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PRODUCT SPECIFICATION
RC7106
5
Functional Description
I/O Pin Operation
Dual Purpose I/O pins such as pin 8 FS0/PCI_F, act as a
logic input upon power up. This allows the determination of
assigned device function. For this example, FS0 along with
the other three select pins will determine the clock frequen-
cies as shown in the table. A short time after power up, the
logic state is latched and the pin becomes a clock output pin.
In this case, pin 8 becomes a PCI free-running clock output.
This feature reduces device pin count by combining clock
outputs with input select pins.
An external 10kohm “strapping” resistor is connected
between the I/O pin and VDD or VSS (ground). A connec-
tion to ground sets a “0” bit and a connection to VDD sets a
“1” bit. See Figure 1.
Upon power up, the first 2mS of operation is used for input
logic selection. The clock output pins are tri-stated, allowing
the output strapping resistor on the I/O pin to pull the pin and
its associated capacitive clock load to either a logic high or
low state. At the end of the 2mS period, the established logic
“0” or “1” condition of the I/O pin is then latched. Next the
output buffer is enabled which converts the I/O pin into an
operating clock output. The 2mS timer is started when VDD
reaches 2.0V. The input bits can only be reset by turning the
VDD off and then back on again.
It should be noted that the strapping resistors have no signifi-
cant effect on clock output signal integrity. The drive imped-
ance of clock output is 20 ohms (nominal) which is
minimally affected by the 10kohm strap to ground or VDD.
As with the series termination resistor, the output strapping
resistor should be placed as close to the I/O pin as possible in
order to keep the interconnecting trace short. The trace from
the resistor to ground or VDD should be kept less than two
inches in length to prevent system noise coupling during
input logic sampling.
Figure 1. Input Logic Selection through Resistor Load Option
VDD
Series
Terminating
Resistor
10K Load
Option 1
10K Load
Option 0
Clock Load
RC7106