AT89C51RB2/RC2
ONCE™ Mode (ON
Chip Emulation)
The ONCE mode facilitates testing and debugging of systems using AT89C51RB2/RC2
without removing the circuit from the board. The ONCE mode is invoked by driving cer-
tain pins of the AT89C51RB2/RC2; 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 AT89C51RB2/RC2 is in ONCE mode, an emulator or test CPU can be used to
drive the circuit. Table 61 shows the status of the port pins during ONCE mode.
Normal operation is restored when normal reset is applied.
Table 61. External Pin Status during ONCE Mode
ALE
PSEN
Port 0
Port 1
Port 2
Port 3
XTAL1/2
Weak pull-up Weak pull-up
Float
Weak pull-up Weak pull-up Weak pull-up
Active
79
4180E–8051–10/06
ATMEL [ ATMEL ]
