DS87C520 datasheet(21/45 Pages) DALLAS

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DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers

21 of 45

how the operation would compare when using the ring, and when starting up normally. The default state

is to exit Stop mode without using the ring oscillator.

The RGSL - Ring Select bit at EXIF.1 (EXIF; 91h) controls this function. When RGSL = 1, the CPU will

use the ring oscillator to exit Stop mode quickly. As mentioned above, the processor will automatically

switch from the ring to the crystal after a delay of 65,536 crystal clocks. For a 3.57MHz crystal, this is

approximately 18ms. The processor sets a flag called RGMD-Ring Mode, located at EXIF.2, that tells

software that the ring is being used. The bit will be a logic 1 when the ring is in use. Attempt no serial

communication or precision timing while this bit is set, since the operating frequency is not precise.

Figure 4. Ring Oscillator Exit from Stop Mode

EMI REDUCTION

One of the major contributors to radiated noise in an 8051-based system is the toggling of ALE. The

microcontroller allows software to disable ALE when not used by setting the ALEOFF (PMR.2) bit to 1.

When ALEOFF = 1, ALE will still toggle during an off-chip MOVX. However, ALE will remain in a

static mode when performing on-chip memory access. The default state of ALEOFF = 0 so ALE toggles

at a frequency of XTAL/4.

STOP MODE WITH RING STARTUP

STOP MODE WITHOUT RING STARTUP

NOTE: DIAGRAM ASSUMES THAT THE OPERATION FOLLOWING STOP REQUIRES

LESS THAN 18ms TO COMPLETE.

DS87C520 Datenblatt(PDF) 21 Page - Dallas Semiconductor