46
0832F–HIREL–02/07
PC7410
e2v semiconductors SAS 2007
14.4
Connection Recommendations
To ensure reliable operation, it is highly recommended to connect unused inputs to an appropriate signal
level. Unused active low inputs should be tied to OV
DD. Unused active high inputs should be connected
to GND. All NC (no-connect) signals must remain unconnected.
Power and ground connections must be made to all external V
DD, OVDD, L2OVDD, and GND pins of the
PC7410.
See “L2 Clock AC Specifications” on page 24 for a discussion of the L2SYNC_OUT and L2SYNC_IN
signals.
14.5
Output Buffer DC Impedance
The PC7410 60x and L2 I/O drivers are characterized over process, voltage and temperature. To mea-
sure Z
0, an external resistor is connected from the chip pad to OVDD or GND. Then the value of each
resistor is varied until the pad voltage is OV
DD/2 (see Figure 14-4).
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
N is trimmed until the voltage at the
pad equals OV
DD/2. RN then becomes the resistance of the pull-down devices. When data is held high,
SW1 is closed (SW2 is open), and R
P is trimmed until the voltage at the pad equals OVDD/2. RP then
becomes the resistance of the pull-up devices. R
P and RN are designed to be close to each other in
value. Then Z
0 = (RP + RN)/2.
Figure 14-4 describes the driver impedance measurement circuit described above.
Figure 14-4. Driver Impedance Measurement Circuit
Alternately, the following is another method to determine the output impedance of the PC7410. A voltage
source, Vforce, is connected to the output of the PC7410, as in Figure 14-4. Data is held low, the voltage
source is set to a value that is equal to (L2)OV
DD/2, and the current sourced by Vforce is measured. The
voltage drop across the pull-down device, which is equal to (L2)OV
DD/2, is divided by the measured cur-
rent to determine the output impedance of the pull-down device, RN. Similarly, the impedance of the pull-
up device is determined by dividing the voltage drop of the pull-up, (L2)OV
DD/2, by the current sank by
the pull-up when the data is high and Vforce is equal to (L2)OV
DD/2. This method can be employed with
either empirical data from a test setup or with data from simulation models, such as IBIS
.
OVDD
OGND
SW2
SW1
RN
RP
Pad
Data
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