DS87C520_1 Datasheet, PDF (19/42 Pages) Dallas Semiconductor – EPROM/ROM High-Speed Micro

DS87C520/DS83C520
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 3.57 MHz crystal, this is
approximately 18 ms. 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.
RING OSCILLATOR EXIT FROM STOP MODE Figure
STOP MODE WITHOUT RING STARTUP
STOP MODE WITH RING STARTUP
Note: Diagram assumes that the operation following Stop requires less than 18 ms 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 a
1. When ALE-OFF= 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.
PERIPHERAL OVERVIEW
The DS87C520/DS83C520 provides several of the most commonly needed peripheral functions in micro-
computer-based systems. These new functions include a second serial port, Power- fail Reset, Power- fail
Interrupt, and a programmable Watchdog Timer. These are described below, and more details are
available in the High-Speed Microcontroller User’s Guide.
SERIAL PORTS
The DS87C520/DS83C520 provides a serial port (UART) that is identical to the 80C52. In addition it
includes a second hardware serial port that is a full duplicate of the standard one. This port optionally
uses pins P1.2 (RXD1) and P1.3 (TXD1). It has duplicate control functions included in new SFR
locations.
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