TMS320DM647_09 Datasheet, PDF (49/181 Pages) Texas Instruments – Digital Media Processor

TMS320DM647
TMS320DM648
www.ti.com
SPRS372F – JANUARY 2010 – REVISED SEPTEMBER 2009
3.3 Pullup/Pulldown Resistors
Proper board design should specify that input pins to the device always be at a valid logic level and not
floating. This may be achieved via pullup/pulldown resistors. The device features internal pullup (IPU) and
internal pulldown (IPD) resistors on most pins to eliminate the need, unless otherwise noted, for external
pullup/pulldown resistors.
An external pullup/pulldown resistor must be used in the following situations:
• Boot and Configuration Pins: If the pin is both routed out and in high-impedance mode, an external
pullup/pulldown resistor must be used, even if the IPU/IPD matches the desired value/state.
• 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.
If the boot and configuration pins are both routed out and in high-impedance mode, it is recommended
that an external pullup/pulldown resistor be used. Although internal pullup/pulldown resistors exist on
these pins and they may match the desired configuration value, providing external connectivity can help
specify that valid logic levels are latched on these important boot configuration pins. In addition, applying
external pullup/pulldown resistors on the boot and configuration pins adds convenience to the user in
debugging and flexibility in switching operating modes.
Tips for choosing an external pullup/pulldown resistor:
• Select a resistor with the largest possible resistance
• Calculate the worst-case leakage current that flows through this external resistor. Worst-case leakage
current can be calculated by adding up all the leakage current at the pin—e.g., the input current (I_I)
from the device, and leakage current from the other device(s) to which this pin is connected.
• Specify that the voltage at the pin stays well within the low-/high-level input voltages (V_IL or V_IH) when
worst-case leakage current is flowing through this external resistor.
– To oppose an IPU and pull the signal to a logic low, the voltage at the pin must stay well below V_IL.
– To oppose an IPD and pull the signal to a logic high, the voltage at the pin must stay well above
V_IH.
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 complement the IPU/IPD on the boot and 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) , see
Section 5.3, Electrical Characteristics Over Recommended Ranges of Supply Voltage and Operating
Temperature.
For the internal pullup/pulldown resistors for all device pins, see the peripheral/system-specific terminal
functions table.
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Device Configuration
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