DSP56321RMSLASHD datasheet(69/88 Pages) NXP

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4-3

Power Consumption Considerations

• Because the DSP output signals have fast rise and fall times, PCB trace lengths should be minimal.

This recommendation particularly applies to the address and data buses as well as the IRQA, IRQB,

IRQC

, IRQD, TA, and BG pins. Maximum PCB trace lengths on the order of 6 inches are recommended.

• Consider all device loads as well as parasitic capacitance due to PCB traces when you calculate

capacitance. This is especially critical in systems with higher capacitive loads that could create higher

transient currents in the VCC and GND circuits.

• All inputs must be terminated (that is, not allowed to float) by CMOS levels except for the three pins

with internal pull-up resistors (TRST, TMS, DE).

• Take special care to minimize noise levels on the VCCP, GNDP, and GNDP1 pins.

• The following pins must be asserted during power-up: RESET and TRST. A stable EXTAL signal

should be supplied before deassertion of RESET. If the VCC reaches the required level before EXTAL

is stable or other “required RESET duration” conditions are met (see Table 2-7), the device circuitry

can be in an uninitialized state that may result in significant power consumption and heat-up. Designs

should minimize this condition to the shortest possible duration.

• Ensure that during power-up, and throughout the DSP56321 operation, VCCQH is always higher or

equal to the VCC voltage level.

• If multiple DSP devices are on the same board, check for cross-talk or excessive spikes on the supplies

due to synchronous operation of the devices.

• The Port A data bus (D[0–23]), HI08, ESSI0, ESSI1, SCI, and timers all use internal keepers to

maintain the last output value even when the internal signal is tri-stated. Typically, no pull-up or

pull-down resistors should be used with these signal lines. However, if the DSP is connected to a

device that requires pull-up resistors (such as an MPC8260), the recommended resistor value is 10 K

Ω

or less. If more than one DSP must be connected in parallel to the other device, the pull-up resistor

value requirement changes as follows:

—2 DSPs = 5 K

Ω (mask sets 0K91M and 1K91M)/7 KΩ (mask set 0K93M) or less

—3 DSPs = 3 K

Ω (mask sets 0K91M and 1K91M)/4 KΩ (mask set 0K93M) or less

—4 DSPs = 2 K

Ω (mask sets 0K91M and 1K91M)/3 KΩ (mask set 0K93M) or less

—5 DSPs = 1.5 K

Ω (mask sets 0K91M and 1K91M)/2 KΩ (mask set 0K93M) or less

—6 DSPs = 1 K

Ω (mask sets 0K91M and 1K91M)/1.5 KΩ (mask set 0K93M) or less

4.3 Power Consumption Considerations

Power dissipation is a key issue in portable DSP applications. Some of the factors affecting current

consumption are described in this section. Most of the current consumed by CMOS devices is alternating

current (ac), which is charging and discharging the capacitances of the pins and internal nodes.

Current consumption is described by this formula:

Equation 3:

Where:

node/pin capacitance

voltage swing

frequency of node/pin toggle

Equation 4:

Example 4-1. Current Consumption

For a Port A address pin loaded with 50 pF capacitance, operating at 3.3 V, with a 66 MHz clock, toggling at its

maximum possible rate (33 MHz), the current consumption is expressed in Equation 4.

–

3.3

5.48 mA

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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