T89C51RD2-3CBC-L ATMEL [ATMEL Corporation], T89C51RD2-3CBC-L Datasheet - Page 45

• Manufacturer Part Number: T89C51RD2-3CBC-L
• Description: 0 to 40MHz Flash Programmable 8-bit Microcontroller
• Manufacturer: ATMEL [ATMEL Corporation]

6.11. ONCE
The ONCE mode facilitates testing and debugging of systems using T89C51RD2 without removing the circuit
from the board. The ONCE mode is invoked by driving certain pins of the T89C51RD2; the following sequence
must be exercised:
While the T89C51RD2 is in ONCE mode, an emulator or test CPU can be used to drive the circuit. Table 23.
shows the status of the port pins during ONCE mode.
Normal operation is restored when normal reset is applied.
(a) "Once" is a registered trademark of Intel Corporation.
Rev. F - 15 February, 2001
Weak pull-up
Pull ALE low while the device is in reset (RST high) and PSEN is high.
Hold ALE low as RST is deactivated.
ALE
(TM)
Weak pull-up
Mode (ON Chip Emulation)
PSEN
Table 23. External Pin Status during ONCE Mode
Port 0
Float
Weak pull-up
Port 1
Weak pull-up
Port 2
T89C51RD2
Weak pull-up
Port 3
XTAL1/2
Active
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