87C54-20 Intel Corporation, 87C54-20 Datasheet - Page 7

• Manufacturer Part Number: 87C54-20
• Description: CHMOS SINGLE-CHIP 8-BIT MICROCONTROLLER WITH 16 KBYTES USER PROGRAMMABLE EPROM
• Manufacturer: Intel Corporation

ONCE MODE
The ONCE (‘‘On-Circuit Emulation’’) Mode facilitates
testing and debugging of systems using the 87C54
without the 87C54 having to be removed from the
circuit 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
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 87C54 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
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 ’’
PSEN is high
Idle
Idle
Power Down
Power Down
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
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
PORT1
AUTOMOTIVE 87C54 87C54-20
Data
Data
Data
Data
Address
PORT2
Data
Data
Data
PORT3
Data
Data
Data
Data
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