Philips Semiconductors

Product specification

87C552

80C51 8-bit microcontroller

1998 May 01

6

PIN DESCRIPTION (Continued)

PIN NO.

MNEMONIC

PLCC

QFP

TYPE

NAME AND FUNCTION

ALE/PROG

48

49

O

Address Latch Enable: Latches the low byte of the address during accesses to external

memory. It is activated every six oscillator periods. During an external data memory

access, one ALE pulse is skipped. ALE can drive up to eight LS TTL inputs and handles

CMOS inputs without an external pull-up. This pin is also the program pulse input (PROG)

during EPROM programming.

49

50

I

External Access: When EA is held at TTL level high, the CPU executes out of the internal

program ROM provided the program counter is less than 8192. When EA is held at TTL

low level, the CPU executes out of external program memory. EA is not allowed to float.

programming.

58

59

I

Analog to Digital Conversion Reference Resistor: Low-end.

59

60

I

Analog to Digital Conversion Reference Resistor: High-end.

60

61

I

Analog Ground

61

63

I

Analog Power Supply

NOTE:

respectively.

OSCILLATOR CHARACTERISTICS

XTAL1 and XTAL2 are the input and output, respectively, of an

inverting amplifier. The pins can be configured for use as an on-chip

oscillator, as shown in the logic symbol.

To drive the device from an external clock source, XTAL1 should be

driven while XTAL2 is left unconnected. There are no requirements

on the duty cycle of the external clock signal, because the input to

the internal clock circuitry is through a divide-by-two flip-flop.

However, minimum and maximum high and low times specified in

the data sheet must be observed.

RESET

A reset is accomplished by holding the RST pin high for at least two

machine cycles (24 oscillator periods), while the oscillator is running.

To insure a good power-on reset, the RST pin must be high long

enough to allow the oscillator time to start up (normally a few

milliseconds) plus two machine cycles. At power-on, the voltage on

IDLE MODE

In the idle mode, the CPU puts itself to sleep while some of the

on-chip peripherals stay active. The instruction to invoke the idle

mode is the last instruction executed in the normal operating mode

before the idle mode is activated. The CPU contents, the on-chip

RAM, and all of the special function registers remain intact during

this mode. The idle mode can be terminated either by any enabled

interrupt (at which time the process is picked up at the interrupt

service routine and continued), or by a hardware reset which starts

the processor in the same manner as a power-on reset.

POWER-DOWN MODE

In the power-down mode, the oscillator is stopped and the

instruction to invoke power-down is the last instruction executed.

Only the contents of the on-chip RAM are preserved. A hardware

reset is the only way to terminate the power-down mode. The control

bits for the reduced power modes are in the special function register

PCON. Table 1 shows the state of the I/O ports during low current

operating modes.

Table 1. External Pin Status During Idle and Power-Down Modes

MODE

PROGRAM

MEMORY

ALE

PSEN

PORT 0

PORT 1

PORT 2

PORT 3

PORT 4

PWM0/

PWM1

Idle

Internal

1

1

Data

Data

Data

Data

Data

High

Idle

External

1

1

Float

Data

Address

Data

Data

High

Power-down

Internal

0

0

Data

Data

Data

Data

Data

High

Power-down

External

0

0

Float

Data

Data

Data

Data

High