MPC745 Motorola Semiconductor Products, MPC745 Datasheet - Page 37

• Manufacturer Part Number: MPC745
• Description: Host Processor
• Manufacturer: Motorola Semiconductor Products

Power and ground connections must be made to all external V
MPC755. Note that power must be supplied to L2OV
used; it is recommended to connect L2OV
(This requirement does not apply to the MPC745 since it has neither an L2 interface nor L2OV
1.8.5
The MPC755 60x and L2 I/O drivers are characterized over process, voltage, and temperature. To measure
Z
resistor is varied until the pad voltage is (L2)OV
The output impedance is the average of two components, the resistances of the pull-up and pull-down
devices. When data is held low, SW2 is closed (SW1 is open), and R
pad equals (L2)OV
SW1 is closed (SW2 is open), and R
becomes the resistance of the pull-up devices.
Figure 22 describes the driver impedance measurement circuit described above.
Alternately, the following is another method to determine the output impedance of the MPC755. A voltage
source, V
source is set to a value that is equal to (L2)OV
voltage drop across the pull-down device, which is equal to (L2)OV
to determine the output impedance of the pull-down device, R
device is determined by dividing the voltage drop of the pull-up, (L2)OV
pull-up when the data is high and V
empirical data from a test setup or with data from simulation models, such as IBIS.
R
Figure 23 describes the alternate driver impedance measurement circuit.
MOTOROLA
0
P
, an external resistor is connected from the chip pad to (L2)OV
and R
N
force
are designed to be close to each other in value. Then Z
Output Buffer DC Impedance
, is connected to the output of the MPC755 as shown in Figure 23. Data is held low, the voltage
DD
/2. R
MPC755 RISC Microprocessor Hardware Specifications
Figure 22. Driver Impedance Measurement Circuit
N
Freescale Semiconductor, Inc.
then becomes the resistance of the pull-down devices. When data is held high,
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Data
force
P
is trimmed until the voltage at the pad equals (L2)OV
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is equal to (L2)OV
DD
to OV
(L2)OV
DD
DD
DD
/2 and the current sourced by V
/2 (see Figure 22).
DD
DD
and L2VSEL to BVSEL if the L2 interface is unused.
even if the L2 interface of the MPC755 will not be
DD
Pad
R
R
(L2)OV
/2. This method can be employed with either
N
P
OGND
DD
N
. Similarly, the impedance of the pull-up
, OV
0
DD
DD
= (R
DD
/2, is divided by the measured current
N
SW2
SW1
DD
P
or GND. Then, the value of each
is trimmed until the voltage at the
, L2OV
+ R
DD
/2, by the current sank by the
System Design Information
N
)/2.
DD
, and GND pins of the
force
is measured. The
DD
DD
/2. R
pins.)
P
then
37