Texas Instruments TMS320C6712D Power-supply sequencing, Characteristics of the Power-Down Modes

SPRS293A − OCTOBER 2005 − REVISED NOVEMBER 2005

Table 32. Characteristics of the Power-Down Modes

PRWD FIELD	POWER-DOWN	WAKE-UP METHOD	EFFECT ON CHIP’S OPERATION
(BITS 15−10)	MODE
000000	No power-down	—	—
001001	PD1	Wake by an enabled interrupt	CPU halted (except for the interrupt logic)
			Power-down mode blocks the internal clock inputs at the
		Wake by an enabled or	boundary of the CPU, preventing most of the CPU’s logic from
010001	PD1		switching. During PD1, EDMA transactions can proceed
		non-enabled interrupt
			between peripherals and internal memory.
			Output clock from PLL is halted, stopping the internal clock
	PD2†		structure from switching and resulting in the entire chip being
011010		Wake by a device reset	halted. All register and internal RAM contents are preserved. All
			functional I/O “freeze” in the last state when the PLL clock is
			turned off.
			Input clock to the PLL stops generating clocks. All register and
			internal RAM contents are preserved. All functional I/O “freeze” in
011100	PD3†	Wake by a device reset	the last state when the PLL clock is turned off. Following reset, the
			PLL needs time to re-lock, just as it does following power-up.
			Wake-up from PD3 takes longer than wake-up from PD2 because
			the PLL needs to be re-locked, just as it does following power-up.
All others	Reserved	—	—

†When entering PD2 and PD3, all functional I/O remains in the previous state. However, for peripherals which are asynchronous in nature or peripherals with an external clock source, output signals may transition in response to stimulus on the inputs. Under these conditions, peripherals will not operate according to specifications.

The device includes a programmable PLL which allows software control of PLL bypass via the PLLEN bit in the PLLCSR register. With this enhanced functionality comes some additional considerations when entering power-down modes.

The power-down modes (PD2 and PD3) function by disabling the PLL to stop clocks to the device. However, if the PLL is bypassed (PLLEN = 0), the device will still receive clocks from the external clock input (CLKIN). Therefore, bypassing the PLL makes the power-down modes PD2 and PD3 ineffective.

Make sure that the PLL is enabled by writing a “1” to PLLEN bit (PLLCSR.0) before writing to either PD3 (CSR.11) or PD2 (CSR.10) to enter a power-down mode.

power-supply sequencing

TI DSPs do not require specific power sequencing between the core supply and the I/O supply. However, systems should be designed to ensure that neither supply is powered up for extended periods of time (>1 second) if the other supply is below the proper operating voltage.

system-level design considerations

System-level design considerations, such as bus contention, may require supply sequencing to be implemented. The core supply should be powered up prior to (and powered down after), the I/O buffers. This is to ensure that the I/O buffers receive valid inputs from the core before the output buffers are powered up, thus, preventing bus contention with other chips on the board.

power-supply design considerations

A dual-power supply with simultaneous sequencing can be used to eliminate the delay between core and I/O power up. A Schottky diode can also be used to tie the core rail to the I/O rail (see Figure 13).

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