Philips Semiconductors

Product specification

80C31/80C51/87C51

CMOS single-chip 8-bit microcontrollers

1996 Aug 16

3-10

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-up 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.

IDLE MODE

In idle mode, the CPU puts itself to sleep while all 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 2 shows the state of I/O ports during low current operating

modes.

Table 2. External Pin Status During Idle and Power-Down Modes

MODE

PROGRAM MEMORY

ALE

PSEN

PORT 0

PORT 1

PORT 2

PORT 3

Idle

Internal

1

1

Data

Data

Data

Data

Idle

External

1

1

Float

Data

Address

Data

Power-down

Internal

0

0

Data

Data

Data

Data

Power-down

External

0

0

Float

Data

Data

Data

ROM CODE SUBMISSION

When submitting ROM code for the 80C51, the following must be specified:

1. 4k byte user ROM data

2. 64 byte ROM encryption key (SC80C51 only)

3. ROM security bits (SC80C51 only).

ADDRESS

CONTENT

BIT(S)

COMMENT

0000H to 0FFFH

DATA

7:0

User ROM Data

1000H to 101FH

KEY

7:0

ROM Encryption Key

1020H

SEC

0

ROM Security Bit 1

1020H

SEC

1

ROM Security Bit 2

Security Bit 1: When programmed, this bit has two effects on masked ROM parts:

1. External MOVC is disabled, and

2. EA# is latched on Reset.

Security Bit 2: When programmed, this bit inhibits Verify User ROM.