Power Consumption Considerations
•
•
•
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.
RESET must be asserted when the chip is powered up. A stable EXTAL signal should be supplied before
deassertion of RESET.
At power-up, ensure that the voltage difference between the 5 V tolerant pins and the chip V never
CC
exceeds 3.5 V.
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: I = C × V × f
Where:
C
V
f
=
=
=
node/pin capacitance
voltage swing
frequency of node/pin toggle
Example 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.
Equation 4: I = 50 × 10–12 × 3.3 × 33 × 106 = 5.48 mA
The maximum internal current (I max) value reflects the typical possible switching of the internal buses on best-
CCI
case operation conditions—not necessarily a real application case. The typical internal current (I
) value
CCItyp
reflects the average switching of the internal buses on typical operating conditions. Perform the following steps for
applications that require very low current consumption:
1. Set the EBD bit when you are not accessing external memory.
2. Minimize external memory accesses, and use internal memory accesses.
3. Minimize the number of pins that are switching.
4. Minimize the capacitive load on the pins.
5. Connect the unused inputs to pull-up or pull-down resistors.
6. Disable unused peripherals.
7. Disable unused pin activity (for example, CLKOUT, XTAL).
One way to evaluate power consumption is to use a current-per-MIPS measurement methodology to minimize
specific board effects (that is, to compensate for measured board current not caused by the DSP). A benchmark
power consumption test algorithm is listed in Appendix A. Use the test algorithm, specific test current
measurements, and the following equation to derive the current-per-MIPS value.
DSP56301 Technical Data, Rev. 10
Freescale Semiconductor
4-3
FREESCALE [ Freescale ]
