8XC52/54/58
Table 2. Status of the External Pins during Idle and Power Down
Program
Mode
ALE
PSEN
PORT0
PORT1
PORT2
PORT3
Memory
Internal
External
Internal
External
Idle
1
1
0
0
1
1
0
0
Data
Float
Data
Float
Data
Data
Data
Data
Data
Address
Data
Data
Data
Data
Data
Idle
Power Down
Power Down
Data
When the idle mode is terminated by a hardware
reset, the device normally resumes program exe-
cution, from where it left off, up to two machine
cycles before the internal reset algorithm takes
control. On-chip hardware inhibits access to inter-
nal 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 re-
set, the instruction following the one that invokes
Idle should not be one that writes to a port pin or
to external memory.
#
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 until the Power Down mode is terminated.
On the 8XC5X either a hardware reset or an 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 on-chip RAM to retain their values.
ONCE MODE
The ONCE (‘‘On-Circuit Emulation’’) Mode facilitates
testing and debugging of systems using the 8XC5X
without the 8XC5X having to be removed from the
circuit. The ONCE Mode is invoked by:
To properly terminate Power Down, the reset or ex-
ternal interrupt should not be executed before V 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).
CC
1) Pull ALE low while the device is in reset and
PSEN is high;
With an external interrupt, INT0 and INT1 must be
enabled 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.
2) Hold ALE low as RST is deactivated.
While the device is in ONCE Mode, the Port 0 pins
float and the other port pins and ALE and PSEN are
weakly pulled high. The oscillator circuit remains ac-
tive. While the 8XC5X 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.
DESIGN CONSIDERATION
The window on the D87C5X must be covered by
#
an opaque label. Otherwise, the DC and AC char-
acteristics may not be met, and the device may
be functionally impaired.
NOTE:
For more detailed information on the reduced power modes refer to current Embedded Microcontrollers and Processors
Handbook Volume I, (Order No. 270645) and Application Note AP-252 (Embedded Applications Handbook, Order No.
270648), ‘‘Designing with the 80C51BH.’’
6
INTEL [ INTEL ]
