
Data Sheet
March 2000
DSP1629 Digital Signal Processor
32
Lucent Technologies Inc.
4 Hardware Architecture
(continued)
Await Bit of the alf Register
Setting the AWAIT bit of the alf register causes the pro-
cessor to go into the standard sleep state or power-sav-
ing standby mode. Operation of the AWAIT bit is the
same as in the DSP1610, DSP1611, DSP1616,
DSP1617, and DSP1618. In this mode, the minimum
circuitry required to process an incoming interrupt re-
mains active, and the PLL remains active if enabled. An
interrupt will return the processor to the previous state,
and program execution will continue. The action result-
ing from setting the AWAIT bit and the action resulting
from setting bits in the powerc register are mostly inde-
pendent. As long as the processor is receiving a clock,
whether slow or fast, the DSP may be put into standard
sleep mode with the AWAIT bit. Once the AWAIT bit is
set, the STOP pin can be used to stop and later restart
the processor clock, returning to the standard sleep
state. If the processor clock is not running, however, the
AWAIT bit cannot be set.
Power Management Sequencing
There are important considerations for sequencing the
power management modes. The small-signal clock in-
put
circuit has a start-up delay which must be taken into ac-
count, and the PLL requires a delay to reach lock-in. Al-
so, the chip may or may not need to be reset following a
return from a low-power state.
Devices with a small-signal input clocking option may
use the XTLOFF bit in the powerc register to power
down the on-chip oscillator or small-signal circuitry,
thereby reducing the power dissipation. When reen-
abling the oscillator or the small-signal circuitry, it is im-
portant to bear in mind that a start-up interval exists
during which time the clocks are not stable.
Two scenarios exist here:
1. Immediate Turn-Off, Turn-On with RSTB: This sce-
nario applies to situations where the target device is
not required to execute any code while the small-sig-
nal input circuit is powered down and where restart
from a reset state can be tolerated. In this case, the
processor clock derived from either the oscillator or
the small-signal input is running when XTLOFF is as-
serted. This effectively stops the internal processor
clock. When the system chooses to reenable the os-
cillator or small-signal input, a reset of the device will
be required. The reset pulse must be of sufficient du-
ration for the small-signal start-up interval to be sat-
isfied (required for the small-signal input circuit to
reach its dc operating point). A minimum reset pulse
of 20 μs will be adequate. The falling edge of the re-
set signal, RSTB, will asynchronously clear the
XTLOFF field, thus reenabling the power to the
small-signal circuitry. The target DSP will then start
execution from a reset state, following the rising
edge of RSTB.
2. Running from Slow Clock While XTLOFF Active: The
second scenario applies to situations where the de-
vice needs to continue execution of its target code.
In this case, the device switches to the slow ring os-
cillator clock first, by enabling the SLOWCKI field.
Then, if the small-signal input is being used, power
down this circuitry by writing a 1 to the XTLOFF field.
Two nops are needed in between the two write oper-
ations to the powerc register. The target device will
then continue execution of its code at slow speed,
while the small-signal input clock is turned off.
Switching from the slow clock back to the high-speed
clock is then accomplished in three user steps. First,
XTLOFF is cleared. Then, a user-programmed rou-
tine sets the internal timer to a delay to wait for the
small-signal input oscillations to become stable.
When the timer counts down to zero, the high-speed
clock is selected by clearing the SLOWCKI field, ei-
ther in the timer's interrupt service routine or follow-
ing a timer polling loop. If PLL operation is desired,
then an additional routine is necessary to enable the
PLL and wait for it to lock.