at89c51rd2-smsum ATMEL Corporation, at89c51rd2-smsum Datasheet - Page 86

• Manufacturer Part Number: at89c51rd2-smsum
• Description: 8-bit Flash Microcontroller
• Manufacturer: ATMEL Corporation

20. ONCE
86
AT89C51RD2/ED2
®
Mode (ON- Chip Emulation)
The ONCE mode facilitates testing and debugging of systems using AT89C51RD2/ED2 without
removing the circuit from the board. The ONCE mode is invoked by driving certain pins of the
AT89C51RD2/ED2; the following sequence must be exercised:
While the AT89C51RD2/ED2 is in ONCE mode, an emulator or test CPU can be used to drive
the circuit. Table 20-1 shows the status of the port pins during ONCE mode.
Normal operation is restored when normal reset is applied.
Table 20-1.
Weak pull-
• Pull ALE low while the device is in reset (RST high) and PSEN is high.
• Hold ALE low as RST is deactivated.
ALE
up
Weak pull-
PSEN
External Pin Status During ONCE Mode
up
Port 0
Float
Weak pull-
Port 1
up
Weak pull-
Port 2
up
Weak pull-
Port 3
up
Port I2
Float
XTALA1/2
Active
4235J–8051–01/08
XTALB1/2
Active