Datasheet

86
4235K–8051–05/08
AT89C51RD2/ED2
20. ONCE
®
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:
• Pull ALE low while the device is in reset (RST high) and PSEN is high.
• Hold ALE low as RST is deactivated.
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. External Pin Status During ONCE Mode
ALE PSEN Port 0 Port 1 Port 2 Port 3 Port I2 XTALA1/2 XTALB1/2
Weak pull-
up
Weak pull-
up
Float
Weak pull-
up
Weak pull-
up
Weak pull-
up
Float Active Active