DS87C530/DS83C530 EPROM/ROM Microcontrollers with Real-Time Clock
The ring oscillator runs at approximately 2MHz to 4MHz but will not be a precise value. Do not conduct real-time precision operations (including serial communication) during this ring period. Figure 6 shows 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 6. Ring Oscillator Exit from Stop Mode
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 DS87C530/DS83C530 allow 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 when performing on-chip memory access. The default state of ALEOFF = 0 so ALE toggles with every instruction cycle.