Intel 83C51FA User Manual
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AUTOMOTIVE 80C51FA/83C51FA
270501±6
Figure 5. External Clock Drive Configuration
IDLE MODE
The user's software can invoke the Idle Mode. When
the microcontroller is in this mode, power consump-
tion is reduced. The Special Function Registers and
the onboard RAM retain their values during Idle, but
the processor stops executing instructions. Idle
Mode will be exited if the chip is reset or if an en-
abled interrupt occurs. The PCA timer/counter can
optionally be left running or paused during Idle
Mode.
POWER DOWN MODE
To save even more power, a Power Down mode can
be invoked by software. In this mode, the oscillator
is stopped and the instruction that invoked Power
Down is the last instruction executed. The on-chip
RAM and Special Function Registers retain their val-
ues if the Power Down mode is terminated with an
interrupt.
On the 83C51FA either a hardware reset or external
interrupt can cause an exit from Power Down. Reset
redefines all the SFRs but does not change the on-
chip RAM. An external interrupt allows both the
SFRs and the on-chip RAM to retain their values.
To properly terminate Power Down the reset or ex-
ternal interrupt should not be executed before V
CC
is
restored to its normal operating level and must be
held active long enough for the oscillator to restart
and stabilize (normally less than 10 ms).
With an external interrupt, INT0 or INT1 must be en-
abled and configured as level-sensitive. Holding the
pin low restarts the oscillator but bringing the pin
back high completes the exit. Once the interrupt is
serviced, the next instruction to be executed after
RETI will be the one following the instruction that put
the device into Power Down.
DESIGN CONSIDERATION
When the Idle mode is terminated by a hardware
reset, the device normally resumes program execu-
tion, from where it left off, up to two machine cycles
before the internal reset algorithm takes control. On-
chip hardware inhibits access to internal RAM in this
event, but access to the port pins is not inhibited. To
eliminate the possibility of an unexpected write when
Idle is terminated by reset, the instruction following
the one that invokes Idle should not be one that
writes to a port pin or to external memory.
ONCE MODE
The ONCE (``On-CircuitEmulation'') Mode facilitates
testing and debugging of systems using the
83C51FA without the 83C51FA having to be re-
moved from the circuit. The ONCE Mode is invoked
by:
1) Pull ALE low while the device is in reset and
PSEN is high;
2) Hold ALE low as RST is deactivated.
While the device is in ONCE Mode, the Port 0 pins
float, the other port pins and ALE and PSEN are
weakly pulled high. The oscillator circuit remains ac-
tive. While the 83C51FA is in this mode, an emulator
or test CPU can be used to drive the circuit. Normal
operation is restored when a normal reset is applied.
Table 2. Status of the External Pins during Idle and Power Down
Mode
Program
ALE
PSEN
PORT0
PORT1
PORT2
PORT3
Memory
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
NOTE:
For more detailed information on the reduced power modes refer to current Embedded Applications Handbook, and Applica-
tion Note AP-252, ``Designing with the 80C51BH.''
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