TMS320C5535_12 Datasheet, PDF (77/156 Pages) Texas Instruments – TMS320C5535, C5534, C5533,C5532 FIXED-POINT DIGITAL SIGNAL PROCESSORS

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4.8 Debugging Considerations
TMS320C5535
TMS320C5534, TMS320C5533, TMS320C5532
SPRS737B – AUGUST 2011 – REVISED MARCH 2012
4.8.1 Pullup/Pulldown Resistors
Proper board design should ensure that input pins to the device DSP always be at a valid logic level and
not floating. This may be achieved via pullup/pulldown resistors. The DSP features internal pullup (IPU)
and internal pulldown (IPD) resistors on many pins, including all GPIO pins, to eliminate the need, unless
otherwise noted, for external pullup/pulldown resistors.
An external pullup/pulldown resistor may need to be used in the following situations:
• Configuration Pins: An external pullup/pulldown resistor is recommended to set the desired value/state
(see the configuration pins listed in Table 4-8, Default Functions Affected by Device Configuration
Pins). Note that some configuration pins must be connected directly to ground or to a specific supply
voltage.
• 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 configuration pins (listed in Table 4-8, Default Functions Affected by Device Configuration Pins), if
they are both routed out and high-impedance state (not driven), it is strongly recommended that an
external pullup/pulldown resistor be implemented. In addition, applying external pullup/pulldown resistors
on the configuration pins adds convenience to the user in debugging and flexibility in switching operating
modes.
When an external pullup or pulldown resistor is used on a pin, the pin’s internal pullup or pulldown resistor
must be disabled through the Pullup/Pulldown Inhibit Registers (PDINHIBR1/2/3) [1C17h, 1C18h, and
1C19h, respectively] to minimize power consumption.
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 (IPU/IPD) 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-kΩ 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-kΩ resistor can be used to compliment the IPU/IPD on the 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 device DSP, see Section 5.3, Electrical Characteristics Over Recommended Ranges of Supply
Voltage and Operating Temperature.
Copyright © 2011–2012, Texas Instruments Incorporated
Device Configuration
77
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