87C51F Intel Corporation, 87C51F Datasheet - Page 7

• Manufacturer Part Number: 87C51F
• Description: CHMOS SINGLE-CHIP 8-BIT MICROCONTROLLER
• Manufacturer: Intel Corporation

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-Circuit Emulation’’) Mode facilitates
testing and debugging of systems using the
87C51FA FB FC without removing it from the cir-
cuit The ONCE Mode is invoked by
1 Pull ALE low while the device is in reset and
2 Hold ALE low as RST is deactivated
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 ’’
Idle
Idle
Power Down
Power Down
PSEN is high
Mode
Table 2 Status of the External Pins during Idle and Power Down
Program
Memory
External
External
Internal
Internal
ALE
1
1
0
0
PSEN
1
1
0
0
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 87C51FA FB FC 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
RFI REDUCTION MODE
The RFI reduction feature can be used only if exter-
nal program memory is not required since this mode
disables the ALE pin during instruction code fetches
By writing a logical one to the LSB of the Auxiliary
Register (address 08EH) the ALE is disabled for in-
struction code fetches and the output is weakly held
high When a logical zero is written the ALE pin is
enabled allowing it to generate the Address Latch
Enable signal This bit is cleared by reset Once dis-
abled ALE remains disabled until it is reset by soft-
ware or until a hardware reset occurs
PORT0
Float
Float
Data
Data
AUTOMOTIVE 87C51FA FB FC FC-20
PORT1
Data
Data
Data
Data
Address
PORT2
Data
Data
Data
PORT3
Data
Data
Data
Data
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