DS87C520 datasheet(13/42 Pages) DALLAS

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DS87C520/DS83C520

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Software is the only mechanism to invoke the PMM. Table 5 illustrates the instruction cycle rate in

PMM for several common crystal frequencies. Since power consumption is a direct function of operating

speed, PMM 1 eliminates most of the power consumption while still allowing a reasonable speed of

processing. PMM 2 runs very slow and provides the lowest power consumption without stopping the

CPU. This is illustrated in Table 6.

Note that PMM provides a lower power condition than Idle mode. This is because in Idle mode, all

clocked functions such as timers run at a rate of crystal divided by 4. Since wake-up from PMM is as fast

as or faster than from Idle and PMM allows the CPU to operate (even if doing NOPs), there is little

reason to use Idle mode in new designs.

MACHINE CYCLE RATE Table 5

CRYSTAL SPEED

FULL OPERATION

(4 CLOCKS)

PMM1

(64 CLOCKS)

PMM2

(1024 CLOCKS)

11.0592 MHz

2.765 MHz

172.8 kHz

10.8 kHz

16 MHz

4.00 MHz

250.0 kHz

15.6 kHz

25 MHz

6.25 MHz

390.6 kHz

24.4 kHz

33 MHz

8.25 MHz

515.6 kHz

32.2 kHz

TYPICAL OPERATING CURRENT IN PMM Table 6

CRYSTAL SPEED

FULL OPERATION

(4 CLOCKS)

PMM1

(64 CLOCKS)

PMM2

(1024 CLOCKS)

11.0592 MHz

13.1 mA

5.3 mA

4.8 mA

16 MHz

17.2 mA

6.4 mA

5.6 mA

25 MHz

25.7 mA

8.1 mA

7.0 mA

33 MHz

32.8 mA

9.8 mA

8.2 mA

CRYSTALESS PMM

A major component of power consumption in PMM is the crystal amplifier circuit. The

DS87C520/DS83C520 allows the user to switch CPU operation to an internal ring oscillator and turn off

the crystal amplifier. The CPU would then have a clock source of approximately 2-4 MHz, divided by

either 4, 64, or 1024. The ring is not accurate, so software can not perform precision timing. However,

this mode allows an additional saving of between 0.5 and 6.0 mA, depending on the actual crystal

frequency. While this saving is of little use when running at 4 clocks per instruction cycle, it makes a

major contribution when running in PMM1 or PMM2.

PMM OPERATION

Software invokes the PMM by setting the appropriate bits in the SFR area. The basic choices are divider

speed and clo ck source. There are three speeds (4, 64, and 1024) and two clock sources (crystal and ring).

Both the decisions and the controls are separate. Software will typically select the clock speed first. Then,

it will perform the switch to ring operation if desired. Lastly, software can disable the crystal amplifier if

desired.

There are two ways of exiting PMM. Software can remove the condition by reversing the procedure that

invoked PMM or hardware can (optionally) remove it. To resume operation at a divide by 4 rate under

software control, simply select 4 clocks per cycle, then crystal based operation if relevant. When

disabling the crystal as the time base in favor of the ring oscillator, there are timing restrictions associated

with restarting the crystal operation. Details are described below.

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