I have a very strange behaviour: after a RESET i put the at89s8253 to sleep, can wake up with INT1, go back to sleep, but can't wake up again !
at89s8253 is running at 5V with a 11MHz crystal and has a supervisor.
Here the very basic code used :

MAIN_STACK EQU 0E0H ;stack pointer initial value

IDLE_MODE EQU 081H ;idle mode (SMOD = 1)
DOWN_MODE EQU 002H ;power down mode

;output control pins definition
LEDR_PIN EQU 0A6H ;(P2.6) LEDR pin: red led output
LEDG_PIN EQU 0A7H ;(P2.7) LEDG pin: green led output
BUZZ_PIN EQU 090H ;(P1.0) BUZ pin: buzzer output

$REGISTERBANK(0) ;register bank 0

$INCLUDE(LP_INT.INC) ;CPU configuration

NAME LP_VECT

EXTRN CODE (MAIN_INIT) ;main initialisation
EXTRN CODE (MAIN_RESET) ;main initialisation
EXTRN CODE (MAIN_INT1) ;main initialisation


;Exeption vectors

;reset vector
CSEG AT 00000H
VECT_RESET: AJMP MAIN_RESET ;main initialisation

;external interrupt 0 vector
CSEG AT 00003H
VECT_INTR0: AJMP MAIN_INIT ;main initialisation

;timer 0 interrupt vector
AJMP MAIN_INIT ;main initialisation

;external interrupt 1 vector
CSEG AT 00013H
VECT_INTR1: AJMP MAIN_INT1 ;not used

;timer 1 interrupt vector
CSEG AT 0001BH
VECT_TMR1: AJMP MAIN_INIT ;not used

;serial port interrupt vector
CSEG AT 00023H
VECT_SER: AJMP MAIN_INIT ;main initialisation

;timer 2/I2C interrupt vector
CSEG AT 0002BH
VECT_TMR2: AJMP MAIN_INIT ;main initialisation

END


$REGISTERBANK(0) ;register bank 0

$INCLUDE(LP_INT.INC) ;CPU definition
$INCLUDE(LP_CST.INC) ;constants definition

NAME LP_MAIN

PUBLIC MAIN_INIT ;main initialisation
PUBLIC MAIN_RESET ;main initialisation
PUBLIC MAIN_INT1 ;main initialisation

;****************************************************************
INT_CODE SEGMENT CODE INBLOCK
;****************************************************************
RSEG INT_CODE


MAIN_RESET: CLR LEDR_PIN ;led red = reset
SJMP MAIN_EXEC

MAIN_INT1: CLR LEDG_PIN ;led green = interrupt 1

MAIN_EXEC: MOV P0,#0E3H ;port 0 initialisation
CLR A ;all zero
MOV IE,A ;disable all interrupts

;system initialisation
MOV SP,#MAIN_STACK-1 ;stack pointer initialisation
MOV PSW,A ;select bank 0
MOV TCON,#00000101B ;external edge interrupt
MOV PCON,#10000100B ;power control disable
MOV 08EH,#000H

CLR BUZZ_PIN ;buzzer on
ACALL MAIN_WAIT ;1 sec
SETB BUZZ_PIN ;buzzer off
ACALL MAIN_WAIT ;1 sec
CLR BUZZ_PIN ;buzzer on
ACALL MAIN_WAIT ;1 sec

MAIN_INIT: CLR A
MOV IE,A ;disable all interrupts
MOV TCON,A ;external level interrupt 1
MOV P0,#022H ;unused bits to 0
MOV P1,#0FFH ;all port high
MOV P2,#0FFH ;all port high
MOV P3,#0FFH ;all port high
MOV IE,#(1 SHL EA_BIT OR 1 SHL EX1_BIT)
MOV PCON,#DOWN_MODE ;power down
MAIN_INIT_0: SJMP MAIN_INIT_0

;wait: about 1 sec
MAIN_WAIT: MOV R4,#10
MAIN_WAIT_0: MOV R3,#200
MAIN_WAIT_1: MOV R2,#250
MAIN_WAIT_2: DJNZ R2,MAIN_WAIT_2
DJNZ R3,MAIN_WAIT_1
DJNZ R4,MAIN_WAIT_0
MAIN_WAIT_3: RET

;****************************************************************
END