TNETV2685FIBZUT9 datasheet(49/191 Pages) TI1

49 / 191 page

TMS320DM647

TMS320DM648

www.ti.com

SPRS372H – MAY 2007 – REVISED APRIL 2012

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 (II)

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 (VIL or VIH) 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 VIL.

–

To oppose an IPD and pull the signal to a logic high, the voltage at the pin must stay well above

VIH.

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 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 (II), and the low-/high-level input voltages (VIL and VIH) , 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.

Device Configuration

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