System Design Information
have a low ESR (equivalent series resistance) rating to ensure the quick response time. They should also
be connected to the power and ground planes through two vias to minimize inductance. Suggested bulk
capacitors are 100–330 µF (AVX TPS tantalum or Sanyo OSCON).
21.4 Connection Recommendations
To ensure reliable operation, connect unused inputs to an appropriate signal level. Unused active low
inputs should be tied to OV , GV , or LV as required. Unused active high inputs should be
DD
DD
DD
connected to GND. All NC (no-connect) signals must remain unconnected.
Power and ground connections must be made to all external V , GV , LV , OV , and GND pins of
DD
DD
DD
DD
the MPC8349EA.
21.5 Output Buffer DC Impedance
The MPC8349EA drivers are characterized over process, voltage, and temperature. For all buses, the
2
driver is a push-pull single-ended driver type (open drain for I C).
To measure Z for the single-ended drivers, an external resistor is connected from the chip pad to OV
0
DD
or GND. Then the value of each resistor is varied until the pad voltage is OV /2 (see Figure 43). The
DD
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 is trimmed until the voltage at the pad equals
P
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 43. 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
with an external precision differential termination resistor of value R . The measured voltage is
× I
. Second, the output voltage is measured while driving logic 1
1
source
source
term
MPC8349EA PowerQUICC II Pro Integrated Host Processor Hardware Specifications, Rev. 13
Freescale Semiconductor
81
FREESCALE [ Freescale ]
