TMS320VC5505_10 Datasheet, PDF (52/135 Pages) Texas Instruments – Fixed-Point Digital Signal Processor

TMS320VC5505
SPRS503B – JUNE 2009 – REVISED JANUARY 2010
www.ti.com
For the configuration pins (listed in Table 4-2, 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/pulldown resistor be implemented. In addition, applying external pullup/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 Pull-up/Pull-down Inhibit Registers (PDINHIBR1/2/3) [1C17h, 1C18h, and
1C19h, respectively] to minimize power consumption.
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 (IPU/IPD) 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 DV_DD rail.
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 compliment the IPU/IPD on the 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 VC5505 DSP, 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 in this document.
4.8.2 CLKOUT Pin
For debug purposes, the DSP includes a CLKOUT pin which can be used to tap different clocks within the
clock generator. The SRC bits of the CLKOUT Control Source Register (CCSSR) can be used to specify
the source for the CLKOUT pin.
Note: the bootloader disables the CLKOUT pin via CLKOFF bit in the ST3_55 CPU register.
For more information on the ST3_55 CPU register, see the TMS320C55x 3.0 CPU Reference Guide
(literature number: SWPU073).
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Device Configuration
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