Texas Instruments TMS320C6454 - Manual (Page 68)

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PRODUCT PREVIEW
TMS320C6454
Fixed-Point Digital Signal Processor
SPRS311A – APRIL 2006 – REVISED DECEMBER 2006
•
Other Input Pins: If the IPU/IPD does not match the desired value/state, use an external
pullup/pulldown resistor to pull the signal to the opposite rail.
For the device configuration pins (listed in Table 3-1 ), if they are both routed out and 3-stated (not driven),
it is strongly recommended that an external pullup/pulldown resistor be implemented. Although, internal
pullup/pulldown resistors exist on these pins and they may match the desired configuration value,
providing external connectivity can help ensure that valid logic levels are latched on these device
configuration pins. In addition, applying external pullup/pulldown resistors on the device configuration pins
adds convenience to the user in debugging and flexibility in switching operating modes.
Tips for choosing an external pullup/pulldown resistor:
•
Consider the total amount of current that may pass through the pullup or pulldown resistor. Make sure
to include the leakage currents of all the devices connected to the net, as well as any internal pullup or
pulldown resistors.
•
Decide a target value for the net. For a pulldown resistor, this should be below the lowest V
IL
level of
all inputs connected to the net. For a pullup resistor, this should be above the highest V
IH
level of all
inputs on the net. A reasonable choice would be to target the V
OL
or V
OH
levels for the logic family of
the limiting device; which, by definition, have margin to the V
IL
and V
IH
levels.
•
Select a pullup/pulldown resistor with the largest possible value; but, which can still ensure that the net
will reach the target pulled value when maximum current from all devices on the net is flowing through
the resistor. The current to be considered includes leakage current plus, any other internal and
external pullup/pulldown resistors on the net.
•
For bidirectional nets, there is an additional consideration which sets a lower limit on the resistance
value of the external resistor. Verify that the resistance is small enough that the weakest output buffer
can drive the net to the opposite logic level (including margin).
•
Remember to include tolerances when selecting the resistor value.
•
For pullup resistors, also remember to include tolerances on the DV
DD
rail.
For most systems, a 1-k
Ω
resistor can be used to oppose the IPU/IPD while meeting the above criteria.
Users should confirm this resistor value is correct for their specific application.
For most systems, a 20-k
Ω
resistor can be used to compliment the IPU/IPD on the device configuration
pins while meeting the above criteria. Users should confirm this resistor value is correct for their specific
application.
For more detailed information on input current (I
I
), and the low-/high-level input voltages (V
IL
and V
IH
) for
the C6454 device, see Section 6.3 , Electrical Characteristics Over Recommended Ranges of Supply
Voltage and Operating Case Temperature.
To determine which pins on the C6454 device include internal pullup/pulldown resistors, see Table 2-3 ,
Terminal Functions.
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