R
N
Pad
Data
SW1
SW2