Datasheet

AT89C51RB2/RC2

4180D–8051–06/05

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