mpc8309 Freescale Semiconductor, Inc, mpc8309 Datasheet - Page 78

• Manufacturer Part Number: mpc8309
• Description: Powerquicc Ii Pro Integrated Communications Processor Family Hardware Specifications
• Manufacturer: Freescale Semiconductor, Inc

System Design Information
to minimize inductance. Suggested bulk capacitors—100 to 330 µF (AVX TPS tantalum or Sanyo
OSCON).
25.4
For all buses, the driver is a push-pull single-ended driver type (open drain for I
To measure Z
or GND. Then, the value of each resistor is varied until the pad voltage is OV
output impedance is the average of two components, the resistances of the pull-up and pull-down devices.
When data is held high, SW1 is closed (SW2 is open) and R
OV
other in value. Then, Z
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
while driving logic 1 with an external precision differential termination resistor of value R
measured voltage is V
R
78
source
DD
MPC8309 PowerQUICC II Pro Integrated Communications Processor Family Hardware Specifications, Rev. 0
/2. R
= R
Output Buffer DC Impedance
term
P
then becomes the resistance of the pull-up devices. R
0
for the single-ended drivers, an external resistor is connected from the chip pad to OV
 (V
1
/V
2
0
2
= (1/(1/R
= (R
– 1). The drive current is then I
P
+ R
Figure 47. Driver Impedance Measurement
1
Data
N
+ 1/R
)/2.
2
))  I
1
source
= R
source
. Solving for the output impedance gives
source
 I
P
Pad
R
source
R
is trimmed until the voltage at the pad equals
N
P
OGND
OV
= V
DD
P
. Second, the output voltage is measured
1
and R
/R
SW2
SW1
source
N
are designed to be close to each
.
DD
2
/2 (see
C).
Freescale Semiconductor
Figure
term
. The
47). The
DD