6.11. ONCE(TM) Mode (ON Chip Emulation)
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:
• Pull ALE low while the device is in reset (RST high) and PSEN is high.
• Hold ALE low as RST is deactivated.
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.
Table 23. External Pin Status during ONCE Mode
(a) "Once" is a registered trademark of Intel Corporation.
Rev. F - 15 February, 2001