AT89C55

4-181

Power Down Mode

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

instruction that invokes power down is the last instruction

executed. The on-chip RAM and Special Function Regis-

ters retain their values until the power down mode is termi-

nated. The only exit from power down is a hardware reset.

Reset redefines the SFRs but does not change the on-chip

restored to its normal operating level and must be held

active long enough to allow the oscillator to restart and sta-

bilize.

Program Memory Lock Bits

The AT89C55 has three lock bits that can be left unpro-

grammed (U) or can be programmed (P) to obtain the addi-

tional features listed in the following table.

When lock bit 1 is programmed, the logic level at the EA pin

is sampled and latched during reset. If the device is pow-

ered up without a reset, the latch initializes to a random

value and holds that value until reset is activated. The

latched value of EA must agree with the current logic level

at that pin in order for the device to function properly.

Lock Bit Protection Modes

Programming the Flash

The AT89C55 is normally shipped with the on-chip Flash

memory array in the erased state (that is, contents = FFH)

and ready to be programmed. The programming interface

accepts either a high-voltage (12-volt) or a low-voltage

mode provides a convenient way to program the AT89C55

inside the user’s system, while the high-voltage program-

ming mode is compatible with conventional third party

Flash or EPROM programmers.

The AT89C55 is shipped with either the high-voltage or

low-voltage programming mode enabled. The respective

top-side marking and device signature codes are listed in

following table.

The AT89C55 code memory array is programmed byte-by-

byte in either programming mode.

To program any non-

blank byte in the on-chip Flash Memory, the entire memory

must be erased using the Chip Erase Mode.

Programming Algorithm: Before programming the

AT89C55, the address, data and control signals should be

set up according to the Flash programming mode table and

Figures 9 and 10. To program the AT89C55, take the fol-

lowing steps:

1.

Input the desired memory location on the address lines.

2.

Input the appropriate data byte on the data lines.

3.

Activate the correct combination of control signals.

4.

mode.

5.

Pulse ALE/PROG once to program a byte in the Flash

array or the lock bits. The byte-write cycle is self-timed

and typically takes no more than 1.5 ms. Repeat steps

1 through 5, changing the address and data for the

entire array or until the end of the object file is reached.

Program Lock Bits

LB1

LB2

LB3

Protection Type

1

U

U

U

No program lock features.

2P

U

U

MOVC instructions executed from external program memory are disabled from fetching code bytes

from internal memory, EA is sampled and latched on reset, and further programming of the Flash

memory is disabled.

3

P

P

U

Same as mode 2, but verify is also disabled.

4

P

P

P

Same as mode 3, but external execution is also disabled.

Top-Side Mark

AT89C55

AT89C55

xxxx

xxxx-5

yyww

yyww

Signature

(030H) = 1EH

(030H) = 1EH

(031H) = 55H

(031H) = 55H

(032H) = FFH

(032H) = 05H