DS87C550 datasheet(21/50 Pages) DALLAS

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DS87C550

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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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