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ADSP-2171

DSP Microcomputer

Analog Devices 

ADSP-2171/ADSP-2172/ADSP-2173

LOW POWER OPERATION

The ADSP-217x has three low power modes that significantly

reduce the power dissipation when the device operates under

standby conditions. These modes are:

• Powerdown

• Idle

• Slow Idle

The CLKOUT pin may also be disabled to reduce external

power dissipation. The CLKOUT pin is controlled by Bit 14 of

SPORT0 Autobuffer Control Register, DM[0x3FF3].

Powerdown

The ADSP-217x processor has a low power feature that lets the

processor enter a very low power dormant state through hard-

ware or software control. Here is a brief list of powerdown fea-

tures. Refer to the ADSP-2100 Family User’s Manual, Chapter 9

“System Interface” for detailed information about the

powerdown feature.

• Powerdown mode holds the processor in CMOS standby with

a maximum current of less than 100 µA in some modes.

• Quick recovery from powerdown. The processor begins ex-

ecuting instructions in as few as 100 CLKIN cycles.

• Support for an externally generated TTL or CMOS processor

clock. The external clock can continue running during

powerdown without affecting the lowest power rating and 100

CLKIN cycle recovery.

• Support for crystal operation includes disabling the oscillator

to save power (the processor automatically waits 4096 CLKIN

cycles for the crystal oscillator to start and stabilize), and let-

ting the oscillator run to allow 100 CLKIN cycle startup.

• Powerdown is initiated by either the powerdown pin (PWD)

or the software powerdown force bit.

• Interrupt support allows an unlimited number of instructions

to be executed before optionally powering down. The

powerdown interrupt also can be used as a non-maskable,

edge sensitive interrupt.

• Context clear/save control allows the processor to continue

where it left off or start with a clean context when leaving the

powerdown state.

• The RESET pin also can be used to terminate powerdown,

and the host software reset feature can be used to terminate

powerdown under certain conditions.

• Powerdown acknowledge pin indicates when the processor has

entered powerdown.

Idle

When the ADSP-217x is in the Idle Mode, the processor waits

indefinitely in a low power state until an interrupt occurs. When

an unmasked interrupt occurs, it is serviced; execution then

continues with the instruction following the IDLE instruction.

Slow Idle

The IDLE instruction is enhanced on the ADSP-217x to let the

processor’s internal clock signal be slowed during IDLE, further

reducing power consumption. The reduced clock frequency, a

programmable fraction of the normal clock rate, is specified by a

selectable divisor given in the IDLE instruction. The format of

the instruction is

IDLE (n);

where n = 16, 32, 64, or 128. This instruction keeps the proces-

sor fully functional, but operating at the slower clock rate. While

it is in this state, the processor’s other internal clock signals,

such as SCLK, CLKOUT, and timer clock, are reduced by the

same ratio. The default form of the instruction, when no clock

divisor is given, is the standard IDLE instruction.

When the IDLE (n) instruction is used, it effectively slows down

the processor’s internal clock and thus its response time to in-

coming interrupts––the 1-cycle response time of the standard

idle state is increased by n, the clock divisor. When an enabled

interrupt is received, the ADSP-217x will remain in the idle

state for up to a maximum of n processor cycles (n = 16, 32, 64,

or 128) before resuming normal operation.

When the IDLE (n) instruction is used in systems that have an

externally generated serial clock (SCLK), the serial clock rate

may be faster than the processor’s reduced internal clock rate.

Under these conditions, interrupts must not be generated at a

faster rate than can be serviced, due to the additional time the

processor takes to come out of the idle state (a maximum of n

processor cycles).

SYSTEM INTERFACE

Figure 3 shows a basic system configuration with the ADSP-

217x, two serial devices, a host processor, a boot EPROM, and

optional external program and data memories. Up to 14K words

of data memory and 16K words of program memory can be sup-

ported. Programmable wait state generation allows the processor

to interface easily to slow memories. The ADSP-217x also pro-

vides one external interrupt and two serial ports or three exter-

nal interrupts and one serial port.

Clock Signals

The ADSP-217x can be clocked by either a crystal or by a TTL-

compatible clock signal.

The CLKIN input cannot be halted, changed during operation,

or operated below the specified frequency during normal opera-

tion. The only exception is while the processor is in the Power-

down State. For additional information, refer to Chapter 9,

ADSP-2100 Family User’s Manual for detailed information on

this powerdown feature.

If an external clock is used, it should be a TTL-compatible sig-

nal running at half the instruction rate. The signal is connected

to the processor’s CLKIN input. When an external clock is

used, the XTAL input must be left unconnected.

The ADSP-217x uses an input clock with a frequency equal to

half the instruction rate; a 16.67 MHz input clock yields a 30 ns

processor cycle (which is equivalent to 33 MHz). Normally, in-

structions are executed in a single processor cycle. All device

timing is relative to the internal instruction clock rate, which is

indicated by the CLKOUT signal when enabled.

–6–

REV. A

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