Data Sheet
FlashFlex51 MCU
SST89E52RD2/RD / SST89E54RD2/RD / SST89E58RD2/RD
SST89V52RD2/RD / SST89V54RD2/RD / SST89V58RD2/RD
61
©2006 Silicon Storage Technology, Inc. S71255-05-000 5/06
12.0 POWER-SAVING MODES
The device provides two power saving modes of operation
for applications where power consumption is critical. The
two modes are idle and power-down, see Table 12-1.
12.1 Idle Mode
Idle mode is entered setting the IDL bit in the PCON regis-
ter. In idle mode, the program counter (PC) is stopped. The
system clock continues to run and all interrupts and periph-
erals remain active. The on-chip RAM and the special func-
tion registers hold their data during this mode.
The device exits idle mode through either a system inter-
rupt or a hardware reset. Exiting idle mode via system
interrupt, the start of the interrupt clears the IDL bit and
exits idle mode. After exit the Interrupt Service Routine, the
interrupted program resumes execution beginning at the
instruction immediately following the instruction which
invoked the idle mode. A hardware reset starts the device
similar to a power-on reset.
12.2 Power-down Mode
The power-down mode is entered by setting the PD bit in
the PCON register. In the power-down mode, the clock is
stopped and external interrupts are active for level sensitive
interrupts only. SRAM contents are retained during power-
down, the minimum V
DD
level is 2.0V.
The device exits power-down mode through either an
enabled external level sensitive interrupt or a hardware
reset. The start of the interrupt clears the PD bit and exits
power-down. Holding the external interrupt pin low restarts
the oscillator, the signal must hold low at least 1024 clock
cycles before bringing back high to complete the exit. Upon
interrupt signal being restored to logic V
IH,
the first instruc-
tion of the interrupt service routine will execute. A hardware
reset starts the device similar to power-on reset.
To exit properly out of power-down, the reset or external
interrupt should not be executed before the V
DD
line is
restored to its normal operating voltage. Be sure to hold
V
DD
voltage long enough at its normal operating level for
the oscillator to restart and stabilize (normally less than
10 ms).
TABLE 12-1: Power Saving Modes
Mode Initiated by State of MCU Exited by
Idle Mode Software
(Set IDL bit in PCON)
MOV PCON, #01H;
CLK is running.
Interrupts, serial port and tim-
ers/counters are active. Pro-
gram Counter is stopped.
ALE and PSEN# signals at a
HIGH level during Idle. All
registers remain unchanged.
Enabled interrupt or hardware reset.
Start of interrupt clears IDL bit and
exits idle mode, after the ISR RETI
instruction, program resumes execu-
tion beginning at the instruction follow-
ing the one that invoked idle mode. A
user could consider placing two or
three NOP instructions after the
instruction that invokes idle mode to
eliminate any problems. A hardware
reset restarts the device similar to a
power-on reset.
Power-down
Mode
Software
(Set PD bit in PCON)
MOV PCON, #02H;
CLK is stopped. On-chip
SRAM and SFR data is main-
tained. ALE and PSEN# sig-
nals at a LOW level during
power -down. External Inter-
rupts are only active for level
sensitive interrupts, if
enabled.
Enabled external level sensitive inter-
rupt or hardware reset. Start of inter-
rupt clears PD bit and exits power-
down mode, after the ISR RETI
instruction program resumes execution
beginning at the instruction following
the one that invoked power-down
mode. A user could consider placing
two or three NOP instructions after the
instruction that invokes power-down
mode to eliminate any problems. A
hardware reset restarts the device sim-
ilar to a power-on reset.
T12-1.0 1255
