DS87C550 datasheet(4/49 Pages) MAXIM

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DS87C550 EPROM High-Speed Microcontroller with ADC and PWM

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PIN DESCRIPTION Table 1

PLCC/

CLCC

QFP

SIGNAL NAME

DESCRIPTION

VCC

VCC - Digital +5V power input.

GND

GND – Digital ground.

RST

RST - I/O. The RST input pin contains a Schmitt voltage input to recognize

external active high Reset inputs. The pin also employs an internal pulldown

resistor to allow for a combination of wired OR external Reset sources. An RC is

not required for power-up, as the DS87C550 provides this function internally. This

pin also acts as an output when the source of the reset is internal to the device (i.e.,

watchdog timer, power-fail, or crystal-fail detect). In this case, the RST pin will be

held high while the processor is in a Reset state, and will return to low as the

processor exits this state. When this output capability is used, the RST pin should

not be connected to an RC network or a logic output driver.

XTAL1

XTAL2

Input - The crystal oscillator pins XTAL1 and XTAL2 provide support for

fundamental mode, parallel resonant, AT cut crystals. XTAL1 acts also as an input

if there is an external clock source in place of a crystal. XTAL2 serves as the

output of the crystal amplifier. Note that this output cannot be used to drive any

additional load when a crystal is attached as this can disturb the oscillator circuit.

PSEN

- Output. The Program Store Enable output. This signal is commonly

connected to optional external ROM memory as a chip enable.

PSEN will provide

an active low pulse during a program byte access, and is driven high when not

accessing external program memory.

ALE

ALE - Output. The Address Latch Enable output functions as a clock to latch the

external address LSB from the multiplexed address/data bus on Port 0. This signal

is commonly connected to the latch enable of an external 373 family transparent

latch. ALE is driven high when the DS87C550 is in a Reset condition. ALE can

also be disabled and forced high using the EMI reduction mode ALEOFF.

- Input. An active low input pin that when connected to ground will force the

DS87C550 to use an external program memory. The internal RAM is still

accessible as determined by register settings. EA should be connected to VCC to

use internal program memory. The input level on this pin is latched at reset.

16-23

10-17

P1.0-P1.7

Port 1 - I/O. Port 1 functions as both an 8-bit, bi-directional I/O port and an

alternate functional interface for several internal resources. The reset condition of

Port 1 is all bits at logic 1. In this state, a weak pullup holds the port high. This

condition allows the pins to serve as both input and output. Input is possible since

any external circuit whose output drives the port will overcome the weak pullup.

When software writes a 0 to any Port 1 pin, the DS87C550 will activate a strong

pulldown that remains on until either a 1 is written or a reset occurs. Writing a 1

after the port has been at 0 will cause a strong transition driver to turn on, followed

by a weaker sustaining pullup. Once the momentary strong driver turns off, the

port again returns to a weakly held high output (and input) state. The alternate

functions of Port 1 pins are detailed below. Note that when the Capture/Compare

functions of timer 2 are used, the interrupt input pins become capture trigger

inputs.

Port

Alternate Function

P1.0

INT2/CT0 External Interrupt 2/Capture Trigger 0

P1.1

INT3/CT1 External Interrupt 3/Capture Trigger 1

P1.2

INT4/CT2 External Interrupt 4/Capture Trigger 2

P1.3

INT5/CT3 External Interrupt 5/Capture Trigger 3

P1.4

External I/O for Timer/Counter 2

P1.5

T2EX

Timer/Counter 2 Capture/Reload Trigger

P1.6

RXD1

Serial Port 1 Input

P1.7

TXD1

Serial Port 1 Output

DS87C550 Datasheet(PDF) 4 Page - Maxim Integrated Products

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