Datasheet

105
AT89C51IC2
4301D–8051–02/08
ONCE
(TM)
Mode (ON
Chip Emulation)
The ONCE mode facilitates testing and debugging of systems using AT89C51IC2 with-
out removing the circuit from the board. The ONCE mode is invoked by driving certain
pins of the AT89C51IC2; 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 AT89C51IC2 is in ONCE mode, an emulator or test CPU can be used to drive
the circuit Table 82 shows the status of the port pins during ONCE mode.
Normal operation is restored when normal reset is applied.
Table 82. External Pin Status during ONCE Mode
(a) "Once" is a registered trademark of Intel Corporation.
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