TMS320C5517_16 Datasheet, PDF (151/197 Pages) Texas Instruments – TMS320C5517 Fixed-Point Digital Signal Processor

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TMS320C5517
SPRS727C – AUGUST 2012 – REVISED APRIL 2014
5.7.20 Emulation and Debug
5.7.20.1 Debugging Considerations
5.7.20.1.1 Pullup and Pulldown Resistors
Proper board design should ensure that input pins to the DSP are always at a valid logic level and not
floating. This may be achieved via pullup and pulldown resistors. The DSP features internal pullup (IPU)
and internal pulldown (IPD) resistors on many pins to eliminate the need, unless otherwise noted, for
external pullup and pulldown resistors.
An external pullup and pulldown resistor may need to be used in the following situations:
• Configuration Pins: An external pullup and pulldown resistor is recommended to set the desired value
or state (see the configuration pins listed in Table 5-5, Default Functions Affected by Device
Configuration Pins). Note that some configuration pins must be connected directly to ground or to a
specific supply voltage.
• Input Pins (I, I/O, I/O/Z): They are required to be driven at all times. To achieve the lowest power, input
pins must not be allowed to float. When they are configured as input or tri-stated, and not driven to a
known state, they may cause an excessive IO-supply current. Prevent this current by externally
terminating it or enabling IPD and IPU, if applicable.
• Other Input Pins: If the IPU and IPD does not match the desired value or state, use an external pullup
and pulldown resistor to pull the signal to the opposite rail.
For the configuration pins (listed in Table 5-5, Default Functions Affected by Device Configuration Pins), if
they are both routed out and 3-stated (not driven), it is strongly recommended that an external pullup and
pulldown resistor be implemented. In addition, applying external pullup and 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
should be disabled through the Pullup and Pulldown Inhibit Registers (PUDINHIBR1, 2, 3, 4, 5, 6, and 7)
to minimize power consumption.
Tips for choosing an external pullup and 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 and 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 and 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 and 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 and 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 and IPD on the configuration pins
while meeting the above criteria. Users should confirm this resistor value is correct for their specific
application.
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