DS87C550 Datasheet, PDF (21/50 Pages) Dallas Semiconductor – EPROM High-Speed Micro with A/D and PWM

DS87C550
ROMSIZE.1 RMS1 Program Memory Select
Bit 1
ROMSIZE.0 RMS0 Program Memory Select
Bit 0
EMI REDUCTION
One of the major contributors to radiated noise in an 8051-based system is the toggling of ALE. The
DS87C550 allows software to disable ALE when not used by setting the ALEOFF (PMR.2) bit to a 1.
When ALEOFF = 1, ALE will still toggle during an off-chip MOVX. However, ALE will remain inactive
when performing on-chip memory access. The default state is ALEOFF = 0 so ALE normally toggles at a
frequency of XTAL/4.
PERIPHERAL OVERVIEW
The DS87C550 provides several of the most commonly needed peripheral functions in microcomputer-
based systems. New functions include a second serial port, power-fail reset, power-fail interrupt flag, and
a programmable watchdog timer. In addition, the DS87C550 contains an analog-to-digital converter and
four channels of pulse width modulation for industrial control and measurement applications. Each of
these peripherals is described below. More details are available in the High-Speed Micro Data Book (or
its most recent addendum).
SERIAL PORTS
The DS87C550 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 P1.6
(RXD1) and P1.7 (TXD1). It has duplicate control functions included in new SFR locations.
Both ports can operate simultaneously but can be at different baud rates or even in different modes. The
second serial port has similar control registers (SCON1, SBUF1) to the original. The new serial port can
only use Timer 1 for timer generated baud rates.
Control for serial port 0 is provided by the SCON0 register while its I/O buffer is SBUF0. The registers
SCON1 and SBUF1 provide the same functions for the second serial port. A full description of the use
and operation of both serial ports may be found in the “High-Speed Microcontroller Data Book.”
ANALOG TO DIGITAL CONVERTER
The DS87C550 contains a 10-bit successive approximation analog-to-digital converter. This converter
provides eight multiplexed channels of analog input and allows the user to select either an external or
internal precision voltage reference to be used for the conversion process. The A/D converter provides
true 9-bit accuracy with a total error of less than ±2 LSBs.
The A/D Converter may be disabled to conserve power by writing a 0 to the SFR ADON bit
(ADCON1.1). At reset, this is the default condition, and the user must write a 1 to this bit prior to using
the A/D Converter.
A/D CONVERTER INPUT
The A/D Converter of the DS87C550 provides eight channels of analog input on device pins ADC7
through ADC0 (P5.7-P5.0). The signals on these pins are input into an analog multiplexer. The magnitude
(and polarity) of these signals is limited by the reference voltage used by the converter (see DC
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