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參數(shù)資料
| 型號： | TMX320TCI6482 |
| 廠商： | Texas Instruments, Inc. |
| 元件分類： | 數(shù)字信號處理 |
| 英文描述： | Communications Infrastructure Digital Signal Processor |
| 中文描述： | 通信基礎(chǔ)設(shè)施的數(shù)字信號處理器 |
| 文件頁數(shù)： | 74/255頁 |
| 文件大?。?/td> | 1893K |
| 代理商： | TMX320TCI6482 |
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www.ti.com
3.7 Pullup/Pulldown Resistors
TMS320TCI6482
Communications Infrastructure Digital Signal Processor
SPRS246F–APRIL 2005–REVISED MAY 2007
Proper board design should ensure that input pins to the TCI6482 device always be at a valid logic level
and not floating. This may be achieved via pullup/pulldown resistors. The TCI6482 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 needs to be used in the following situations:
Device Configuration Pins
: If the pin is both routed out and 3-stated (not driven), 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.
For the device configuration pins (listed in
Table 3-1
), 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
configuration pins. In addition, applying external pullup/pulldown resistors on the device 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 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-k
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
resistor can be used to compliment the IPU/IPD on the device 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 TCI6482 device, see
Section 7.3
,
Electrical Characteristics Over Recommended Ranges of Supply
Voltage and Operating Case Temperature
.
To determine which pins on the TCI6482 device include internal pullup/pulldown resistors, see
Table 2-3
,
Terminal Functions
.
Device Configuration
74
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