DS87C520 datasheet(14/45 Pages) DALLAS

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DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers

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POWER MANAGEMENT

Along with the standard Idle and power down (Stop) modes of the standard 80C52, the

DS87C520/DS83C520 provide a new Power Management Mode. This mode allows the processor to

continue functioning, yet to save power compared with full operation. The DS87C520/DS83C520 also

feature several enhancements to Stop mode that make it more useful.

POWER MANAGEMENT MODE (PMM)

Power Management Mode offers a complete scheme of reduced internal clock speeds that allow the CPU

to run software but to use substantially less power. During default operation, the DS87C520/DS83C520

use four clocks per machine cycle. Thus the instruction cycle rate is Clock/4. At 33MHz crystal speed,

the instruction cycle speed is 8.25MHz (33/4). In PMM, the microcontroller continues to operate but uses

an internally divided version of the clock source. This creates a lower power state without external

components. It offers a choice of two reduced instruction cycle speeds (and two clock sources - discussed

below). The speeds are (Clock/64) and (Clock/1024).

Software is the only mechanism to invoke the PMM. Table 4 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 5.

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.

Table 4. Machine Cycle Rate

CRYSTAL SPEED

(MHz)

FULL OPERATION

(4 CLOCKS)

(MHz)

PMM1

(64 CLOCKS)

(kHz)

PMM2

(1024 CLOCKS)

(kHz)

11.0592

2.765

172.8

10.8

4.00

250.0

15.6

6.25

390.6

24.4

8.25

515.6

32.2

Table 5. Typical Operating Current in PMM

CRYSTAL SPEED

(MHz)

FULL OPERATION

(4 CLOCKS)

(mA)

PMM1

(64 CLOCKS)

(mA)

PMM2

(1024 CLOCKS)

(mA)

11.0592

13.1

5.3

4.8

17.2

6.4

5.6

25.7

8.1

7.0

32.8

9.8

8.2

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