DS87C520-QNL Datasheet, PDF (21/43 Pages) Maxim Integrated Products – EPROM/ROM High-Speed Microcontrollers

DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers
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
STOP MODE WITHOUT RING STARTUP
STOP MODE WITH RING STARTUP
NOTE: DIAGRAM ASSUMES THAT THE OPERATION FOLLOWING STOP REQUIRES
LESS THAN 18ms TO COMPLETE.
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.
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