AT89C51ID2-SMSIM ATMEL [ATMEL Corporation], AT89C51ID2-SMSIM Datasheet - Page 110

• Manufacturer Part Number: AT89C51ID2-SMSIM
• Description: 8-bit Flash Microcontroller
• Manufacturer: ATMEL [ATMEL Corporation]

ONCE
Chip Emulation)
110
(TM)
AT89C51ID2
Mode (ON
The ONCE mode facilitates testing and debugging of systems using AT89C51ID2 with-
out removing the circuit from the board. The ONCE mode is invoked by driving certain
pins of the AT89C51ID2; the following sequence must be exercised:
•
•
While the AT89C51ID2 is in ONCE mode, an emulator or test CPU can be used to drive
the circuit Table 83 shows the status of the port pins during ONCE mode.
Normal operation is restored when normal reset is applied.
Table 83. External Pin Status during ONCE Mode
(a) "Once" is a registered trademark of Intel Corporation.
pull-up
Weak
ALE
Pull ALE low while the device is in reset (RST high) and PSEN is high.
Hold ALE low as RST is deactivated.
PSEN
pull-up
Weak
Port 0
Float
Port 1
pull-up
Weak
Port 2
pull-up
Weak
Port 3
pull-up
Weak
Port I2
Float
XTALA1/2
Active
4289A–8051–09/03
XTALB1/2
Active