TNETV2664FIBZWT datasheet(121/249 Pages) TI1

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TMS320C6424

www.ti.com

SPRS347D – MARCH 2007 – REVISED DECEMBER 2009

3.9

Debugging Considerations

3.9.1

Pullup/Pulldown Resistors

Proper board design should ensure that input pins to the C642x DSP device always be at a valid logic

level and not floating. This may be achieved via pullup/pulldown resistors. The C642x DSP 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 needs to be used in the following situations:

•

Boot and Configuration Pins: If the pin is both routed out and 3-stated (not driven), an external

pullup/pulldown resistor is strongly recommended, 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.

•

EMIFA Chip Select Outputs: On C6424, the EMIFA chip select pins (EM_CS2, EM_CS3, EM_CS4,

and EM_CS5) feature an internal pulldown (IPD) resistor. If these pins are connected and used as an

EMIFA chip select signal, for proper device operation, an external pullup resistor must be used to

ensure the EM_CSx function defaults to an inactive (high) state.

For the boot and configuration pins (listed in Table 2-7, Boot Terminal Functions), 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 boot and 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:

•

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 VIL level of

all inputs connected to the net. For a pullup resistor, this should be above the highest VIH level of all

inputs on the net. A reasonable choice would be to target the VOL or VOH levels for the logic family of

the limiting device; which, by definition, have margin to the VIL and VIH 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 DVDD rail.

Device Configurations

121

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