at89s8253-24psu ATMEL Corporation, at89s8253-24psu Datasheet - Page 35

• Manufacturer Part Number: at89s8253-24psu
• Description: At89s8253 8-bit Microcontroller With 12k Bytes Flash And 2k Bytes Eeprom
• Manufacturer: ATMEL Corporation

18. Power-down Mode
19. Program Memory Lock Bits
3286K–MICRO–12/06
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 Registers retain their
values until the power-down mode is terminated. Exit from power-down can be initiated either by
a hardware reset or by an enabled external interrupt. Reset redefines the SFRs but does not
change the on-chip RAM. The reset should not be activated before V
operating level and must be held active long enough to allow the oscillator to restart and
stabilize.
To exit power-down via an interrupt, external interrupt pin P3.2 or P3.3 must be kept low for at
least the specified required crystal oscillator start up time. Afterwards, the interrupt service rou-
tine starts at the rising edge of the external interrupt pin if the SFR bit AUXR.1 is set. If AUXR.1
is reset (cleared), execution starts after a self-timed interval of 2 ms (nominal) from the falling
edge of the external interrupt pin.
The user should not attempt to enter (or re-enter) the power-down mode for a minimum of 4 µs
until after one of the following conditions has occurred: Start of code execution (after any type of
reset), or Exit from power-down mode.
The AT89S8253 has three lock bits that can be left unprogrammed (U) or can be programmed
(P) to obtain the additional features listed in
When lock bit 1 is programmed, the logic level at the EA pin is sampled and latched during reset.
If the device is powered 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.
Once programmed, the lock bits can only be unprogrammed with the Chip Erase operation in
either the parallel or serial modes.
Table 19-1.
Note:
1
2
3
4
Program Lock Bits
1. U = Unprogrammed
LB1
U
P
P
P
P = Programmed
Lock Bit Protection Modes
LB2
U
U
P
P
LB3
U
U
U
P
Protection Type
No internal memory lock feature.
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
(parallel or serial mode) is disabled.
Same as Mode 2, but parallel or serial verify are also disabled.
Same as Mode 3, but external execution is also disabled.
(1)
Table
19-1.
CC
is restored to its normal
AT89S8253
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