DSP56321 datasheet(63/84 Pages) FREESCALE

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Power Consumption Considerations

DSP56321 Technical Data, Rev. 11

Freescale Semiconductor

4-3

•

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).

•

The following pins must be asserted during the power-up sequence: 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 VCCQL 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

Note:

Refer to EB610/D DSP56321/DSP56321T Power-Up Sequencing Guidelines for detailed information

about minimizing power consumption during startup.

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

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.

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