To help DSP developers maximize battery life and enable power-sensitive systems in portable, low-power applications, Texas Instruments Inc. created the TMS320C5509A, TMS320C5507 and TMS320C5503 digital signal processors. To ensure that this DSP will consume only as much power as needed, however, it is important to understand how to minimize power consumption and take advantage of idling all the unused peripherals.

For example, effective system-level board design requires good termination of some specific pins, idling some peripherals, and taking advantage of idle and power-down modes on the DSP. This can be achieved by dynamically turning off all peripherals and internal functional units when they are not in use. Minimizing power consumption will also help reduce cost, heat and component density.

If input pins float, then every time the floating pin's logic state toggles, additional I/O current consumption occurs. By controlling unused and floating pins, developers save power while maintaining optimum functionality. Idling or correctly configuring unused DSP peripherals such as real-time clock, A/D converter, MMC/SD cards and USB can also save power for critical, power-sensitive applications. Here are some pointers on how to optimize power consumption by terminating peripherals' unused pins on the C5509A, C5507 and C5503 DSPs.

Do

• Put the DSP in standby mode--this is a critical part of power optimization. To achieve this, a developer can activate the idle configurations, which include idling CPU, all peripherals, clock gen and on-chip oscillator.

• Terminate unused input pins by pull-up/down and make sure that unused "output" pins are not connected to any resistive loads Enabling the bus keepers on the EMIF bus will also save power.

• The real-time clock (RTC) is the only module that is completely isolated from the rest of the chip. It can be powered while the rest of the chip is not powered, and vice versa. Idling of other peripherals and modules on the DSP will not affect the RTC. To minimize power dissipation, the RTC module should be powered up when it is not being used.

• Program the on-chip analog-to-digital converter clock to the lowest possible clock rate of 4 MHz, so the maximum possible conversion clock frequency is 2 MHz. This will minimize power consumption of the A/D converter state machine. If an on-chip A/D converter module is not used, then the A/D converter connect supply and ground should be connected to the supply and ground suggested by the data sheet.

• Idle the USB module on the DSP by setting the individual bits in the USBIDLECTL register. To disable the USB port, write 1 to the idle enable (IDLEEN) bit in USBIDLECTL, and then write 1 to the PERI bit in ICR. Make sure the USBCLK is set to 48 MHz. Make sure to leave DP and DN pins floating. Finally, pull up the USB pin PU high with 10 kohms, so the USB does not reset and wake up the on-chip oscillator.

Don't

• Attach the emulator or use debugging functions with the Code Composer Studio integrated development environment to put the DSP in sleep or Idle3 mode. Don't have any pending interrupts, and make sure unused input pins are pulled high. Don't do any DMA transfer or have RTDX enabled. All of these actions will reactivate the Idle3 mode. (Example code that pus the DSP in Idle3 power-down mode is available from TI.)

• Have an external interrupt or an interrupt generated by the on-chip real-time clock. Don't have USB host asserts resume or system hardware reset, as these events will wake up the internal oscillator from idle or sleep mode.

• Terminate the data or address bus pins, since these have holder features to reduce the static power dissipation caused by floating unused pins. The bus holders will eliminate the need for external resistors on unused pins.

• Use an external oscillator. To consume the least amount of power, consider utilizing the on-chip oscillator.

• Leave RTC pins floating even if you are not using them, since it is common to keep the RTC running when the DSP is turned off. Leaving RTC pins floating will increase switching, which will consequently consume more power.

By Naser Salameh (nsalameh@ ti.com), DSP applications engineer, Texas Instruments Inc. (Dallas)

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